# RabbitFarm

### 2023-01-29

The Weekly Challenge 201 (Prolog Solutions)

*The examples used here are from the weekly challenge problem statement and demonstrate
the working solution.*

## Part 1

*You are given an array of unique numbers. Write a script to find out all missing numbers
in the range 0..$n where $n is the array size.*

### Solution

```
missing_number(Numbers, Missing):-
length(Numbers, NumbersLength),
between(0, NumbersLength, Missing),
\+ member(Missing, Numbers).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- missing_number([0, 1, 3], Missing).
Missing = 2 ?
(1 ms) yes
| ?- missing_number([0, 1], Missing).
Missing = 2
yes
| ?-
```

### Notes

`missing_number/2`

will only find one missing number at a time. In the examples that come
with the original problem statement there is only ever one missing number. If multiple
missing numbers are required backtracking with `findall/3`

is all you need!

## Part 2

*You are given an integer, $n > 0. Write a script to determine the number of ways of
putting $n pennies in a row of piles of ascending heights from left to right.*

### Solution

```
sum(Coins):-
sum([], Coins, 0).
sum(Coins, Coins, 5).
sum(Partial, Coins, Sum):-
Sum < 5,
between(1, 5, X),
S is Sum + X,
sum([X | Partial], Coins, S).
main:-
findall(Coins, sum(Coins), C),
maplist(msort, C, CS),
sort(CS, CoinsSorted),
write(CoinsSorted), nl,
halt.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- main.
[[1,1,1,1,1],[1,1,1,2],[1,1,3],[1,2,2],[1,4],[2,3],[5]]
```

### Notes

The approach here is the same that I used for the *Coins Sum* problem from
TWC 075. The same as for the
Perl solution to the same problem.

## References

posted at: 18:39 by: Adam Russell | path: /prolog | permanent link to this entry

### 2023-01-15

The Weekly Challenge 199 (Prolog Solutions)

*The examples used here are from the weekly challenge problem statement and demonstrate
the working solution.*

## Part 1

*You are given a list of integers, @list. Write a script to find the total count of Good
airs.*

### Solution

```
good_pair(Numbers, Pair):-
length(Numbers, L),
fd_domain(I, 1, L),
fd_domain(J, 1, L),
I #<# J,
fd_labeling([I, J]),
fd_element(I, Numbers, Ith),
fd_element(J, Numbers, Jth),
Ith #= Jth,
Pair = [I, J].
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- good_pair([1, 2, 3, 1, 1, 3], Pair).
Pair = [1,4] ? a
Pair = [1,5]
Pair = [3,6]
Pair = [4,5]
no
| ?- good_pair([1, 2, 3], Pair).
no
| ?- good_pair([1, 1, 1, 1], Pair).
Pair = [1,2] ? a
Pair = [1,3]
Pair = [1,4]
Pair = [2,3]
Pair = [2,4]
Pair = [3,4]
yes
| ?-
```

### Notes

I take a clpfd approach to this problem and the next. This allows a pretty concise
solution. Here we get the length of the list of numbers, constrain the indices for the
pair and then specify the additional conditions of a *Good Pair*.

## Part 2

*You are given an array of integers, @array and three integers $x,$y,$z. Write a script to
find out total Good Triplets in the given array.*

### Solution

```
good_triplet(Numbers, X, Y, Z, Triplet):-
length(Numbers, I),
fd_domain(S, 1, I),
fd_domain(T, 1, I),
fd_domain(U, 1, I),
S #<# T, T #<# U,
fd_labeling([S, T, U]),
fd_element(S, Numbers, Sth),
fd_element(T, Numbers, Tth),
fd_element(U, Numbers, Uth),
Ast is abs(Sth - Tth), Ast #=<# X,
Atu is abs(Tth - Uth), Atu #=<# Y,
Asu is abs(Sth - Uth), Asu #=<# Z,
Triplet = [Sth, Tth, Uth].
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- good_triplet([3, 0, 1, 1, 9, 7], 7, 2, 3, Triplet).
Triplet = [3,0,1] ? a
Triplet = [3,0,1]
Triplet = [3,1,1]
Triplet = [0,1,1]
no
| ?- good_triplet([1, 1, 2, 2, 3], 0, 0, 1, Triplet).
no
| ?-
```

### Notes

Again for part 2 a clpfd solution ends up being fairly concise. In fact, the approach here is virtually identical to part 1. The differences are only that we are looking for a triple, not a pair, and slightly different criteria.

## References

posted at: 15:16 by: Adam Russell | path: /prolog | permanent link to this entry

### 2023-01-08

The Weekly Challenge 198 (Prolog Solutions)

*The examples used here are from the weekly challenge problem statement and demonstrate
the working solution.*

## Part 1

*You are given a list of integers, @list. Write a script to find the total pairs in the
sorted list where 2 consecutive elements has the max gap. If the list contains less
then 2 elements then return 0.*

### Solution

```
largest_gap(Numbers, LargestGap):-
largest_gap(Numbers, -1, LargestGap).
largest_gap([], LargestGap, LargestGap).
largest_gap([_|[]], LargestGap, LargestGap).
largest_gap([A, B|Numbers], Gap, LargestGap):-
G is B - A,
(G > Gap, largest_gap([B|Numbers], G, LargestGap));
largest_gap([B|Numbers], Gap, LargestGap).
gap_pairs(Numbers, GapPairs):-
length(Numbers, L),
L =< 2,
GapPairs = 0.
gap_pairs(Numbers, GapPairs):-
length(Numbers, L),
L > 2,
largest_gap(Numbers, LargestGap),
gap_pairs(Numbers, LargestGap, 0, GapPairs).
gap_pairs([], _, GapPairs, GapPairs).
gap_pairs([_|[]], _, GapPairs, GapPairs).
gap_pairs([A, B|Numbers], LargestGap, Pairs, GapPairs):-
LargestGap #= B - A,
succ(Pairs, P),
gap_pairs([B|Numbers], LargestGap, P, GapPairs).
gap_pairs([A, B|Numbers], LargestGap, Pairs, GapPairs):-
LargestGap #\= B - A,
gap_pairs([B|Numbers], LargestGap, Pairs, GapPairs).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- gap_pairs([3], Pairs).
Pairs = 0 ?
(1 ms) yes
| ?- gap_pairs([2, 5, 8, 1], Pairs).
Pairs = 2 ?
yes
| ?-
```

### Notes

At first glance this code may look more complex than it really is. All we are doing is , first, computing the largest gap between any two adjacent numbers. Then, second, seeing which pairs have exactly this gap.

## Part 2

*You are given an integer $n > 0. Write a script to print the count of primes less
than $n.*

### Solution

```
primes_under_n(N, NumberPrimes):-
findall(Prime, (between(2, N, I), fd_prime(I), Prime = I), Primes),
length(Primes, NumberPrimes).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- primes_under_n(10, Primes).
Primes = 4
yes
| ?- primes_under_n(15, Primes).
Primes = 6
yes
| ?- primes_under_n(1, Primes).
Primes = 0
yes
| ?- primes_under_n(25, Primes).
Primes = 9
yes
```

### Notes

This solution is short and sweet! No reason or the writeup to be longer than the code itself, right?

## References

posted at: 13:29 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-12-18

The Weekly Challenge 195 (Prolog Solutions)

## Part 1

*You are given a positive integer, $n > 0. Write a script to print the count of all
special integers between 1 and $n.*

### Solution

```
code2digit(C, D):-
number_codes(D, [C]).
special(N):-
number_codes(N, NCodes),
maplist(code2digit, NCodes, Digits),
sort(Digits, DigitsSorted),
length(Digits, NumberDigits),
length(DigitsSorted, NumberDigits).
number_special(N, NumberSpecial):-
findall(I, (between(1, N, I), special(I)), Specials),
length(Specials, NumberSpecial).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- number_special(15, NumberSpecial).
NumberSpecial = 14
(1 ms) yes
| ?- number_special(35, NumberSpecial).
NumberSpecial = 32
(1 ms) yes
| ?-
```

### Notes

The definition of a *special integer* for this problem is an integer whose digits are
unique. To determine this specialness we split the number into its digits using
`number_codes/2`

and a `maplist/3`

which uses a small helper predicate to convert the
codes back to the corresponding digit.

After getting set with identifying special integers all the is left is to count up all the ones found in the given range.

## Part 2

*You are given a list of numbers, @list. Write a script to find most frequent even numbers
in the list. In case you get more than one even numbers then return the smallest even
integer. For all other case, return -1.*

### Solution

```
even(N, Even):-
(0 is mod(N, 2), Even = N);
(Even = nil).
frequency(ListNumbers, N, Frequency):-
delete(ListNumbers, N, ListDeleted),
length(ListNumbers, L),
length(ListDeleted, LD),
Frequency is L - LD.
most_frequent_even(ListNumbers, MostFrequentEven):-
maplist(even, ListNumbers, EN),
delete(EN, nil, EvenNumbers),
length(EvenNumbers, LengthEvens),
LengthEvens > 0,
maplist(frequency(ListNumbers), EvenNumbers, Frequencies),
msort(Frequencies, FS),
reverse(FS, FrequenciesSorted),
((
nth(1, FrequenciesSorted, F1),
nth(2, FrequenciesSorted, F2),
F1 \== F2,
nth(N, Frequencies, F1),
nth(N, EvenNumbers, MostFrequentEven)
);
(
nth(1, FrequenciesSorted, F1),
nth(2, FrequenciesSorted, F2),
F1 == F2,
findall(MFE, (member(FX, FrequenciesSorted), FX == F1, nth(N, Frequencies, FX), nth(N, EvenNumbers, MFE)), MostFrequentEvens),
sort(MostFrequentEvens, MostFrequentEvensSorted),
nth(1, MostFrequentEvensSorted, MostFrequentEven)
)
), !.
most_frequent_even(_, MostFrequentEven):-
MostFrequentEven = -1, !.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- most_frequent_even([1, 1, 2, 6, 2], MostFrequentEven).
MostFrequentEven = 2
yes
| ?- most_frequent_even([1, 3, 5, 7], MostFrequentEven).
MostFrequentEven = -1
yes
| ?- most_frequent_even([6, 4, 4, 6, 1], MostFrequentEven).
MostFrequentEven = 4
yes
| ?-
```

### Notes

The code here may look a bit more convoluted than it really is. Well my use of the
disjunction in `most_frequent/2`

may only be against my own personal sense of aesthetics!
Also, in balance the use of `maplist/3`

cleans things up a bit.

The main ideas here are:

Remove all odd numbers and check to see if any numbers remain.

Compute the frequency of each remaining even number.

Sort and see if there is a tie for most frequent.

If there is no tie in (3) then we're done in the first part of the disjunction. Otherwise, in the second part of the disjunction, find the smallest of the numbers tied for most frequent.

## References

posted at: 16:05 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-12-03

The Weekly Challenge 193 (Prolog Solutions)

## Part 1

*You are given an integer, $n > 0. Write a script to find all possible binary numbers of
size $n.*

### Solution

```
binary --> [].
binary --> digit, binary.
digit --> [0]; [1].
binary_numbers_size_n(N, BinaryNumbers):-
length(Binary, N),
findall(Binary, phrase(binary, Binary), BinaryNumbers).
main:-
binary_numbers_size_n(2, BinaryNumbers),
write(BinaryNumbers), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- main.
[[0,0],[0,1],[1,0],[1,1]]
(1 ms) yes
| ?- binary_numbers_size_n(3, BinaryNumbers).
BinaryNumbers = [[0,0,0],[0,0,1],[0,1,0],[0,1,1],[1,0,0],[1,0,1],[1,1,0],[1,1,1]]
yes
| ?- binary_numbers_size_n(4, BinaryNumbers).
BinaryNumbers = [[0,0,0,0],[0,0,0,1],[0,0,1,0],[0,0,1,1],[0,1,0,0],[0,1,0,1],[0,1,1,0],[0,1,1,1],[1,0,0,0],[1,0,0,1],[1,0,1,0],[1,0,1,1],[1,1,0,0],[1,1,0,1],[1,1,1,0],[1,1,1,1]]
yes
```

### Notes

This challenge presented a perfect use for a DCG, right!?!? For convenience we wrap the
DCG in a predicate `binary_numbers_size_n/2`

to make sure we set a list of the correct
size.

## Part 2

*You are given a list of strings of same length, @s. Write a script to find the odd string
in the given list. Use positional alphabet values starting
with 0, i.e. a = 0, b = 1, ... z = 25.*

### Solution

```
string_differences(String, Differences):-
atom_codes(String, Codes),
string_differences(Codes, [], Differences).
string_differences([_|[]], Differences, Differences).
string_differences([C0, C1|T], DifferenceAccum, Differences):-
Difference is C1 - C0,
string_differences([C1|T], [Difference|DifferenceAccum], Differences).
odd_string(Strings, OddString):-
maplist(string_differences, Strings, Differences),
member(Difference, Differences),
delete(Differences, Difference, UpdatedDifferences),
length(UpdatedDifferences, UpdatedDifferencesLength),
UpdatedDifferencesLength > 1,
nth(N, Differences, Difference),
nth(N, Strings, OddString).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- odd_string([adc, wzy, abc], OddString).
OddString = abc ?
yes
| ?- odd_string([aaa, bob, ccc, ddd], OddString).
OddString = bob ?
(1 ms) yes
| ?- odd_string([aaaa, bbob, cccc, dddd], OddString).
OddString = bbob ?
yes
```

### Notes

The approach here is:

1) Compute all the differences for each string using `maplist/3`

`with string_differences/2`

, a helper predicate I wrote.

2) Once done we use `member/2`

identify a difference from the list

3) Delete the difference and see if the list has been reduced to size one.

4) If the list has not been reduced to one element then we know we have found the uniquely odd string!

5) By position, determine the OddString from the original list.

## References

posted at: 19:58 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-11-27

The Weekly Challenge 192 (Prolog Solutions)

## Part 1

*You are given a positive integer, $n. Write a script to find the binary flip.*

### Solution

```
bits(N, Bits):-
bits(N, [], Bits).
bits(0, Bits, Bits):-!.
bits(N, BitsAccum, Bits):-
B is N /\ 1,
N0 is N >> 1,
bits(N0, [B|BitsAccum], Bits).
bits_reverse(B, RB):-
Flipped is xor(B, 1),
number_chars(Flipped, [RB]).
binary_flip(N, NBitsReversed):-
bits(N, Bits),
maplist(bits_reverse, Bits, BitsReversed),
number_chars(NBitsReversed, ['0','b'|BitsReversed]).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- binary_flip(5, BinaryFlip).
BinaryFlip = 2
yes
| ?- binary_flip(4, BinaryFlip).
BinaryFlip = 3
yes
| ?- binary_flip(6, BinaryFlip).
BinaryFlip = 1
yes
```

### Notes

I learned a slightly obscure bit (no pun intended!) of information about Prolog's handling
of binary numbers this week. If you prepend '0','b' to a list of characters, or the ascii
code equivalents to a list of codes, representing binary digits then `number_chars/2`

(or
`number_codes/2`

) will automatically convert to decimal.

The solution to the whole problem is:

Convert the number to a list of bits (

`bits/2`

).Flip the bits with an xor

Convert the list of flipped bits to decimal using the

`number_chars/2`

method.

## Part 2

*You are given a list of integers greater than or equal to zero, @list. Write a script to
distribute the number so that each members are same. If you succeed then print the total
moves otherwise print -1.*

### Solution

```
:-dynamic(moves/1).
moves(0).
equal_distribution(ListIntegers, _):-
length(ListIntegers, L),
sum_list(ListIntegers, S),
Average is S / L,
F is floor(Average),
C is ceiling(Average),
F \== C,
fail.
equal_distribution(ListIntegers, ListEqualDistribution):-
length(ListIntegers, L),
sum_list(ListIntegers, S),
Average is S / L,
F is floor(Average),
C is ceiling(Average),
F == C,
length(ListEqualDistribution, L),
equal_distribution(ListIntegers, F, ListEqual),
delete(ListEqual, F, ListEqualAverageDeleted),
length(ListEqualAverageDeleted, ListEqualAverageDeletedLength),
((ListEqualAverageDeletedLength == 0,
ListEqualDistribution = ListEqual);
equal_distribution(ListEqual, ListEqualDistribution)), !.
distribute(Average, [X, Y], [S, T]):-
X < Average,
X < Y,
S is X + 1,
T is Y - 1,
moves(Moves),
succ(Moves, M),
retract(moves(Moves)),
asserta(moves(M)).
distribute(Average, [X, Y], [S, T]):-
X > Average,
X > Y,
S is X - 1,
T is Y + 1,
moves(Moves),
succ(Moves, M),
retract(moves(Moves)),
asserta(moves(M)).
distribute(Average, [X, Y], [S, T]):-
((X == Average; X == Y);
(X < Average, X > Y)
),
S = X,
T = Y.
equal_distribution([A, B|[]], Average, [X, Y|[]]):-
maplist(distribute(Average),[[A, B]], [[X, Y]]).
equal_distribution(ListIntegers, Average, [X|T]):-
append([A, B], RestIntegers, ListIntegers),
maplist(distribute(Average),[[A, B]], [[X, Y]]),
equal_distribution([Y|RestIntegers], Average, T).
main(ListIntegers, Moves):-
retract(moves(_)),
asserta(moves(0)),
(equal_distribution(ListIntegers, _), moves(Moves), !);
Moves = -1.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- main([1, 0, 5], Moves).
Moves = 4
(1 ms) yes
| ?- main([0, 2, 0], Moves).
Moves = -1
yes
| ?- main([0, 3, 0], Moves).
Moves = 2
yes
```

### Notes

The rules that must be followed are:

1) You can only move a value of '1' per move

2) You are only allowed to move a value of '1' to a direct neighbor/adjacent cell.

This code ended up being a little more complex than I had originally thought it would be.
At the heart of the solution is what I consider a pretty nice application of `maplist/3`

to drive the `distribution/3`

, which is the implementation of the given rules.

We need to track the number of moves taken, not just the final resulting list. Rather than
track the moves using a variable passed to the various predicates handling the
re-distribution it seemed a bit cleaner to me to instead not have so many variables and
`asserta/1`

and `retract/1`

the updated number of moves.

I generally try and avoid the use of the disjunction `(;)/2`

with the exception of small
cases where to use it would unnaturally increase the amount of code. In this problem there
are several such small cases such as whether we set the number of Moves to -1 in the case
of an impossible re-distribution or the condition for detecting that we are done
re-distributing.

## References

posted at: 19:05 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-11-20

The Weekly Challenge 191 (Prolog Solutions)

## Part 1

*You are given an encoded string consisting of a sequence $s of numeric characters: 0..9.
Write a script to find the all valid different decodings in sorted order.*

### Solution

```
twice_greater(X, Y, TwiceGreater):-
X \== Y,
TwiceY is 2 * Y,
X >= TwiceY,
TwiceGreater = -1.
twice_greater(X, Y, TwiceGreater):-
TwiceY is 2 * Y,
X < TwiceY,
TwiceGreater = 1.
twice_largest(List):-
max_list(List, Max),
maplist(twice_greater(Max), List, TwiceGreater),
delete(TwiceGreater, -1, TwiceGreaterOneDeleted),
length(TwiceGreaterOneDeleted, TwiceGreaterOneDeletedLength),
TwiceGreaterOneDeletedLength == 1, !.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- twice_largest([1, 2, 3, 4]).
no
| ?- twice_largest([1, 2, 0, 5]).
yes
| ?- twice_largest([2, 6, 3, 1]).
yes
| ?- twice_largest([4, 5, 2, 3]).
no
```

### Notes

There are a few ways one could approach this problem. I thought this implementation was
nice and concise, albeit unconventional. `maplist/3`

is used to generate a list with
entries corresponding to whether or not the respective element, times two, in the original
List was more or less than the List Max. If we find only one element fails this, the List
Max itself, then `twice_largest/1`

is true.

## Part 2

*You are given an integer, 0 < $n <= 15. Write a script to find the number of orderings
of numbers that form a cute list.*

### Solution

```
cute(_, _) --> [].
cute(N, CuteList) --> [X], {between(1, N, X), \+ member(X, CuteList),
length(CuteList, CuteListLength),
succ(CuteListLength, I),
(0 is mod(X, I); 0 is mod(I, X)),
append(CuteList, [X], CuteListUpdated)},
cute(N, CuteListUpdated).
main:-
N = 15,
findall(Cute, (length(Cute, N), phrase(cute(N, []), Cute)), C),
sort(C, CuteList),
length(CuteList, NumberCuteList),
write(NumberCuteList), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- main.
24679
```

### Notes

This is a somewhat convoluted use of a DCG and, in turn, the DCG code itself might be a
bit convoluted. Here the DCG will generate all lists which conform to the given rules.
There ends up being some duplication which is natural given the condition. To remove the
duplicates I `sort/2`

the results.

## References

posted at: 16:05 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-11-13

The Weekly Challenge 190 (Prolog Solutions)

## Part 1

*You are given a string with alphabetic characters only: A..Z and a..z. Write a script to
find out if the usage of Capital is appropriate if it satisfies at least one of the
rules.*

### Solution

```
all_small([]).
all_small([H|T]):-
H >= 97,
H =< 122,
all_small(T).
all_capitals([]).
all_capitals([H|T]):-
H >= 65,
H =< 90,
all_capitals(T).
capital_detection([]).
capital_detection([H|T]):-
H >= 65,
H =< 90,
all_capitals(T).
capital_detection([H|T]):-
H >= 65,
H =< 90,
all_small(T).
capital_detection([H|T]):-
H >= 97,
H =< 122,
all_small(T).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- capital_detection("Perl").
true ?
yes
| ?- capital_detection("TPF").
true ?
(1 ms) yes
| ?- capital_detection("PyThon").
no
| ?- capital_detection("raku").
yes
```

### Notes

The rules to be satisfied are:

```
1) Only first letter is capital and all others are small.
2) Every letter is small.
3) Every letter is capital.
```

While I know it is not everyone's favorite way of handling strings in Prolog, I prefer to stick with the codes representation of double quotes strings. Here the solution is pretty straightforward, with the code being in most ways a direct translation of the rules.

## Part 2

*You are given an encoded string consisting of a sequence $s of numeric characters: 0..9.
Write a script to find the all valid different decodings in sorted order.*

### Solution

```
digits([1, 2]).
alphabet(1, 'A').
alphabet(2, 'B').
alphabet(3, 'C').
alphabet(4, 'D').
alphabet(5, 'E').
alphabet(6, 'F').
alphabet(7, 'G').
alphabet(8, 'H').
alphabet(9, 'I').
alphabet(10,'J').
alphabet(11, 'K').
alphabet(12, 'L').
alphabet(13, 'M').
alphabet(14, 'N').
alphabet(15, 'O').
alphabet(16, 'P').
alphabet(17, 'Q').
alphabet(18, 'R').
alphabet(19, 'S').
alphabet(20, 'T').
alphabet(21, 'U').
alphabet(22, 'V').
alphabet(23, 'W').
alphabet(24, 'X').
alphabet(25, 'Y').
alphabet(26, 'Z').
list_chunks(_, [], []).
list_chunks(List, [H|T], [PrefixNumber|RestNumbers]):-
length(Prefix, H),
prefix(Prefix, List),
append(Prefix, Rest, List),
number_codes(PrefixNumber, Prefix),
list_chunks(Rest, T, RestNumbers).
sum(Digits, Length):-
sum([], Digits, Length, 0).
sum(Digits, Digits, _, _).
sum(Partial, Digits, Length, Sum):-
Sum < Length,
digits(L),
member(X, L),
S is Sum + X,
sum([X | Partial], Digits, Length, S).
decode(Encoded, Decoded):-
number_codes(Encoded, EncodedCodes),
length(EncodedCodes, EncodedLength),
findall(Digits,(
sum(Digits, EncodedLength),
sum_list(Digits, EncodedLength)), DigitList),
findall(Chunks, (
member(ChunkSizes, DigitList),
list_chunks(EncodedCodes, ChunkSizes, Chunks)), ChunkList),
findall(DecodedChunk,(
member(C, ChunkList),
maplist(alphabet, C, DecodedChunkChars),
atom_chars(DecodedChunk, DecodedChunkChars)), Decoded).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- decode(11, Decoded).
Decoded = ['AA','K']
yes
| ?- decode(1115, Decoded).
Decoded = ['AAAE','KAE','AKE','AAO','KO']
(1 ms) yes
| ?- decode(127, Decoded).
Decoded = ['ABG','LG']
yes
```

### Notes

There is an element of this task which reminded me of a much older problem presented back in TWC 075. In brief, the question was how many ways could coins be used in combination to form a target sum. My solution used a mix of Prolog and Perl since Prolog is especially well suited for elegant solutions to these sorts of combinatorial problems.

I recognized that this week we have a similar problem in how we may separate the given encoded string into different possible chunks for decoding. Here we know that no chunk may have value greater than 26 and so we can only choose one or two digits at a time. How many ways we can make these one or two digit chunks is the exact same problem, somewhat in hiding, as in TWC 075!

For the Perl Solutions to this problem I re-used much of that older code mentioned above. Also, that Prolog code which was embedded in the Perl code was used as the basis for this pure Prolog solution.

`decode/2`

is the main predicate. We need to work across all possible splits of the
list of the number and it's easiest to use `findall/3`

to cover the splitting of the
number, the chunking, and the final decoding of all possible combinations.

## References

posted at: 21:12 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-11-06

The Weekly Challenge 189 (Prolog Solutions)

## Part 1

*You are given an array of characters (a..z) and a target character. Write a script to
find out the smallest character in the given array lexicographically greater than the
target character.*

### Solution

```
greater_than_character(Target, C0, C1):-
C0 > Target,
C1 = C0.
greater_than_character(Target, C0, C1):-
\+ C0 > Target,
C1 = nil.
greater_character(Characters, Target, Greater):-
maplist(greater_than_character(Target), Characters, GreaterCharacters),
delete(GreaterCharacters, nil, GreaterCharactersNonNil),
length(GreaterCharactersNonNil, GreaterCharactersNonNilLength),
GreaterCharactersNonNilLength > 0,
min_list(GreaterCharactersNonNil, GreaterChar),
char_code(Greater, GreaterChar), !.
greater_character(Characters, [Target], Greater):-
maplist(greater_than_character(Target), Characters, GreaterCharacters),
delete(GreaterCharacters, nil, GreaterCharactersNonNil),
length(GreaterCharactersNonNil, GreaterCharactersNonNilLength),
GreaterCharactersNonNilLength == 0,
char_code(Greater, Target), !.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- greater_character("emug", "b", LeastGreaterCharacter).
LeastGreaterCharacter = e
(1 ms) yes
| ?- greater_character("dcef", "a", LeastGreaterCharacter).
LeastGreaterCharacter = c
yes
| ?- greater_character("jar", "o", LeastGreaterCharacter).
LeastGreaterCharacter = r
(1 ms) yes
| ?- greater_character("dcaf", "a", LeastGreaterCharacter).
LeastGreaterCharacter = c
(1 ms) yes
| ?- greater_character("tgal", "v", LeastGreaterCharacter).
LeastGreaterCharacter = v
yes
```

### Notes

First off, I should note that the use of double quoted strings here might confuse some
people. To clarify, in GNU Prolog what look like double quoted strings are handled as
lists of character codes. Character codes, not characters! So the use of *Character*
in some variable names may seem a little distracting. They are accurate in intent, but if
you are unaware of what's going on it may seem a little strange.

For the solution itself I use a trick with `maplist/3`

that I have used
previously.
`greater_than_character/3`

will either return the character greater than the target or
`nil`

otherwise. `min_list/2`

is then used to get the smallest character which is
greater than the target, as required. Also, note that in the case no character is found to
be greater than the target we instead supply the target itself.

## Part 2

*You are given an array of 2 or more non-negative integers. Write a script to find out the
smallest slice, i.e. contiguous subarray of the original array, having the degree of the
given array.*

### Solution

```
array_degree(Array, Degree):-
array_degree(Array, Array, 0, Degree), !.
array_degree([], _, Degree, Degree).
array_degree([H|T], Array, DegreeAccum, Degree):-
length(Array, ArrayLength),
delete(Array, H, ArrayWithout),
length(ArrayWithout,ArrayWithoutLength),
CurrentDegree is ArrayLength - ArrayWithoutLength,
CurrentDegree > DegreeAccum,
array_degree(T, Array, CurrentDegree, Degree).
array_degree([H|T], Array, DegreeAccum, Degree):-
length(Array, ArrayLength),
delete(Array, H, ArrayWithout),
length(ArrayWithout,ArrayWithoutLength),
CurrentDegree is ArrayLength - ArrayWithoutLength,
\+ CurrentDegree > DegreeAccum,
array_degree(T, Array, DegreeAccum, Degree).
least_slice_degree(Array, LeastDegreeSlice):-
array_degree(Array, ArrayDegree),
findall(Sublist, (prefix(Prefix, Array), suffix(Suffix, Array),
sublist(Sublist, Array), length(Sublist, SublistLength),
SublistLength >= 2, flatten([Prefix, Sublist, Suffix], Array)), Sublists),
sort(Sublists, Slices),
findall(DegreeSlice, (member(Slice, Slices), array_degree(Slice, Degree), DegreeSlice = Degree-Slice), DegreeSlices),
findall(MatchingSlice, (member(DegreeSlice, DegreeSlices), ArrayDegree-MatchingSlice = DegreeSlice), MatchingSlices),
findall(LengthSlice, (member(MatchingDegreeSlice, MatchingSlices), length(MatchingDegreeSlice, MatchingDegreeSliceLength), LengthSlice = MatchingDegreeSliceLength-MatchingDegreeSlice), LengthSlices),
keysort(LengthSlices),
[_-LeastDegreeSlice|_] = LengthSlices.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- least_slice_degree([1, 3, 3, 2], LeastDegreeSlice).
LeastDegreeSlice = [3,3]
(6 ms) yes
| ?- least_slice_degree([1, 2, 1, 3], LeastDegreeSlice).
LeastDegreeSlice = [1,2,1]
(6 ms) yes
| ?- least_slice_degree([1, 3, 2, 1, 2], LeastDegreeSlice).
LeastDegreeSlice = [2,1,2]
(19 ms) yes
| ?- least_slice_degree([1, 1, 2, 3, 2], LeastDegreeSlice).
LeastDegreeSlice = [1,1]
(19 ms) yes
| ?- least_slice_degree([2, 1, 2, 1, 1], LeastDegreeSlice).
LeastDegreeSlice = [1,2,1,1]
(20 ms) yes
```

### Notes

This is the most I've used `findall/3`

in a long time! The main complexity here, I feel,
is in that we must compute all contiguous slices of the original list. `sublist/2`

is not
useful by itself here as it returns sublists which are not contiguous, only ordered. With
the help of `prefix/2`

and `suffix/2`

we are able to identify contiguous sublists.

`findall/3`

is used, then, to obtain and then process all of these contiguous slices to
ultimately identify the one that meets all criteria. First we identify all slices, then we
obtain all degree/slice pairs, and then finally we examine the lengths.

## References

posted at: 20:57 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-10-30

The Weekly Challenge 188 (Prolog Solutions)

## Part 1

*You are given list of integers @list of size $n and divisor $k. Write a script to
find out count of pairs in the given list that satisfies a set of rules.*

### Solution

```
divisible_pair(Numbers, K, Pair):-
length(Numbers, NumbersLength),
between(1, NumbersLength, I),
succ(I, INext),
between(INext, NumbersLength, J),
nth(I, Numbers, Ith),
nth(J, Numbers, Jth),
IJModK is (Ith + Jth) mod K,
IJModK == 0,
Pair = [I, J].
divisible_pairs(Numbers, K, Pairs):-
findall(Pair, divisible_pair(Numbers, K, Pair), Pairs).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- divisible_pairs([4, 5, 1, 6], 2, Pairs), length(Pairs, NumberPairs).
NumberPairs = 2
Pairs = [[1,4],[2,3]]
(1 ms) yes
| ?- divisible_pairs([1, 2, 3, 4], 2, Pairs), length(Pairs, NumberPairs).
NumberPairs = 2
Pairs = [[1,3],[2,4]]
(1 ms) yes
| ?- divisible_pairs([1, 3, 4, 5], 3, Pairs), length(Pairs, NumberPairs).
NumberPairs = 2
Pairs = [[1,4],[3,4]]
yes
| ?- divisible_pairs([5, 1, 2, 3], 4, Pairs), length(Pairs, NumberPairs).
NumberPairs = 2
Pairs = [[1,4],[2,4]]
yes
| ?- divisible_pairs([7, 2, 4, 5], 4, Pairs), length(Pairs, NumberPairs).
NumberPairs = 1
Pairs = [[1,4]]
yes
```

### Notes

The rules, if not clear from the above code are : the pair (i, j) is eligible if and only if

0 <= i < j < len(list)

list[i] + list[j] is divisible by k

There really is not too much here beyond translating the rules into Prolog. The first
condition on i and j are handled by default using `between/3`

. Once we define how to find
one such pair we use `findall/3`

to obtain them all.

## Part 2

*You are given two positive integers $x and $y. Write a script to find out the number of
operations needed to make both ZERO.*

### Solution

```
count_zero(X, Y, Count):-
count_zero(X, Y, 0, Count), !.
count_zero(0, 0, Count, Count).
count_zero(X, Y, CountAccum, Count):-
X > Y,
XNew is X - Y,
succ(CountAccum, CountAccumSucc),
count_zero(XNew, Y, CountAccumSucc, Count).
count_zero(X, Y, CountAccum, Count):-
Y > X,
YNew is Y - X,
succ(CountAccum, CountAccumSucc),
count_zero(X, YNew, CountAccumSucc, Count).
count_zero(X, Y, CountAccum, Count):-
X == Y,
XNew is X - Y,
YNew is Y - X,
succ(CountAccum, CountAccumSucc),
count_zero(XNew, YNew, CountAccumSucc, Count).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- count_zero(5, 4, Count).
Count = 5
(1 ms) yes
| ?- count_zero(4, 6, Count).
Count = 3
yes
| ?- count_zero(2, 5, Count).
Count = 4
yes
| ?- count_zero(3, 1, Count).
Count = 3
yes
| ?- count_zero(7, 4, Count).
Count = 5
yes
```

### Notes

The operations are dictated by these rules:

`$x = $x - $y if $x >= $y`

or

`$y = $y - $x if $y >= $x (using the original value of $x)`

Be carefully examining the rules we can see that we can arrange `count_zero/4`

predicates
in a somewhat concise way. I find this preferable to Prolog's if/else syntax which
absolutely could have been used here. I would argue that the slightly longer form here is
worthwhile in that it is much more readable.

One thing I found especially convenient was that due to the immutable nature of Prolog
variables we don;t have to do any extra accounting for the possibly changed value of `X`

when computing an updated `Y`

. The
Perl solution to this
requires a temporary variable, for example.

## References

posted at: 18:55 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-10-23

The Weekly Challenge 187 (Prolog Solutions)

## Part 2

*You are given a list of positive numbers, @n, having at least 3 numbers.
Write a script to find the triplets (a, b, c) from the given list that satisfies
a set of rules.*

### Solution

```
magical_triple_sum(Numbers, Triple, TripleSum):-
sublist([A, B, C], Numbers),
A + B > C,
B + C > A,
A + C > B,
Triple = [A, B, C],
sum_list(Triple, TripleSum).
magical_triple(Numbers, Triple):-
fd_maximize(magical_triple_sum(Numbers, Triple, TripleSum), TripleSum).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- magical_triple([4, 2, 3], Triple).
Triple = [4,2,3] ?
(1 ms) yes
| ?- magical_triple([1, 3, 2], Triple).
no
| ?- magical_triple([1, 1, 3, 2], Triple).
no
| ?- magical_triple([1, 2, 3, 2], Triple).
Triple = [2,3,2] ?
yes
```

### Notes

The "magical" rules, if not clear from the above code are:

a + b > c

b + c > a

a + c > b

a + b + c is maximum.

I don't routinely do a lot of constraint programming, but I don't think a smaller solution for this can be written! Indeed, the code here is largely a rewriting of the given rules in slightly modified Prolog form.

## References

posted at: 17:25 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-10-16

The Weekly Challenge 186 (Prolog Solutions)

## Part 1

*You are given two lists of the same size. Write a predicate that
merges the two lists.*

### Solution

```
zip([], [], []).
zip([Ha|Ta], [Hb|Tb], [Ha, Hb|Tc]):-
zip(Ta, Tb, Tc).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- zip([1,2,3],[a,b,c],C).
C = [1,a,2,b,3,c]
```

### Notes

This is a great simple example of recursion in Prolog. If there is any doubt
about what is happening here is what `trace/0`

shows:

```
| ?- trace.
The debugger will first creep -- showing everything (trace)
yes
{trace}
| ?- zip([1,2,3],[a,b,c],C).
1 1 Call: zip([1,2,3],[a,b,c],_38) ?
2 2 Call: zip([2,3],[b,c],_73) ?
3 3 Call: zip([3],[c],_102) ?
4 4 Call: zip([],[],_131) ?
4 4 Exit: zip([],[],[]) ?
3 3 Exit: zip([3],[c],[3,c]) ?
2 2 Exit: zip([2,3],[b,c],[2,b,3,c]) ?
1 1 Exit: zip([1,2,3],[a,b,c],[1,a,2,b,3,c]) ?
```

Given that we know the lists are of the same size we have an extremely elegant solution. That is, we state that the first argument is an element, followed by the rest of the list, and the same goes for the second argument. For the third argument we state that its head is the combined initial two elements from the first and second lists and this unifies our uninstantiated variable.

## References

posted at: 20:00 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-09-04

The Weekly Challenge 180 (Prolog Solutions)

## Part 1

*You are given a string. Write a script to find out the first unique character in the
given string and print its index (0-based).*

### Solution

```
index_first_unique(Words, IndexUnique):-
index_first_unique(Words, 0, IndexUnique).
index_first_unique(String, I, IndexUnique):-
succ(I, Index),
length(String, Length),
nth(Index, String, Character),
delete(String, Character, Deleted),
length(Deleted, LengthDeleted),
LengthDifference is Length - LengthDeleted,
LengthDifference == 1, !,
IndexUnique = I.
index_first_unique(String, I, IndexUnique):-
succ(I, Index),
length(String, Length),
nth(Index, String, Character),
delete(String, Character, Deleted),
length(Deleted, LengthDeleted),
LengthDifference is Length - LengthDeleted,
\+ LengthDifference == 1,
succ(I, X),
index_first_unique(String, X, IndexUnique).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- index_first_unique("Long Live Perl", IndexUnique).
IndexUnique = 1
yes
| ?- index_first_unique("Perl Weekly Challenge", IndexUnique).
IndexUnique = 0
yes
| ?- index_first_unique("aabbcc", IndexUnique).
no
| ?- index_first_unique("Prolog Solution to Perl Weekly Challenge", IndexUnique).
IndexUnique = 7
yes
```

### Notes

The main steps here are to check to see if after a character is deleted from the list if the new list length only varies from the original by 1. If so, then we are done. In the case the list is exhausted without finding a unique character the predicate simply fails.

Also note that GNU Prolog's `nth/3`

assumes 1 indexing and so to get the correct 0 based
answer we do an extra `succ/2`

.

## Part 2

*You are given list of numbers and an integer. Write a script to trim the given list when
an element is less than or equal to the given integer.*

### Solution

```
trimmer(X, Y, Z):-
X < Y,
Z = Y.
trimmer(X, Y, Z):-
X >= Y,
Z = 'trimmed'.
trim_list(Numbers, I, TrimmedList):-
maplist(trimmer(I), Numbers, PartialTrimmedList),
delete(PartialTrimmedList, 'trimmed', TrimmedList).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- trim_list([1, 4, 2, 3, 5], 3, TrimmedList).
TrimmedList = [4,5] ?
yes
| ?- trim_list([9, 0, 6, 2, 3, 8, 5], 4, TrimmedList).
TrimmedList = [9,6,8,5] ?
yes
```

### Notes

`maplist/3`

is always tempting to use when required to process a list. In order to get the
effect that we want, however, requires that the predicate used in the maplist succeed for
each value of the list to be processed. Here `trimmer/3`

will either succeed with the
numerical value that passes that comparison or succeed and provide the atom `trimmed`

for
values in the list that fail the comparison. The resulting list is then used with
`delete/3`

to get the final list containing only the numerical values required.

## References

posted at: 16:18 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-08-14

The Weekly Challenge 177 (Prolog Solutions)

## Part 1

*You are given a positive number, $n. Write a script to validate the given number
against the included check digit.*

### Solution

```
damm_matrix(0, [0, 7, 4, 1, 6, 3, 5, 8, 9, 2]).
damm_matrix(1, [3, 0, 2, 7, 1, 6, 8, 9, 4, 5]).
damm_matrix(2, [1, 9, 0, 5, 2, 7, 6, 4, 3, 8]).
damm_matrix(3, [7, 2, 6, 0, 3, 4, 9, 5, 8, 1]).
damm_matrix(4, [5, 1, 8, 9, 0, 2, 7, 3, 6, 4]).
damm_matrix(5, [9, 5 ,7, 8, 4, 0, 2, 6, 1, 3]).
damm_matrix(6, [8, 4, 1, 3, 5, 9, 0, 2, 7, 6]).
damm_matrix(7, [6, 8, 3, 4, 9, 5, 1, 0, 2, 7]).
damm_matrix(8, [4, 6, 5, 2, 7, 8, 3, 1, 0, 9]).
damm_matrix(9, [2, 3, 9, 6, 8, 1, 4, 7, 5, 0]).
damm_validate(N):-
number_chars(N, NChars),
damm_validate(NChars, 0).
damm_validate([], InterimDigit):-
InterimDigit == 0.
damm_validate([H|T], InterimDigit):-
number_chars(N, [H]),
damm_matrix(N, DammRow),
succ(InterimDigit, I),
nth(I, DammRow, NextInterimDigit),
damm_validate(T, NextInterimDigit).
```

### Sample Run

```
| ?- damm_validate(5727).
(1 ms) no
| ?- damm_validate(5724).
yes
```

### Notes

The beauty of Prolog is when it's implicit features such as backtracking and unification are exclusively leveraged to obtain a beautiful solution. This is not one of those cases! This is pure recursion, more functional than logical. I can't think of a more elegant way to express this Damm Validation process, however. Essentially we are performing a series of lookups from a table. I could see this being done with a DCG, perhaps. But in that case we wouldn't really be making anything more elegant, I would argue, just removing the explicit recursion while still having to do the same sort of lookups. It would probably end up being more code than this!

Anyway, Damm Validation is pretty succinctly expressed, it's just that the algorithm doesn't seem to really lend itself to being made much more logically implemented.

## Part 2

*Write a script to generate first 20 Palindromic Prime Cyclops Numbers.*

### Solution

```
cyclops(X):-
number_chars(X, XChars),
length(XChars, XCharsLength),
XCharsLengthMinusOne is XCharsLength - 1,
delete(XChars, '0', XCharsNoZero),
length(XCharsNoZero, NoZeroLength),
NoZeroLength == XCharsLengthMinusOne,
append(Beginning, ['0'|End], XChars),
length(Beginning, BeginningLength),
length(End, EndLength),
BeginningLength == EndLength.
palindrome(X):-
number_codes(X, C),
reverse(C, CR),
number_codes(X, CR).
palindrome_prime(Prime):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(100, MAX_INTEGER, Prime),
palindrome(Prime),
fd_prime(Prime).
palindromic_prime_cyclops(_) --> [].
palindromic_prime_cyclops(Seen) --> [X], {palindrome_prime(X), cyclops(X), \+ member(X, Seen)}, palindromic_prime_cyclops([X|Seen]).
main:-
length(PalindromicPrimeCyclops, 5),
phrase(palindromic_prime_cyclops([]), PalindromicPrimeCyclops),
write(PalindromicPrimeCyclops), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- length(PalindromicPrimeCyclops, 20), phrase(palindromic_prime_cyclops([]), PalindromicPrimeCyclops).
PalindromicPrimeCyclops = [101,16061,31013,35053,38083,73037,74047,91019,94049,1120211,1150511,1160611,1180811,1190911,1250521,1280821,1360631,1390931,1490941,1520251] ?
```

### Notes

This borrows a quite a bit of code from previous challenges! Probably the newest thing
here is the checking of the cyclops condition. In `cyclops/1`

we first check to see if
there is a single zero. This is done by removing all zeroes with `delete/3`

and then
confirming, by checking lengths, that only one was removed. After that we split the list
around the '0' using `append/3`

and if the sublists before and after the '0' are of the
same size we know we have a cyclops number. Notice that it only matters that they are
equal. Whether they are of even length or odd length doesn't matter since we know we have
just a single '0' and so the length of the entire list is necessarily of odd length, as
required.

## References

posted at: 17:37 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-08-07

The Weekly Challenge 176 (Prolog Solutions)

## Part 1

*Write a script to find the smallest integer x such that x, 2x, 3x, 4x, 5x and 6x are
permuted multiples of each other.*

### Solution

```
permuted(X, Y):-
number_chars(X, XChars),
number_chars(Y, YChars),
msort(XChars, XCharsSorted),
msort(YChars, YCharsSorted),
XCharsSorted == YCharsSorted.
permuted_multiple(Permuted):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(1, MAX_INTEGER, X),
X2 is 2 * X,
X3 is 3 * X,
X4 is 4 * X,
X5 is 5 * X,
X6 is 6 * X,
permuted(X, X2), permuted(X2, X3),
permuted(X3, X4), permuted(X4, X5), permuted(X5, X6),
Permuted = X.
```

### Sample Run

```
| ?- permuted_multiple(Permuted).
Permuted = 142857 ?
(2150 ms) yes
```

### Notes

I implemented solutions for this problem in multiple languages and compared side by side on the same system this Prolog solution ran the fastest. I think that's because in Prolog the logic can be (naturally!) expressed very succinctly and so the underlying instructions getting executed are most efficient. Other implementations seem to require a bit more overhead, such as when deconstructing an integer into a list of digits for example.

The task here is for us to generate the smallest integer with this permutable property.
Technically the code here will generate all such numbers, however in search of other
solutions it seems there is just the one found before we exceed the bounds of
`MAX_INTEGER`

.

## Part 2

*Write a script to find out all Reversible Numbers below 100.*

### Solution

```
all_odd([]).
all_odd([H|T]):-
number_chars(Digit, [H]),
M is mod(Digit, 2),
M == 1,
all_odd(T).
reversible(X):-
number_chars(X, XChars),
reverse(XChars, XCharsReversed),
number_chars(R, XCharsReversed),
Sum is X + R,
number_chars(Sum, SumChars),
all_odd(SumChars).
reversible_under_n(N, Reversible):-
between(1, N, X),
reversible(X),
Reversible = X.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- reversible_under_n(100, Reversibles).
Reversibles = 10 ? a
Reversibles = 12
Reversibles = 14
Reversibles = 16
Reversibles = 18
Reversibles = 21
Reversibles = 23
Reversibles = 25
Reversibles = 27
Reversibles = 30
Reversibles = 32
Reversibles = 34
Reversibles = 36
Reversibles = 41
Reversibles = 43
Reversibles = 45
Reversibles = 50
Reversibles = 52
Reversibles = 54
Reversibles = 61
Reversibles = 63
Reversibles = 70
Reversibles = 72
Reversibles = 81
Reversibles = 90
(10 ms) no
```

### Notes

Here we also use Prolog's ability to identify multiple solutions to generate all solutions
less than `100`

, as required.

## References

posted at: 11:54 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-07-24

The Weekly Challenge 174 (Prolog Solutions)

## Part 1

*Write a script to generate the first 19 Disarium Numbers.*

### Solution

```
disariums(_) --> [].
disariums(Seen) --> [X], {disarium(X), \+ member(X, Seen)}, disariums([X|Seen]).
sum_power(Digits, Sum):-
sum_power(Digits, 0, 0, Sum).
sum_power([], _, Sum, Sum).
sum_power([H|T], I, PartialSum, Sum):-
succ(I, N),
number_chars(X, [H]),
Partial is PartialSum + round(X ** N),
sum_power(T, N, Partial, Sum).
disarium(X):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(0, MAX_INTEGER, X),
number_chars(X, Chars),
sum_power(Chars, Sum),
Sum == X.
n_disariums(N, Disariums):-
length(Disariums, N),
phrase(disariums([]), Disariums).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- n_disariums(19, Diariums).
Diariums = [0,1,2,3,4,5,6,7,8,9,89,135,175,518,598,1306,1676,2427,2646798] ?
```

## References

posted at: 19:34 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-07-03

The Weekly Challenge 171 (Prolog Solutions)

## Part 1

*Write a script to generate the first twenty Abundant Odd Numbers.*

### Solution

```
proper_divisors(X, Divisors):-
Half is X // 2,
findall(Divisor,(
between(1, Half, Divisor),
M is mod(X, Divisor),
M == 0
), Divisors).
abundants_odd(_) --> [].
abundants_odd(Previous) --> [X], {abundant_odd(Previous, X)}, abundants_odd(X).
abundant_odd(Previous, X):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(Previous, MAX_INTEGER, X),
X > Previous,
M is mod(X, 2),
M == 1,
proper_divisors(X, Divisors),
sum_list(Divisors, DivisorsSum),
DivisorsSum > X.
n_abundants(N, Abundants):-
length(Abundants, N),
phrase(abundants_odd(-1), Abundants).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- n_abundants(20, Abundants).
Abundants = [945,1575,2205,2835,3465,4095,4725,5355,5775,5985,6435,6615,6825,7245,7425,7875,8085,8415,8505,8925] ?
```

### Notes

The use of a DCG here seems appropriate as we are generating a sequence of numbers of a DCG will allow us to reason on such lists. The logic for inclusion in the sequence is a bit complex and so it further seems natural to break that into its own predicate.

## Part 2

*Create sub compose($f, $g) which takes in two parameters $f and $g as subroutine refs
and returns subroutine ref i.e. compose($f, $g)->($x) = $f->($g->($x)).*

### Solution

```
f(S, T):-
T is S + S.
g(S, T):-
T is S * S.
compose(F, G, H):-
asserta((h(X, Y) :- call(G, X, X0), call(F, X0, Y))),
H = h.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- compose(f, g, H), A =.. [H, 7, X], A.
A = h(7,98)
H = h
X = 98
yes
| ?-
```

### Notes

This challenge is posed as being Perl specific and while most any language is able to do more or less what is asked for, this is a bit of a strange thing to do in Prolog.

In general, Prolog would dicate the use of a *meta-interpreter* for this sort of thing,
instead of this sort of Functional Programming practice. Sticking to the letter of the
challenge I was able to cobble something together with `asserta/1`

and the `univ`

operator
`(=..)/2`

.

An assumption made in my code is that we know ahead of time the number of arguments to
predicates `F`

and `G`

and that we also know which variables are instantiated or not.
Those assumptions greatly simplify things and we can compose the two predicates in this
way. Without that assumption the code would explode in complexity as we would need to
examine whether variables are instantiated or not and then make possibly incorrect
new assumptions that they, in fact, should have been or not.

## References

posted at: 13:18 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-06-19

The Weekly Challenge 169 (Prolog Solutions)

## Part 1

*Write a script to generate the first 20 Brilliant Numbers.*

### Solution

```
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).
brilliants(_) --> [].
brilliants(Seen) --> [X], {brilliant(X), \+ member(X, Seen)}, brilliants([X|Seen]).
brilliant(X):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(1, MAX_INTEGER, X),
prime_factors(X, Factors),
length(Factors, 2),
nth(1, Factors, First),
nth(2, Factors, Second),
number_chars(First, FirstChars),
number_chars(Second, SecondChars),
length(FirstChars, FirstCharsLength),
length(SecondChars, SecondCharsLength),
FirstCharsLength == SecondCharsLength.
n_brilliants(N, Brilliants):-
length(Brilliants, N),
phrase(brilliants([]), Brilliants).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- n_brilliants(20, Brilliants).
Brilliants = [4,6,9,10,14,15,21,25,35,49,121,143,169,187,209,221,247,253,289,299] ?
```

### Notes

The use of a DCG here seems appropriate as we are generating a sequence of numbers of a DCG will allow us to reason on such lists. The logic for inclusion in the sequence is a bit complex and so it further seems natural to break that into its own predicate. That is not required, of course, but in terms of pure style it seems the DCG starts to look clunky or overstuffed when containing a lot of Prolog code in curly braces. Perhaps that is especially true here where we further need additional predicates for computing the prime factors.

## Part 2

*Write a script to generate the first 20 Achilles Numbers.*

### Solution

```
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).
powerful(N, X):-
M is mod(N, X * X),
M == 0.
imperfect(N):-
Sqrt is round(sqrt(N)),
S is Sqrt - 1,
length(I, S),
fd_domain(I, 2, Sqrt),
fd_all_different(I),
fd_labeling(I),!,
maplist(imperfect(N), I).
imperfect(N, X):-
D is log(N) / log(X),
Check is abs(D - round(D)),
\+ Check < 0.000001.
achilles(_) --> [].
achilles(Seen) --> [X], {current_prolog_flag(max_integer, MAX_INTEGER),
between(2, MAX_INTEGER, X), \+ member(X, Seen), achilles(X)},
achilles([X|Seen]).
achilles(X):-
prime_factors(X, Factors),
maplist(powerful(X), Factors),
imperfect(X).
n_achilles(N, Achilles):-
length(Achilles, N),
phrase(achilles([]), Achilles).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- n_achilles(20, Achilles).
Achilles = [72,108,200,288,392,432,500,648,675,800,864,968,972,1125,1152,1323,1352,1372,1568,1800] ?
```

### Notes

The approach here for the second task is similar to that of the first. Somewhat
surprisingly while the conditions of this sequence are more complex the code itself is
represented in a cleaner way. I attribute that to the use of `maplist/2`

which streamlines
the checking of lists for the two criteria of Achilles numbers: that they are *powerful*
but *imperfect*.

## References

posted at: 12:39 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-06-12

The Weekly Challenge 168 (Prolog Solutions)

## Part 1

*Calculate the first 13 Perrin Primes.*

### Solution

```
perrin_primes(A, B, C) --> {D is B + A, fd_not_prime(D)},
perrin_primes(B, C, D).
perrin_primes(A, B, C) --> {D is B + A, fd_prime(D), D \== C},
[D], perrin_primes(B, C, D).
perrin_primes(A, B, C) --> {D is B + A, fd_prime(D), D == C},
[], perrin_primes(B, C, D).
perrin_primes(_, _, _) --> [], !.
n_perrin_primes(N, PerrinPrimes):-
length(PerrinPrimes, N),
phrase(perrin_primes(3, 0, 2), PerrinPrimes).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- n_perrin_primes(13, PerrinPrimes).
PerrinPrimes = [3,2,5,7,17,29,277,367,853,14197,43721,1442968193,792606555396977] ?
```

### Notes

This is a pretty cut and dry use of a DCG to generate this interesting mathematical
sequence. A couple of things that stand out are (1) the condition that `D \== C`

which is
to remove the duplicate `5`

which occurs naturally at the beginning of the sequence.
Afterwards all of the terms strictly increase. Also, (2) although the first two terms are
indeed `3, 2`

it is a convention to sort these and present them as `2, 3`

although I did
not happen to do so here.

## Part 2

*You are given an integer greater than 1. Write a script to find the home prime of the
given number.*

### Solution

```
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).
factor_concat(Factors, Atom):-
factor_concat(Factors, '', Atom).
factor_concat([], Atom, Atom).
factor_concat([H|T], AtomAccum, Atom):-
number_atom(H, A),
atom_concat(AtomAccum, A, UpdatedAtomAccum),
factor_concat(T, UpdatedAtomAccum, Atom).
home_prime(N, HomePrime):-
prime_factors(N, Factors),
factor_concat(Factors, A),
number_atom(Number, A),
fd_not_prime(Number),
home_prime(Number, HomePrime).
home_prime(N, HomePrime):-
prime_factors(N, Factors),
factor_concat(Factors, A),
number_atom(Number, A),
fd_prime(Number),
HomePrime = Number.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- home_prime(16, HomePrime).
HomePrime = 31636373 ?
(4 ms) yes
| ?- home_prime(54, HomePrime).
HomePrime = 2333 ?
(1 ms) yes
| ?- home_prime(108, HomePrime).
HomePrime = 23971 ?
yes
```

### Notes

Here we are asked to compute the *Home Prime* of any given number. The process
for doing so is, given `N`

to take the prime factors for `N`

and concatenate them
together. If the result is prime then we are done, that is the *Home Prime* of `N`

,
typically written `HP(N)`

. This is an easy process to repeat, and in many cases the
computation is a very quick one. However, in some cases, the size of the interim numbers
on the path to HP(N) grow extremely large and the computation bogs down. I have used the
prime factorization code here in several other weekly challenges and it is quite
performant but even this runs rather slowly as we are faced with extremely large numbers.

## References

posted at: 19:21 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-22

The Weekly Challenge 165 (Prolog Solutions)

## Part 1

*Plot lines and points in SVG format.*

### Solution

```
svg-->svg_begin, svg_body, svg_end.
svg_body-->[].
svg_body-->svg_line(_, _, _, _), svg_body.
svg_body-->svg_point(_, _), svg_body.
svg_begin-->['<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"><svg height="100%" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">'].
svg_point(X, Y)-->['<circle cx="', X,'" cy="', Y, '" r="1" />'].
svg_line(X1, Y1, X2, Y2)-->['<line x1="', X1, '" x2="', X2, '" y1="', Y1, '" y2="', Y2, '" style="stroke:#006600;" />'].
svg_end-->[''].
plot([], SVGAccum, SVG):-
phrase(svg_begin, Begin),
flatten([Begin|SVGAccum], SVG).
plot([H|T], SVGAccum, SVG):-
length(H, 2),
[X, Y] = H,
phrase(svg_point(X, Y), Point),
plot(T, [Point|SVGAccum], SVG).
plot([H|T], SVGAccum, SVG):-
length(H, 4),
[X1, Y1, X2, Y2] = H,
phrase(svg_line(X1, Y1, X2, Y2), Line),
plot(T, [Line|SVGAccum], SVG).
plot(Lines, SVG):-
phrase(svg_end, End),
plot(Lines, [End], SVG).
main:-
plot([[53,10], [53, 10, 23, 30], [23, 30]], SVG),
maplist(write, SVG), nl,
halt.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"><svg height="100%" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><circle cx="23" cy="30" r="1" /><line x1="53" x2="23" y1="10" y2="30" /><circle cx="53" cy="10" r="1" /></svg>
```

### Notes

SVG is an XML based format, and so the use of a DCG like this may be unexpected. After all, the "grammar" is really dictated by a known XML schema. Still, the DCG is helpful in that we can describe the sequence and formatting of the statements we expect. From this we can generate the required SVG and even do some basic validation.

## Part 2

*Given a list of numbers, generate the skip summations.*

### Solution

```
avg_difference(Avg, V, Difference):-
Difference is V - Avg.
square(X, XSquared):-
XSquared is X * X.
xy(X, Y, XY):-
XY is X * Y.
linear_regression(Points, RegressionLineEndpoints):-
length(Points, NumberPoints),
% 1. Calculate average of your X variable.
maplist(nth(1), Points, Xs),
msort(Xs, XSorted),
nth(NumberPoints, XSorted, XMax),
sum_list(Xs, XSum),
XAvg is XSum / NumberPoints,
% 2. Calculate the difference between each X and the average X.
maplist(avg_difference(XAvg), Xs, XDifferences),
% 3. Square the differences and add it all up. This is Sx.
maplist(square, XDifferences, XDifferencesSquared),
sum_list(XDifferencesSquared, Sx),
% 4. Calculate average of your Y variable.
maplist(nth(2), Points, Ys),
sum_list(Ys, YSum),
YAvg is YSum / NumberPoints,
% 5. Multiply the differences (of X and Y from their respective averages) and add them all together. This is Sxy.
maplist(avg_difference(YAvg), Ys, YDifferences),
maplist(xy, XDifferences, YDifferences, XY),
sum_list(XY, Sxy),
% 6. Using Sx and Sxy, you calculate the intercept by subtracting Sx / Sxy * AVG(X) from AVG(Y).
M is Sxy / Sx,
B is YAvg - (Sxy / Sx * XAvg),
EndX is XMax + 10,
EndY is M * EndX + B,
RegressionLineEndpoints = [0, B, EndX, EndY].
main:-
Points = [[333,129], [39, 189], [140, 156], [292, 134], [393, 52], [160, 166], [362, 122], [13, 193], [341, 104], [320, 113], [109, 177], [203, 152], [343, 100], [225, 110], [23, 186], [282, 102], [284, 98], [205, 133], [297, 114], [292, 126], [339, 112], [327, 79], [253, 136], [61, 169], [128, 176], [346, 72], [316, 103], [124, 162], [65, 181], [159, 137], [212, 116], [337, 86], [215, 136], [153, 137], [390, 104], [100, 180], [76, 188], [77, 181], [69, 195], [92, 186], [275, 96], [250, 147], [34, 174], [213, 134], [186, 129], [189, 154], [361, 82], [363, 89]],
linear_regression(Points, RegressionLine),
write(RegressionLine), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p --consult-file prolog/ch-2.p
| ?- Points = [[333,129], [39, 189], [140, 156], [292, 134], [393, 52], [160, 166], [362, 122], [13, 193], [341, 104], [320, 113], [109, 177], [203, 152], [343, 100], [225, 110], [23, 186], [282, 102], [284, 98], [205, 133], [297, 114], [292, 126], [339, 112], [327, 79], [253, 136], [61, 169], [128, 176], [346, 72], [316, 103], [124, 162], [65, 181], [159, 137], [212, 116], [337, 86], [215, 136], [153, 137], [390, 104], [100, 180], [76, 188], [77, 181], [69, 195], [92, 186], [275, 96], [250, 147], [34, 174], [213, 134], [186, 129], [189, 154], [361, 82], [363, 89]],
linear_regression(Points, RegressionLine), append(Points, [RegressionLine], PointsLine), plot(PointsLine, SVG), maplist(write, SVG), nl.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"><svg height="100%" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><line x1="0" x2="403" y1="200.13227253558171" y2="79.249802930305563" style="stroke:#006600;" /><circle cx="363" cy="89" r="1" /><circle cx="361" cy="82" r="1" /><circle cx="189" cy="154" r="1" /><circle cx="186" cy="129" r="1" /><circle cx="213" cy="134" r="1" /><circle cx="34" cy="174" r="1" /><circle cx="250" cy="147" r="1" /><circle cx="275" cy="96" r="1" /><circle cx="92" cy="186" r="1" /><circle cx="69" cy="195" r="1" /><circle cx="77" cy="181" r="1" /><circle cx="76" cy="188" r="1" /><circle cx="100" cy="180" r="1" /><circle cx="390" cy="104" r="1" /><circle cx="153" cy="137" r="1" /><circle cx="215" cy="136" r="1" /><circle cx="337" cy="86" r="1" /><circle cx="212" cy="116" r="1" /><circle cx="159" cy="137" r="1" /><circle cx="65" cy="181" r="1" /><circle cx="124" cy="162" r="1" /><circle cx="316" cy="103" r="1" /><circle cx="346" cy="72" r="1" /><circle cx="128" cy="176" r="1" /><circle cx="61" cy="169" r="1" /><circle cx="253" cy="136" r="1" /><circle cx="327" cy="79" r="1" /><circle cx="339" cy="112" r="1" /><circle cx="292" cy="126" r="1" /><circle cx="297" cy="114" r="1" /><circle cx="205" cy="133" r="1" /><circle cx="284" cy="98" r="1" /><circle cx="282" cy="102" r="1" /><circle cx="23" cy="186" r="1" /><circle cx="225" cy="110" r="1" /><circle cx="343" cy="100" r="1" /><circle cx="203" cy="152" r="1" /><circle cx="109" cy="177" r="1" /><circle cx="320" cy="113" r="1" /><circle cx="341" cy="104" r="1" /><circle cx="13" cy="193" r="1" /><circle cx="362" cy="122" r="1" /><circle cx="160" cy="166" r="1" /><circle cx="393" cy="52" r="1" /><circle cx="292" cy="134" r="1" /><circle cx="140" cy="156" r="1" /><circle cx="39" cy="189" r="1" /><circle cx="333" cy="129" r="1" /></svg>
```

### Notes

This is mainly an implementation of the same linear regression procedure as used in the Perl solution to the same problem. By consulting the solution to the first problem we can then re-use the same plotting code.

## References

posted at: 23:28 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-15

The Weekly Challenge 164 (Prolog Solutions)

## Part 1

*Write a script to find all prime numbers less than 1000, which are also palindromes in base 10.*

### Solution

```
:-initialization(main).
palindrome(X):-
fd_labeling(X),
number_codes(X, C),
reverse(C, CR),
number_codes(X, CR).
palindrome_primes(N, PalindromePrimes, NumberPrimes):-
fd_labeling(NumberPrimes),
length(Primes, NumberPrimes),
fd_domain(Primes, 1, N),
fd_all_different(Primes),
maplist(palindrome, Primes),
maplist(fd_prime, Primes),
fd_labeling(Primes),
PalindromePrimes = Primes.
palindrome_primes(N, Primes):-
NP is N // 2,
fd_domain(NumberPrimes, 1, NP),
fd_maximize(palindrome_primes(N, Primes, NumberPrimes), NumberPrimes).
palindrome_prime(N, Prime):-
between(1, N, Prime),
palindrome(Prime),
fd_prime(Prime).
pp(_, _) --> [].
pp(N, Seen) --> [X], {palindrome_prime(N, X), \+ member(X, Seen)}, pp(N, [X|Seen]).
main:-
findall(Prime, palindrome_prime(1000, Prime), PalindromePrimes),
write(PalindromePrimes), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
[2,3,5,7,11,101,131,151,181,191,313,353,373,383,727,757,787,797,919,929]
| ?- phrase(pp(1000, []), PalindromePrimes).
PalindromePrimes = [] ? ;
PalindromePrimes = [2] ? ;
PalindromePrimes = [2,3] ? ;
PalindromePrimes = [2,3,5] ? ;
PalindromePrimes = [2,3,5,7] ? ;
PalindromePrimes = [2,3,5,7,11] ? ;
PalindromePrimes = [2,3,5,7,11,101] ? ;
PalindromePrimes = [2,3,5,7,11,101,131] ? ;
PalindromePrimes = [2,3,5,7,11,101,131,151] ? ;
PalindromePrimes = [2,3,5,7,11,101,131,151,181] ? ;
PalindromePrimes = [2,3,5,7,11,101,131,151,181,191] ? ;
PalindromePrimes = [2,3,5,7,11,101,131,151,181,191,313] ?
.
.
.
```

### Notes

I experimented with a few different ways to generate Palindrome Primes. The quickest and
most efficient way is what is used in `main/0`

. Now, suppose we wanted to reason about
lists of such numbers versus just generating them there is also a DCG option as shown
which will generate or validate all possible lists of Palindrome Primes. Finally, there is
the extremely inefficient method using constraints to maximize the size of the list of
Palindrome Primes under 1000. This works! Now, does it work "well"? Absolutely not! This
is not a very good method for performing the task and is many orders of magnitude slower
than the other two. I will admit to an odd satisfaction to getting this unusual approach
work, however.

## Part 2

*Given a list of numbers, generate the skip summations.*

### Solution

```
:-initialization(main).
pdi(0, Total, Total).
pdi(N, Total_, Total):-
N_ is N // 10,
Total__ is Total_ + round(mod(N, 10) ** 2),
pdi(N_, Total__, Total).
pdi(N, Total):-
pdi(N, 0, Total).
happy(1, _).
happy(N, Seen):-
\+ member(N, Seen),
pdi(N, Total),!,
N_ is Total,
happy(N_, [N|Seen]).
happy(N):-
happy(N, []).
happy(_) --> [].
happy(Seen) --> [X], {between(1, 100, X), \+ member(X, Seen), happy(X)}, happy([X|Seen]).
main:-
length(Happy, 8),
phrase(happy([]), Happy),
write(Happy), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
[1,7,10,13,19,23,28,31]
```

### Notes

As with the code in the first part I also implemented this as a DCG. Here a DCG is more practical since we are asked specifically to generate a list of the first 8 Happy Numbers. This is more of a "list reasoning" task than how the Palindrome Prime question was asked.

## References

posted at: 23:58 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-08

The Weekly Challenge 163 (Prolog Solutions)

## Part 1

*You are given a list of numbers. Write a script to calculate the sum of the bitwise &
operator for all unique pairs.*

### Solution

```
and(N, X, And):-
And is N /\ X.
sum_and([], 0).
sum_and([H|T], SumAnd):-
sum_and(T, Sum),
maplist(and(H), T, Ands),
sum_list(Ands, AndSum),
SumAnd is Sum + AndSum.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- sum_and([1, 2, 3], SumAnd).
SumAnd = 3
yes
| ?- sum_and([2, 3, 4], SumAnd).
SumAnd = 2
yes
| ?- sum_and([1, 2, 3], 99).
no
| ?- sum_and([1, 2, 3], 3).
yes
```

### Notes

It is not too often you see the bitwise operators used in Prolog code! I am kind of a fan
of the `/\`

operator, I mean, it even looks like an upper case 'A', right?

Other than the novelty of the bitwise and I found this task to be well suited for the use
of `maplist/3`

for computing the pairwise and operations across the list of numbers. This
is something of a "nested loop" in the sense that recursively the head of the list of
numbers is removed and upon each recursive step the maplist is re-performed on the
increasingly shorter list.

## Part 2

*Given a list of numbers, generate the skip summations.*

### Solution

```
skip_sum(Numbers, N, Sum):-
append(Left, [N|_], Numbers),
sum_list(Left, SumLeft),
Sum is N + SumLeft.
skip_summations(Numbers, Summations):-
skip_summations(Numbers, [Numbers], Summations).
skip_summations([], Summations, Summations).
skip_summations([_|T], SummationsAccum, Summations):-
maplist(skip_sum(T), T, Sums),
skip_summations(Sums, [Sums|SummationsAccum], Summations).
print_summation(S):-
write(S),
write(' ').
print_summations([]).
print_summations([H|T]):-
print_summations(T),
maplist(print_summation, H), nl.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- skip_summations([1, 2, 3, 4, 5], Summations), print_summations(Summations).
1 2 3 4 5
2 5 9 14
5 14 28
14 42
42
Summations = [[],[42],[14,42],[5,14,28],[2,5,9,14],[1,2,3,4,5]] ?
yes
| ?- skip_summations([1, 3, 5, 7, 9], Summations), print_summations(Summations).
1 3 5 7 9
3 8 15 24
8 23 47
23 70
70
Summations = [[],[70],[23,70],[8,23,47],[3,8,15,24],[1,3,5,7,9]] ?
yes
| ?- skip_summations([1, 3, 5, 7, 9], [[],[70],[23,70],[8,23,47],[3,8,15,24],[1,3,5,27,9]]).
no
| ?- skip_summations([1, 3, 5, 7, 9], [[],[70],[23,70],[8,23,47],[3,8,15,24],[1,3,5,7,9]]).
true ?
yes
```

### Notes

Much like the task above we use a `maplist/3`

within a recursive step. Here the
`maplist/3`

is used to do the partial sums for each "line" of the output. There is also a
`maplist/2`

used to print the output lines with a space between each element.

Satisfyingly, as shown above, the same code not only generates the skip summations but also validates them as well. This sort of intrinsic Prolog behavior brings joy!

## References

posted at: 13:52 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-01

The Weekly Challenge 162 (Prolog Solutions)

## Part 1

*Write a script to generate the check digit of a given ISBN-13 code.*

### Solution

```
weight(-1, 1).
weight(1, 3).
check_sum([], _, 0).
check_sum([H|T], I, CheckSum):-
N is I * -1,
check_sum(T, N, C),
weight(I, Weight),
CheckSum is H * Weight + C.
isbn_check_digit(ISBN, CheckDigit):-
check_sum(ISBN, -1, CheckSum),
Check is mod(CheckSum, 10),
CheckDigit is 10 - Check.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- isbn_check_digit([9, 7, 8, 0, 3, 0, 6, 4, 0, 6, 1, 5], CheckDigit).
CheckDigit = 7
(1 ms) yes
$ gprolog --consult-file prolog/ch-1.p
| ?- isbn_check_digit([9, 7, 8, 0, 3, 0, 6, 4, 0, 6, 1, 5], 3).
(1 ms) no
$ gprolog --consult-file prolog/ch-1.p
| ?- isbn_check_digit([9, 7, 8, 0, 3, 0, 6, 4, 0, 6, 1, 5], 7).
yes
```

### Notes

Sometimes when writing this sort of code I feel the urge to really unleash the power of Prolog and make the most general solution. What would that mean here though? Generate ISBNs for which a given check digit would be valid? While possible it seems like a kind of weird thing to do. Is it interesting to reason about ISBNs in this way? I seem to think that is unlikely.

This code does what seems to reasonable: generate a check digit given an ISBN, or, given both an ISBN and a check digit confirm that the given check digit is the correct one.

## References

posted at: 14:34 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-04-24

The Weekly Challenge 161 (Prolog Solutions)

## Part 1

*Output or return a list of all abecedarian words in the dictionary, sorted in decreasing
order of length.*

### Solution

```
check_and_read(10, [] ,_):-
!.
check_and_read(13, [], _):-
!.
check_and_read(32, [], _):-
!.
check_and_read(44, [], _):-
!.
check_and_read(end_of_file, [], _):-
!.
check_and_read(Char, [Char|Chars], Stream):-
get_code(Stream, NextChar),
check_and_read(NextChar, Chars, Stream).
read_data(Stream, []):-
at_end_of_stream(Stream).
read_data(Stream, [X|L]):-
\+ at_end_of_stream(Stream),
get_code(Stream, Char),
check_and_read(Char, Chars, Stream),
atom_codes(X, Chars),
read_data(Stream, L).
abecedarian(Words, Abecedarian):-
member(Word, Words),
atom_chars(Word, Chars),
sort(Chars, SortedChars),
atom_chars(W, SortedChars),
W = Word,
Abecedarian = Word.
word_length(Word, LengthWord):-
atom_chars(Word, Chars),
length(Chars, Length),
LengthWord = Length-Word.
abecedarians(Words, Abecedarians):-
findall(Abecedarian, abecedarian(Words, Abecedarian), A),
maplist(word_length, A, AL),
keysort(AL, ALSorted),
reverse(ALSorted, Abecedarians).
main:-
open('dictionary', read, Stream),
read_data(Stream, Dictionary),
close(Stream),
abecedarians(Dictionary, Abecedarians),
write(Abecedarians), nl,
halt.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p --entry-goal main
[6-chintz,6-chimps,6-begins,6-almost,6-abhors,5-glory,5-ghost,5-forty,5-flops,5-first,5-filmy,5-films,5-empty,5-dirty,5-deity,5-chops,5-chips,5-chins,5-chimp,5-below,5-begin,5-befit,5-aglow,5-adopt,5-adept,5-abort,5-abhor,4-nosy,4-most,4-mops,4-lost,4-lops,4-know,4-knot,4-imps,4-host,4-hops,4-hips,4-hint,4-hims,4-hilt,4-gory,4-glow,4-gist,4-gins,4-gilt,4-foxy,4-fort,4-flux,4-flow,4-flop,4-fist,4-firs,4-fins,4-film,4-envy,4-elms,4-egos,4-dirt,4-dips,4-dins,4-dims,4-deny,4-dent,4-dens,4-defy,4-deft,4-crux,4-cost,4-copy,4-cops,4-clot,4-city,4-chow,4-chop,4-chip,4-chin,4-cent,4-blow,4-blot,4-bins,4-best,4-bent,4-belt,4-begs,4-amps,4-alms,4-airy,4-airs,4-aims,4-ails,4-ahoy,4-aces,4-ably,4-abet,3-pry,3-opt,3-now,3-not,3-nor,3-mow,3-mop,3-low,3-lot,3-lop,3-joy,3-jot,3-ivy,3-ins,3-imp,3-how,3-hot,3-hop,3-hit,3-his,3-hip,3-him,3-guy,3-got,3-gnu,3-gin,3-fry,3-fox,3-for,3-fly,3-flu,3-fix,3-fit,3-fir,3-fin,3-elm,3-ego,3-dry,3-dot,3-dos,3-dip,3-din,3-dim,3-dew,3-den,3-cry,3-coy,3-cox,3-cow,3-cot,3-cop,3-chi,3-buy,3-boy,3-box,3-bow,3-bop,3-bit,3-bin,3-bet,3-beg,3-art,3-apt,3-any,3-ant,3-amp,3-air,3-aim,3-ail,3-ago,3-ads,3-ado,3-act,3-ace,2-qt,2-ox,2-or,2-no,2-my,2-mu,2-ms,2-ix,2-iv,2-it,2-(is),2-in,2-ho,2-hi,2-go,2-em,2-eh,2-do,2-cs,2-by,2-be,2-ax,2-at,2-as,2-an,2-am,2-ah,2-ad,1-x,1-m,1-a]
```

### Notes

Most of the code here is just for reading the provided dictionary of words. Once that is
complete Prolog really shines. `abecedarian/2`

is the majority of the logic: if a word's
characters when sorted and re-assembled are the original word then it is an Abecedarian.

`abecedarians/2`

is necessary only to fulfill the requirements of the problem
specification which is that all Abecedarians be sorted by length and returned in
descending order.

## References

posted at: 14:27 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-04-17

#### The Weekly Challenge 160 (Prolog Solutions)

## Part 1

*You are given a positive number, $n < 10. Write a script to generate english text
sequence starting with the English cardinal representation of the given number, the word
"is" and then the English cardinal representation of the count of characters that made up
the first word, followed by a comma. Continue until you reach four.*

### Solution

```
cardinal(1, one).
cardinal(2, two).
cardinal(3, three).
cardinal(4, four).
cardinal(5, five).
cardinal(6, six).
cardinal(7, seven).
cardinal(8, eight).
cardinal(9, nine).
cardinal(10, ten).
four_is_magic(N) --> {between(1, 9, N), \+N == 4,
cardinal(N, Cardinal),
atom_codes(Cardinal, Codes),
length(Codes, Count)},
[N-Count], four_is_magic(Count).
four_is_magic(N) --> {between(1, 9, N), N == 4}, [N-magic].
print_magic([]):-
nl.
print_magic([H|T]):-
N-Count = H,
\+ N == 4,
cardinal(N, Cardinal),
cardinal(Count, CountCardinal),
format("~a is ~a, ", [Cardinal, CountCardinal]),
print_magic(T).
print_magic([H|T]):-
N-_ = H,
N == 4,
cardinal(N, Cardinal),
format("~a is ~a", [Cardinal, magic]),
print_magic(T).
main(N) :-
phrase(four_is_magic(N), FourIsMagic),
print_magic(FourIsMagic).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- main(6).
six is three, three is five, five is four, four is magic
true ?
(1 ms) yes
| ?- main(_).
one is three, three is five, five is four, four is magic
true ? ;
two is three, three is five, five is four, four is magic
true ? ;
three is five, five is four, four is magic
true ? ;
five is four, four is magic
true ? ;
six is three, three is five, five is four, four is magic
true ? ;
seven is five, five is four, four is magic
true ? ;
eight is five, five is four, four is magic
true ?
(1 ms) yes
```

### Notes

Prolog has an interesting history of generating text. This is one of the more simple applications I will admit. Here I use a DCG to generate the sequence and then a recursive set of predicates to print the associated text.

In typical Prolog fashion, as shown in the sample output, we can not only generate a sequence with a specified starting point, but all such sequences.

## Part 2

*You are give an array of integers, @n. Write a script to find out the Equilibrium Index
of the given array, if found.*

### Solution

```
equilibrium_index(Numbers, I):-
member(N, Numbers),
append(Left, [N|Right], Numbers),
sum_list(Left, SumLeft),
sum_list(Right, SumRight),
SumLeft == SumRight,
nth(I, Numbers, N).
main:-
(equilibrium_index([1, 3, 5, 7, 9], I), format("~d~n", [I]); format("-1~n", _)),
(equilibrium_index([1, 2, 3, 4, 5], J), format("~d~n", [J]); format("-1~n", _)),
(equilibrium_index([2, 4, 2], K), format("~d~n", [K]); format("-1~n", _)), halt.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- main.
4
-1
2
```

### Notes

This problem was very "Prolog shaped" in my opinion! Just from reading the problem
statement I could picture the standard Prolog predicates that could be used here. Prolog
is not a language which naturally lends itself to *code golf* contests but in terms of
simple elegance this is quite a compact solution!

## References

posted at: 09:59 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-03-20

#### The Weekly Challenge 156 (Prolog Solutions)

## Part 1

*Write a script to generate the first 10 Pernicious Numbers.*

### Solution

```
pernicious(_) --> [].
pernicious(Seen) --> [X], x(Seen, X), {set_bits(X, Bits), fd_prime(Bits)}, pernicious([X|Seen]).
x(Seen, X) --> {between(1, 100, X), \+ member(X, Seen)}.
set_bits(N, X):-
set_bits(N, 0, X).
set_bits(0, X, X).
set_bits(N, X_Acc, X):-
B is N /\ 1,
X0 is X_Acc + B,
N0 is N >> 1,
set_bits(N0, X0, X), !.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- length(Pernicious, 10), phrase(pernicious([]), Pernicious).
Pernicious = [3,5,6,7,9,10,11,12,13,14] ?
(115 ms) yes
| ?- phrase(pernicious([]), [3, 5, 6]).
true ?
(95 ms) yes
```

### Notes

DCGs are great aren't they? The ability to have two modes, one to test and the other to create is a joy! The logic here is pretty straightforward and more or less follows straight fromt he definition.

## Part 2

*Write a script to compute the first 10 distinct Padovan Primes.*

### Solution

```
weird(_) --> [].
weird(Seen) --> [X], x(Seen, X), {
findall(F, factor(X, F), Factors), flatten([1, Factors], FlatFactors),
sum_list(FlatFactors, FactorSum),
FactorSum > X,
powerset(FlatFactors, FactorSets),
maplist(sum_list, FactorSets, FactorSetSums),
\+ member(X, FactorSetSums)
},
weird([X|Seen]).
x(Seen, X) --> {between(1, 1000, X), \+ member(X, Seen)}.
powerset(X,Y):- bagof(S, subseq(S,X), Y).
subseq([], []).
subseq([], [_|_]).
subseq([X|Xs], [X|Ys] ):- subseq(Xs, Ys).
subseq([X|Xs], [_|Ys] ):- append(_, [X|Zs], Ys), subseq(Xs, Zs).
factor(N, Factors):-
S is round(sqrt(N)),
fd_domain(X, 2, S),
R #= N rem X,
R #= 0,
Q #= N // X,
Q #\= X,
fd_labeling([Q, X]),
Factors = [Q, X].
factor(N, Factors):-
S is round(sqrt(N)),
fd_domain(X, 2, S),
R #= N rem X,
R #= 0,
Q #= N // X,
Q #= X,
fd_labeling([Q]),
Factors = [Q].
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- phrase(weird([]), [70]).
true ?
yes
| ?- length(Weird, 1), phrase(weird([]), Weird).
Weird = [70] ?
(4 ms) yes
```

### Notes

This solution follows the same *generate and test* approach I used in the
Perl Solution, as far as the
testing of the powerset of divisors is concerned anyway. (I'll admit I was too lazy to
write my own powerset code so I grabbed someone else's. See the references for a link to
the source.)

In my ongoing attempts to improve my DCG skills I implemented this as a DCG which is a bit of overkill for this problem, but it is always nice to be able to generate the sequence as well as validate.

## References

posted at: 18:26 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-03-06

The Weekly Challenge 154 (Prolog Solutions)

## Part 1

*Write a script to compute the first 10 distinct Padovan Primes.*

### Solution

```
:-initialization(main).
make_lists([], []).
make_lists([Word|Words], [List|Rest]):-
atom_chars(Word, List),
make_lists(Words, Rest).
missing_permutation(Word, Permutations, Missing):-
atom_chars(Word, Chars),
permutation(Chars, Permutation),
\+ member(Permutation, Permutations),
atom_chars(Missing, Permutation).
main:-
make_lists(['PELR', 'PREL', 'PERL', 'PRLE', 'PLER', 'PLRE', 'EPRL', 'EPLR', 'ERPL',
'ERLP', 'ELPR', 'ELRP', 'RPEL', 'RPLE', 'REPL', 'RELP', 'RLPE', 'RLEP',
'LPER', 'LPRE', 'LEPR', 'LRPE', 'LREP'], Permutations),
missing_permutation('PERL', Permutations, Missing),
write(Missing), nl,
halt.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
LERP
```

### Notes

This is a nice place where Prolog really shines compared to the Perl solution
to the same problem. That approach requires a good deal of care to properly generalize.
The Prolog solution is completely general without any extra work! Here we need only split
the starting word into characters and then backtrack through any possible missing
permutations with `permutation/2`

and `member/2`

. Elegant!

## Part 2

*Write a script to compute the first 10 distinct Padovan Primes.*

### Solution

```
padovan_primes(Size, Primes, PrimesAccum, A, B, C) --> {D is B + A, fd_not_prime(D)}, [A], padovan_primes(Size, Primes, PrimesAccum, B, C, D).
padovan_primes(Size, Primes, PrimesAccum, A, B, C) --> {D is B + A, fd_prime(D), append(PrimesAccum, [D], NewPrimesAccum), length(NewPrimesAccum, L), L < Size}, [A], padovan_primes(Size, Primes, NewPrimesAccum, B, C, D).
padovan_primes(Size, Primes, PrimesAccum, A, B, _) --> {D is B + A, fd_prime(D), append(PrimesAccum, [D], NewPrimesAccum), length(NewPrimesAccum, L), L >= Size, Primes = NewPrimesAccum}, [D].
n_padovan_primes(N, Primes):-
succ(N, X),
phrase(padovan_primes(X, PadovanPrimes, [], 1, 1, 1), _),
[_|Primes] = PadovanPrimes.
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- n_padovan_primes(7, Primes).
Primes = [2,3,5,7,37,151,3329] ?
(113 ms) yes
| ?-
```

### Notes

If you watch any of the videos on The Power of Prolog YouTube channel you'll learn
from Markus Triska that a DCG is the preferable way to handle this sort of problem.
Not just because DCGs are a convenient way to process a list in Prolog, but because they
can be used to both generate and test solutions. Excellent advice! This code above shows
this for a somewhat complicated problem. We must generate the sequence and also determine
which of the sequence terms are prime. Primality testing is performed by GNU Prolog's
`fd_prime/1`

and `fd_not_prime/1`

. As the primes are found they are added, along with the
most recently computed three sequence terms, as extra arguments.

This solution is very similar to a previous bit of code for Fibonacci Strings.

## References

posted at: 19:25 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-02-06

#### The Weekly Challenge 150 (Prolog Solutions)

## Part 1

*You are given two strings having the same number of digits, $a and $b. Write a script to
generate Fibonacci Words by concatenation of the previous two strings. Print the 51st
of the first term having at least 51 digits.*

### Solution

```
fibonacci_words(Size, A, B) --> {atom_concat(A, B, C), atom_chars(C, Chars), length(Chars, N), N < Size}, [A], fibonacci_words(Size, B, C).
fibonacci_words(Size, A, B) --> {atom_concat(A, B, C), atom_chars(C, Chars), length(Chars, N), N >= Size}, [C].
fibonacci_words_nth_character(A, B, N, NthChar) :-
phrase(fibonacci_words(N, A, B), FibonacciWords),
last(FibonacciWords, LongestTerm),
atom_chars(LongestTerm, LongestTermChars),
nth(N, LongestTermChars, NthChar).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-1.p
| ?- fibonacci_words_nth_character('1234', '5678', 51, N).
N = '7' ?
(2 ms) yes
| ?-
```

### Notes

This little bit of code might be the thing I am proudest of in a while! You see, modern
Prolog style generally promotes the use of DCGs for almost any list processing you might
want to do. The reasons for this are that DCG code is usually easily bimodal and also
testing of code is much easier. Furthermore, DCG code can be said to rely more on the
Prolog engine itself backtracking versus more manual recursive style code. Of course these
are general statements which are not always necessarily true. In any event I default to
not using DCGs so generally, but have been making a conscious attempt to do so. This
*Fibonacci Words* task is well suited for DCGs.

`Size`

, `A`

, and `B`

are passed as extra arguments to the DCG. Some parts of the DCG might
appear a little mysterious so here is what the DCG code looks like when expanded

```
fibonacci_words(A, B, C, D, E) :-
atom_concat(B, C, F),
atom_chars(F, G),
length(G, H),
H < A,
D = [B|I],
fibonacci_words(A, C, F, I, E).
fibonacci_words(A, B, C, D, E) :-
atom_concat(B, C, F),
atom_chars(F, G),
length(G, H),
H >= A,
D = [F|E].
```

while the variable names are changed during the term expansion you can see the behavior
which may at first seem odd. The first three variables for each predicate are the "extra"
ones which carry the size information, and the most recent two terms of the sequence which
are used to compute the next term. In the DCG when we have `[A]`

you can see what happens,
that it is expanded to unify with the first implicit argument. In the expanded version
we see `D = [B|I]`

, and in this way we create the recursive relationship to build the
sequence list. Well, of course the recursive call which follows is important too, but see
how we've now passed as the first implicit argument `I`

which represents the
uninstantiated tail of a list and on the next call will recursively be added to.

## Part 2

*Write a script to generate all square-free integers <= 500.*

### Solution

```
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).
square_free(N, SquareFree):-
findall(X,
(between(1, N, X),
prime_factors(X, PrimeFactors),
sort(PrimeFactors, PrimeFactorsSorted),
msort(PrimeFactors, PrimeFactorsMSorted),
length(PrimeFactorsSorted, SortedLength),
length(PrimeFactorsMSorted, MSortedLength),
SortedLength == MSortedLength), SquareFree).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- square_free(500, SquareFree).
SquareFree = [1,2,3,5,6,7,10,11,13,14,15,17,19,21,22,23,26,29,30,31,33,34,35,37,38,39,41,42,43,46,47,51,53,55,57,58,59,61,62,65,66,67,69,70,71,73,74,77,78,79,82,83,85,86,87,89,91,93,94,95,97,101,102,103,105,106,107,109,110,111,113,114,115,118,119,122,123,127,129,130,131,133,134,137,138,139,141,142,143,145,146,149,151,154,155,157,158,159,161,163,165,166,167,170,173,174,177,178,179,181,182,183,185,186,187,190,191,193,194,195,197,199,201,202,203,205,206,209,210,211,213,214,215,217,218,219,221,222,223,226,227,229,230,231,233,235,237,238,239,241,246,247,249,251,253,254,255,257,258,259,262,263,265,266,267,269,271,273,274,277,278,281,282,283,285,286,287,290,291,293,295,298,299,301,302,303,305,307,309,310,311,313,314,317,318,319,321,322,323,326,327,329,330,331,334,335,337,339,341,345,346,347,349,353,354,355,357,358,359,362,365,366,367,370,371,373,374,377,379,381,382,383,385,386,389,390,391,393,394,395,397,398,399,401,402,403,406,407,409,410,411,413,415,417,418,419,421,422,426,427,429,430,431,433,434,435,437,438,439,442,443,445,446,447,449,451,453,454,455,457,458,461,462,463,465,466,467,469,470,471,473,474,478,479,481,482,483,485,487,489,491,493,494,497,498,499]
(26 ms) yes
| ?-
```

### Notes

I am re-using the prime factorization code I have use din the past, most recently in
Challenge 123.
Getting the factors is really the hardest part. Once that is doine we need to only check
to see if a number has duplicate prime factors indicating a square. To do that we sort
using `msort/2`

and `sort/2`

and see if the resulting lists are the same length. Recall
that `sort/2`

will remove duplicates whereas `msort/2`

does not. If the results of both
are the same length then we can conclude there were no square factors.

## References

posted at: 16:44 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-01-16

#### The Weekly Challenge 147 (Prolog Solutions)

## Part 1

*Write a script to generate first 20 left-truncatable prime numbers in base 10.*

### Solution

```
:-initialization(main).
left_truncatable(X):-
fd_labeling(X),
number_codes(X, C),
\+ member(48, C),
length(C, L),
findall(Truncatable, (
between(1, L, N),
length(T, N),
append(_, T, C),
number_codes(Truncatable, T),
fd_prime(Truncatable)), Truncatables),
length(Truncatables, NumberTruncatable),
L == NumberTruncatable.
first_twenty_left_truncatable(FirstTwenty):-
length(FirstTwenty, 20),
fd_domain(FirstTwenty, 1, 200),
fd_all_different(FirstTwenty),
maplist(left_truncatable, FirstTwenty),
fd_labeling(FirstTwenty).
main:-
first_twenty_left_truncatable(FirstTwenty),
write(FirstTwenty), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
[2,3,5,7,13,17,23,37,43,47,53,67,73,83,97,113,137,167,173,197]
```

### Notes

I thought quite a while on how to best approach this problem in Prolog. The code here works well. Some knowledge of the size of the left truncatable primes lets us set the upper bound of the domain pretty tightly, but at best that is a very small optimization, if we can even really claim it as such. The change which might most effect performance is to start with a pre-generated list of primes. Especially since the density of primes is much more sparse as the numbers increase the number of unnecessary checks would be greatly reduced.

## Part 2

*Write a script to find the first pair of Pentagon Numbers whose sum and difference are
also a Pentagon Number.*

### Solution

```
n_pentagon_numbers(0, []).
n_pentagon_numbers(N, [H|T]):-
H #= N * (3 * N - 1) / 2,
Next #= N - 1,
n_pentagon_numbers(Next, T).
first_pair_pentagon(FirstPair):-
n_pentagon_numbers(10000, Pentagons),
fd_domain([X, Y, Sum, AbsoluteDifference], Pentagons),
Sum #= X + Y,
Difference #= X - Y,
((
Difference #< 0,
AbsoluteDifference #= -1 * Difference
); AbsoluteDifference #= Difference),
fd_labeling([X, Y]),
FirstPair = [X, Y].
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- first_pair_pentagon(FirstPair).
FirstPair = [7042750,1560090] ?
```

### Notes

Apparently GNU Prolog does not define an absolute value function for FD vars. Perhaps because of some theoretical limitation I am unaware of? No matter, some extra code takes care of that. Frankly, the bigger issue is that in this case the use of an FD solver doesn't really help much beyond a pure Prolog one. Again, I may be unaware of what is happening under the hood, but performance wise it doesn't seem any better to constrain the domains of the variables versus an outright "generate and test".

## References

posted at: 13:06 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-12-19

The Weekly Challenge 143 (Prolog Solutions)

## Part 1

*_You are given a string, $s, containing mathematical expression. Write a script to print
the result of the mathematical expression. To keep it simple, please only accept
+ - * ().*

### Solution

```
:-initialization(main).
expression(Answer) --> term(Answer).
expression(Answer) --> term(Answer0), [(+)], expression(Answer1), {Answer is Answer0 + Answer1}.
expression(Answer) --> term(Answer0), [(-)], expression(Answer1), {Answer is Answer0 - Answer1}.
term(Answer) --> operand(Answer).
term(Answer) --> operand(Answer0), [(*)], term(Answer1), {Answer is Answer0 * Answer1}.
term(Answer) --> operand(Answer0), [(/)], term(Answer1), {Answer is Answer0 / Answer1}.
operand(X) --> [X], {number(X)}.
operand(Answer) --> ['('], expression(Answer), [')'].
calculator(Expression, Answer):-
phrase(expression(Answer), Expression).
main:-
calculator([10, (+), 20, (-), 5], AnswerA),
write(AnswerA), nl,
calculator(['(', 10, (+), 20, (-), 5, ')', (*), 2], AnswerB),
write(AnswerB), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
25
50
```

### Notes

This is the sort of problem which is just so clean and straightforward to implement in Prolog. A DCG is used to describe the expected infix notation of the calculator and that pretty much takes care of it.

## Part 2

*You are given a positive number, $n. Write a script to find out if the given number is a
Stealthy Number.*

### Solution

```
:-initialization(main).
stealthy(N):-
fd_domain(S, 2, N),
fd_domain(T, 2, N),
fd_domain(U, 2, N),
fd_domain(V, 2, N),
S * T #= N,
U * V #= N,
S + T #= U + V + 1,
fd_labeling([S, T, U, V]).
main:-
(stealthy(36), format("~d~n", [1]);format("~d~n", [0])),
(stealthy(12), format("~d~n", [1]);format("~d~n", [0])),
(stealthy(6), format("~d~n", [1]);format("~d~n", [0])),
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
1
1
0
```

### Notes

Much like Part 1 of this weeks challenge Prolog really shines in terms of providing a short clean solution. Here we describe the desired property in terms of finite domain variables and Prolog let's us know if any values exist which match those constraints.

## References

posted at: 19:56 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-12-13

Constructing a Plot in gnuplot Using Gnu Prolog

## Part 1

*Advent of Code 2021 Day 13, in summary, involves the simulation of folding a transparent
piece of paper. When "folded" properly a correct solution yields points which when plotted
reveal a code which is the solution to a puzzle!*

Not wanting to spend too much time on plotting I first considered dumping the points to a
file and then using a plotting program like gnuplot directly. In the spirit of having a
complete self-contained solution, however, I explored what could be done be interfacing
with gnuplot using GNU Prolog's `popen/3`

. This worked very well!

### Solution

```
:-dynamic(dots/1).
:-initialization(main).
check_and_read(10, [] ,_):-
!.
check_and_read(13, [], _):-
!.
check_and_read(32, [], _):-
!.
check_and_read(44, [], _):-
!.
check_and_read(end_of_file, [], _):-
!.
check_and_read(Char, [Char|Chars], Stream):-
get_code(Stream, NextChar),
check_and_read(NextChar, Chars, Stream).
read_data(Stream, []):-
at_end_of_stream(Stream).
read_data(Stream, [X|L]):-
\+ at_end_of_stream(Stream),
get_code(Stream, Char),
check_and_read(Char, Chars, Stream),
atom_codes(X, Chars),
read_data(Stream, L).
gnuplot_command(Command, PlotStream):-
repeat, % START REPEAT
length(Command, CommandLength),
between(1, CommandLength, N),
nth(N, Command, CommandCode),
put_code(PlotStream, CommandCode),
N == CommandLength, % END REPEAT
nl(PlotStream).
plot_configuration(PlotStream):-
popen('/usr/pkg/bin/gnuplot', 'write', PlotStream),
gnuplot_command("set terminal postscript eps size 9, 2 enhanced color font 'Courier, 11'", PlotStream),
gnuplot_command("set output '13.eps'", PlotStream),
gnuplot_command("set yrange [:] reverse", PlotStream),
gnuplot_command("plot '-' u 1:2 t 'Code' with points pointtype 7 pointsize 2", PlotStream).
plot_dots(PlotStream):-
findall(_,(
dots(X-Y),
atom_codes(X, CodesA),
atom_codes(Y, CodesY),
append(CodesA, [32|CodesY], DataCodes),
length(DataCodes, CodesLength),
findall(_,(
between(1, CodesLength, N),
nth(N, DataCodes, DataCode),
put_code(PlotStream, DataCode)
),_),
nl(PlotStream)
), _).
plot:-
plot_configuration(PlotStream),
plot_dots(PlotStream),
[Exit] = "e",
put_code(PlotStream, Exit),
nl(PlotStream),
close(PlotStream).
make_transparency(Records, Folds):-
make_transparency(Records, [], Folds).
make_transparency([], Folds, Folds).
make_transparency([fold, along, Fold|Records], FoldAccum, Folds):-
make_transparency(Records, [Fold|FoldAccum], Folds).
make_transparency([H|Records], FoldAccum, Folds):-
atom_codes(H, C),
C \== [],
append([Y], Rest, Records),
asserta(dots(H-Y)),
make_transparency(Rest, FoldAccum, Folds).
make_transparency([H|Records], FoldAccum, Folds):-
atom_codes(H, C),
C == [],
make_transparency(Records, FoldAccum, Folds).
fold_up(Line):-
findall(Yn, (dots(X-Y), number_atom(Yn, Y)), Yns),
max_list(Yns, MaxY),
Half is div(MaxY, 2),
number_atom(LineN, Line),
findall(_,(
dots(X-Y),
number_atom(Yn, Y),
Yn > LineN,
Y0 is LineN - Yn + Half,
number_atom(Y0, Ya),
retract(dots(X-Y)),
retractall(dots(X-Ya)),
asserta(dots(X-Ya))
), _).
fold_left(Line):-
findall(Xn, (dots(X-Y), number_atom(Xn, X)), Xns),
max_list(Xns, MaxX),
Half is div(MaxX, 2),
number_atom(LineN, Line),
findall(_,(
dots(X-Y),
number_atom(Xn, X),
Xn > LineN,
X0 is LineN - Xn + Half,
number_atom(X0, Xa),
retract(dots(X-Y)),
retractall(dots(Xa-Y)),
asserta(dots(Xa-Y))
), _).
fold_transparency([], _).
fold_transparency(Folds, Count):-
append(F, [Fold], Folds),
atom_codes(Fold, [Axis, 61|Location]),
atom_codes(Direction, [Axis]),
atom_codes(Line, Location),
((Direction == x, fold_left(Line))
;
(Direction == y, fold_up(Line))),
findall(X-Y,dots(X-Y), Dots),
length(Dots, Count),
fold_transparency(F, _).
transparency(Records, Count):-
make_transparency(Records, Folds),
fold_transparency(Folds, Count),
plot.
main:-
open('data', read, Stream),
read_data(Stream, Records),
close(Stream),
transparency(Records, _),
halt.
```

### The results

### Notes

When first plotted the data I did not set the image size and so the default resulted in a
squared image which obscured the characters which are supposed to be visible. The
`size 9, 2`

part of the terminal configuration is to set an aspect ratio for easy reading
of the resulting characters. `set yrange [:] reverse`

is necessary to re-orient the
gnuplot axis to match the axis of the puzzle data.

Anyway, I've never used `popen/3`

before and was very happy to find that it was so easy to
use to come up quickly with fully working solution. Especially when rushing to complete a
coding puzzle as quickly as I could. Another testament to GNU Prolog's clean design!

## References

posted at: 15:42 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-11-28

#### The Weekly Challenge 140 (Prolog Solutions)

## Part 1

*You are given two decimal-coded binary numbers, $a and $b. Write a script to simulate the
addition of the given binary numbers.*

### Solution

```
:-initialization(main).
sum_carry(0, 0, 1, 1, 1).
sum_carry(1, 0, 1, 0, 0).
sum_carry(0, 1, 1, 1, 0).
sum_carry(0, 1, 0, 1, 1).
sum_carry(0, 0, 0, 0, 0).
sum_carry(1, 0, 0, 0, 1).
sum_carry(0, 1, 1, 0, 1).
sum_carry(1, 0, 0, 1, 0).
sum_carry(0, 1, 1, 1).
sum_carry(1, 0, 0, 1).
sum_carry(0, 0, 0, 0).
sum_carry(1, 0, 1, 0).
add_binary(A, B, Sum):-
number_chars(A, AChars),
number_chars(B, BChars),
reverse(AChars, ACharsReverse),
reverse(BChars, BCharsReverse),
add_binary(ACharsReverse, BCharsReverse, 0, [], Sum).
add_binary([], [], 0, SumAccum, Sum):-
number_chars(Sum, SumAccum).
add_binary([], [], 1, SumAccum, Sum):-
number_chars(Sum, ['1'|SumAccum]).
add_binary([H|T], [], Carry, SumAccum, Sum):-
number_chars(D, [H]),
sum_carry(S, C, D, Carry),
number_chars(S, [N]),
add_binary(T, [], C, [N | SumAccum], Sum).
add_binary([H0|T0], [H1|T1], Carry, SumAccum, Sum):-
number_chars(D0, [H0]),
number_chars(D1, [H1]),
sum_carry(S, C, D0, D1, Carry),
number_chars(S, [N]),
add_binary(T0, T1, C, [N | SumAccum], Sum).
main:-
add_binary(11, 1, X0),
write(X0), nl,
add_binary(101, 1, X1),
write(X1), nl,
add_binary(100, 11, X2),
write(X2), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
100
110
111
```

### Notes

The approach here may seem just a little strange for "simulating a binary addition", but it seemed like a fun idea, given the small number of combinations, to just store all the intermediate results and then retrieve them as needed. This all seems to work ok, except that maybe going back and forth between numbers and chars is a little clunky.

## Part 2

*You are given 3 positive integers, $i, $j and $k. Write a script to print the $kth
element in the sorted multiplication table of $i and $j.*

### Solution

```
:-initialization(main).
multiply(I, J, N):-
between(1, I, Ith),
between(1, J, Jth),
N is Ith * Jth.
multiplication_table(I, J, Table):-
bagof(N, multiply(I, J, N), Table).
nth_from_table(I, J, K, N):-
multiplication_table(I, J, Table),
msort(Table, SortedTable),
nth(K, SortedTable, N).
main:-
nth_from_table(2, 3, 4, N0),
write(N0), nl,
nth_from_table(3, 3, 6, N1),
write(N1), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
3
4
```

### Notes

It's maybe a little confusing, to me anyway, that GNU Prolog's `msort/2`

does *not* merge
duplicates but `sort/2`

does. Other than that I have to say that I really like this bit of
Prolog. It seems very clean to me in that no recursion was required, everything is handled
via Prolog itself.

## References

posted at: 16:59 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-11-21

#### The Weekly Challenge 139 (Prolog Solutions)

## Part 1

*You are given a list of numbers. Write a script to implement JortSort. It should return
true/false depending if the given list of numbers are already sorted.*

### Solution

```
:-initialization(main).
jort([]).
jort([H0, H1|[]]):-
H1 >= H0.
jort([H0, H1|T]):-
H1 >= H0,
jort([H1|T]).
main:-
(jort([1, 2, 3, 4, 5]), format("1~n", _); format("0~n", _)),
(jort([1, 3, 2, 4, 5]), format("1~n", _); format("0~n", _)),
(jort([1, 2, 3, 4, 5, 6]), format("1~n", _); format("0~n", _)),
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
1
0
1
```

### Notes

I had never heard of a Jort Sort before this week. Once I understand what it was, a joke function which just returns true or false based on whether a given list is already sorted, I am still not sure I really get it. Or, at least, I don't really get the "joke". Apparently it started as a JavaScript thing so maybe there is something inherently funny about the JavaScript code for it.

Anyway, this is pretty easily done in Prolog especially in this case where we only are
to be given a list of numbers. The code as written only goes in one direction in that it
only verifies a list as requested. This could go in the other direction with a little
use of `between/3`

and generate a sorted list too. But would that ruin the joke? Make it
funnier? I found the whole exercise so unamusing I didn't bother!

Maybe the amusing part of this whole "joke" sort was, ironically, just how stupid I found the whole thing.

## References

posted at: 16:33 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-10-31

#### The Weekly Challenge 136 (Prolog Solutions)

## Part 1

*You are given 2 positive numbers, $m and $n. Write a script to find out if the given two
numbers are Two Friendly.*

### Solution

```
:-initialization(main).
two_friendly(M, N):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(1, MAX_INTEGER, M),
between(1, MAX_INTEGER, N),
GCD is gcd(M, N),
P is log(2, GCD),
P0 is ceiling(P),
P1 is floor(P),
P0 == P1.
main:-
(two_friendly(8, 24), format("1~n", _); format("0~n", _)),
(two_friendly(26, 39), format("1~n", _); format("0~n", _)),
(two_friendly(4, 10), format("1~n", _); format("0~n", _)),
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
1
0
1
```

### Notes

Evn after years of writing Prolog I still get quite a kick out of its inherent power. Here
the `two_friendly/2`

predicate not only verifies for any `M`

and `N`

but also is
multi-modal and can generate pairs for any `M`

and `N`

as well!

## Part 2

*You are given a positive number $n. Write a script to find how many different sequences
you can create using Fibonacci numbers where the sum of unique numbers in each sequence
are the same as the given number.*

### Solution

```
fibonaccis_below_n(N, Fibonaccis):-
fibonaccis_below_n(N, Fibonaccis, 0, [1, 1]).
fibonaccis_below_n(-1, Fibonaccis, _, Fibonaccis):- !.
fibonaccis_below_n(N, Fibonaccis, Sum, PartialFibonaccis):-
[H0, H1 | _] = PartialFibonaccis,
F is H0 + H1,
F < N,
fibonaccis_below_n(N, Fibonaccis, Sum, [F|PartialFibonaccis]).
fibonaccis_below_n(N, Fibonaccis, Sum, PartialFibonaccis):-
[H0, H1 | _] = PartialFibonaccis,
F is H0 + H1,
F > N,
fibonaccis_below_n(-1, Fibonaccis, Sum, PartialFibonaccis).
sum_x([], 0).
sum_x([X|Xs], X+Xse):-
sum_x(Xs, Xse).
sum_lists(X, N, Xsub):-
sublist(Xsub, X),
sum_x(Xsub, Xe),
N #= Xe.
fibonacci_sum_clp(N, Summands):-
fibonaccis_below_n(N, Fibonaccis),
sum_lists(Fibonaccis, N, Summands).
```

### Sample Run

```
$ gprolog --consult-file prolog/ch-2.p
| ?- setof(Summands, fibonacci_sum_clp(16, Summands), S).
S = [[8,5,2,1],[8,5,3],[13,2,1],[13,3]]
(1 ms) yes
```

### Notes

Instead of using a pre-computed list of Fibonacci numbers we generate them as needed. No
particular reason other than it's a little more fun, and also it allows us to flexibly
allow for virtually any value for `N`

.

I just realized as I am looking at the code that I may have slightly misnamed the
`fibonacci_sum_clp/2`

predicate! I was experimenting with different approaches including
a clpfd one. This is clearly not really using clpfd though! Instead `sublist/2`

is used
to generate and test all possible sublists of the Fibonacci subsequence with values
less than `N`

.

## References

posted at: 20:09 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-10-24

#### The Weekly Challenge 135 (Prolog Solutions)

## Part 1

*You are given an integer. Write a script find out the middle 3-digits of the given
integer, if possible, otherwise show a sensible error message.*

### Solution

```
:-initialization(main).
middle_three(N, Middle3):-
number_chars(N, Chars),
N > 0,
length(Chars, Length),
Length > 2,
IsOdd is Length mod 2, IsOdd == 1,
length(M3, 3),
PrefixLength is ceiling(Length / 2) - 2,
length(Prefix, PrefixLength),
append(Prefix, Middle, Chars),
append(M3, _, Middle),
number_chars(Middle3, M3).
middle_three(N, Middle3):-
(N < 0, N0 is abs(N), middle_three(N0, Middle3));
(number_chars(N, Chars), length(Chars, Length), Length < 3, format("too short~n", _));
(number_chars(N, Chars), length(Chars, Length), IsOdd is Length mod 2, IsOdd == 0, format("even number of digits~n", _));
middle_three(N, Middle3).
main:-
middle_three(1234567, Middle3),
((nonvar(Middle3), format("~d~n", [Middle3]), halt);
halt).
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
345
```

### Notes

Interestingly this is one of the rare cases where a Prolog solution follows a fairly similar approach a [Perl solution to the same problem0(http://www.rabbitfarm.com/cgi-bin/blosxom/2021/10/24/perl).

## Part 2

*You are given 7-characters alphanumeric SEDOL. Write a script to validate the given
SEDOL. Print 1 if it is a valid SEDOL otherwise 0.*

### Solution

```
:-initialization(main).
weight(1, 1).
weight(2, 3).
weight(3, 1).
weight(4, 7).
weight(5, 3).
weight(6, 9).
base --> alphanumeric, alphanumeric, alphanumeric, alphanumeric, alphanumeric, alphanumeric.
alphanumeric --> [AlphaNumeric], {letter_or_digit(AlphaNumeric)}.
sedol(Sedol):-
var(Sedol),
sedol(_, Sedol).
sedol(Sedol):-
nonvar(Sedol),
length(Base, 6),
append(Base, [CheckDigit], Sedol),
phrase(base, Base),
compute_check(Base, ComputedCheckDigit),
CheckDigit == ComputedCheckDigit.
sedol(Base, Sedol):-
phrase(base, Base),
check_digit(Base, Sedol).
letter_or_digit(A):-
nonvar(A),
atom_codes(A, C),
((C >= 66, C =< 90);
(C >= 48, C =< 57)).
letter_or_digit(A):-
var(A),
((between(66, 90, C)); %B through Z
(between(48, 57, C))), %0-9
atom_codes(A, [C]).
compute_check(Base, CheckSum):-
compute_check(Base, 1, CheckSum, 0).
compute_check([], _, CheckSum, PartialCheckSum):-
CheckSum is mod(10 - mod(PartialCheckSum, 10), 10).
compute_check([H|T], Index, CheckSum, PartialCheckSum):-
atom_codes(H, [C]),
between(66, 90, C),
weight(Index, Weight),
LetterValue is C - 64 + 9,
Partial is PartialCheckSum + (LetterValue * Weight),
succ(Index, I),
compute_check(T, I, CheckSum, Partial).
compute_check([H|T], Index, CheckSum, PartialCheckSum):-
atom_codes(H, [C]),
between(48, 57, C),
weight(Index, Weight),
NumeralValue is C - 48,
Partial is PartialCheckSum + (NumeralValue * Weight),
succ(Index, I),
compute_check(T, I, CheckSum, Partial).
check_digit(Base, BaseCheckDigit):-
compute_check(Base, CheckDigit),
append(Base, [CheckDigit], BaseCheckDigit).
main:-
(sedol(['2','9','3','6','9','2',1]), format("1~n", _);
format("0~n", _)),
(sedol(['1','2','3','4','5','6',7]), format("1~n", _);
format("0~n", _)),
(sedol(['B','0','Y','B','K','L',9]), format("1~n", _);
format("0~n", _)),
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
1
0
1
```

### Notes

I had originally hoped to have all code for this using almost exclusively DCGs. Things got a bit unwieldy with the checksum computation and so I dialed that back a bit, so to speak. Here the DCG part is for validating or generating a SEDOL base 6 digits sequence and the check digit is computed using regular Prolog predicates.

The rules around SEDOLs are a bit more complex than this problem lets on. I won't recount them all here, but suffice to say we are dealing with a quite idealized set of validations here. For example, prior to 2004 only numerals were allowed, but since then letters are allowed. But only a numeral can follow a letter. Again, though, those are only rules that apply for a certain time range.

Here we are just checking on length, whether or not the SEDOl contains all numerals and/or (uppercase) letter, and the checksum validation.

## References

posted at: 15:17 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-10-17

#### The Weekly Challenge 134 (Prolog Solutions)

## Part 1

*Write a script to generate first 5 Pandigital Numbers in base 10.*

### Solution

```
:-initialization(first_5_pandigitals).
pandigital(Pandigitals):-
Digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
Pandigitals = [A, B, C, D, E, F, G, H, I, J],
fd_domain([A, B, C, D, E, F, G, H, I, J], Digits),
A #= 1,
B #= 0,
fd_labeling(Pandigitals).
first_5_pandigitals:-
setof(P, pandigital(P), Pandigitals),
sort(Pandigitals, [A, B, C, D, E | _ ]),
print_pandigitals([A, B, C, D, E]).
print_pandigitals([]).
print_pandigitals([H|T]):-
maplist(number_codes, H, Codes),
flatten(Codes, DigitCodes),
number_codes(Number, DigitCodes),
write(Number), nl,
print_pandigitals(T).
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
1023456789
1023456798
1023456879
1023456897
1023456978
```

### Notes

Rather than recursively iterate over numbers like in the Perl solution I instead use a bit of constraint programming to generate and test a large number of candidates. I then take only the first five as required. A bit of intuition, based on the definition, reduces the number of candidates generated: the first digit will clearly be a 1, the second a 0.

This runs pretty quickly, yet another testimonial to the design and implementation of GNU Prolog's FD solver. On my 2018 Mac Mini it ran to completion in about second. This is much faster than the brute force Perl solution which takes about 20s on the same machine.

## Part 2

*You are given 2 positive numbers, $m and $n. Write a script to generate multiplication
table and display count of distinct terms.*

### Solution

```
:-initialization(main).
table(M, N, K):-
between(1, M, I),
between(1, N, J),
K is I * J.
print_table(M, N, Distinct):-
findall(K, table(M, N, K), Ks),
setof(K, table(M, N, K), Distinct),
print_header(M, N),
print_seperator(M, N),
print_rows(M, N, Ks),
maplist(number_chars, Distinct, DistinctRow),
maplist(add_space, DistinctRow, DistinctSpaced),
flatten(DistinctSpaced, DistinctSpacedFlattened),
format("~nDistinct Terms: ~S~n", [DistinctSpacedFlattened]),
length(Distinct, Count),
format("Count: ~d ~n", [Count]).
print_rows(0, _, _).
print_rows(M, N, Products):-
length(Row, N),
append(Rest, Row, Products),
M0 is M - 1,
print_rows(M0, N, Rest),
maplist(number_chars, Row, RowChars),
maplist(add_space, RowChars, CharsSpaced),
flatten(CharsSpaced, CharsSpacedFlattened),
format(" ~n ~d | ~S ", [M, CharsSpacedFlattened]).
print_header(_, N):-
format(" x | ", _),
print_header(N).
print_header(0).
print_header(N):-
N0 is N - 1,
print_header(N0),
format("~d ", [N]).
print_seperator(_, N):-
format("~n --+-", _),
print_seperator(N).
print_seperator(0).
print_seperator(N):-
N0 is N - 1,
print_seperator(N0),
format("~a", ['--']).
add_space(C, CS):-
flatten(C, F),
append(F, [' '], FS),
atom_chars(A, FS),
atom_chars(A, CS).
main:-
print_table(3, 3, _), nl, nl,
print_table(3, 5, _),
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
x | 1 2 3
--+-------
1 | 1 2 3
2 | 2 4 6
3 | 3 6 9
Distinct Terms: 1 2 3 4 6 9
Count: 6
x | 1 2 3 4 5
--+-----------
1 | 1 2 3 4 5
2 | 2 4 6 8 10
3 | 3 6 9 12 15
Distinct Terms: 1 2 3 4 5 6 8 9 10 12 15
Count: 11
```

### Notes

This was an interesting exercise in formatting output in Prolog! As can be seen, the vast
majority of the code here is to format the table in the required way. I haven't ever done
all that much with `format/2`

, but it is quite versatile. As far as formatting output goes
it provides a solid set of primitives in the spirit of C's `printf`

that allow for
us to do whatever we want, albeit with a bit of work.

The actual computation of the value takes no more than the first half dozen or so lines of
code in `table/3`

and `print_table/3`

.

## References

posted at: 12:55 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-08-29

#### The Weekly Challenge 127 (Prolog Solutions)

## Part 1

*You are given two sets with unique numbers. Write a script to figure out if they are
disjoint.*

### Solution

```
:-initialization(main).
conflicts(List0, List1):-
member(X, List0),
member(X, List1).
disjoint(List0, List1):-
\+ conflicts(List0, List1).
main:-
((disjoint([1, 2, 5, 3, 4], [4, 6, 7, 8, 9]), write(1)); (write(0))),
nl,
((disjoint([1, 3, 5, 7, 9], [0, 2, 4, 6, 8]), write(1)); (write(0))),
nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
0
1
```

### Notes

The `conflcts/2`

predicate establishes whether or not if there is any overlapping elements
in the two lists. To establish whether the sets are disjoint `disjoint/2`

examines the
negation of this result.

Remember, Prolog only backtracks on failure and not success. This fits nicely for what we
want. We only care if the lists have any overlap whatsoever. So using backtracking
`conflicts/2`

will backtrack for all members of List0 until it either succeeds in finding
an element that is also in List1 or, if none, fails completely indicating a disjoint set.
Since we actually consider the discovery of disjoint sets a success we negate the result
with `\+`

in `disjoint/2`

.

## Part 2

*You are given a list of intervals. Write a script to determine conflicts between the
intervals.*

### Solution

```
:-initialization(main).
conflicts(Intervals, Conflicts):-
conflicts(Intervals, Conflicts, []).
conflicts([_|[]], Conflicts, ConflictsAccum):-
sort(ConflictsAccum, Conflicts).
conflicts([H|T], Conflicts, ConflictsAccum):-
last(T, Last),
append(Intervals, [Last], T),
append([H], Intervals, CompareIntervals),
comparisons(Last, CompareIntervals, HasConflicts),
append(ConflictsAccum, HasConflicts, ConflictsAccumUpdated),
conflicts(CompareIntervals, Conflicts, ConflictsAccumUpdated).
comparisons(Interval, CompareIntervals, Conflicts):-
comparisons(Interval, CompareIntervals, Conflicts, []).
comparisons(_, [], Conflicts, Conflicts).
comparisons(Interval, [[H0|T0]|T], Conflicts, ConflictsAccum):-
[I|J] = Interval,
I >= H0,
I =< T0,
comparisons([I|J], T, Conflicts, [Interval|ConflictsAccum]).
comparisons([I|J], [_|T], Conflicts, ConflictsAccum):-
comparisons([I|J], T, Conflicts, ConflictsAccum).
main:-
conflicts([[1, 4], [3, 5], [6, 8], [12, 13], [3, 20]], Conflicts0),
write(Conflicts0), nl,
conflicts([[3, 4], [5, 7], [6, 9], [10, 12], [13, 15]], Conflicts1),
write(Conflicts1), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
[[3,5],[3,20]]
[[6,9]]
```

### Notes

The examples given in the problem statement are with the `[minimum, maximum]`

intervals
sorted by the maximum value. This makes the problem a bit easier since then we need only
check to see if, when working down the sorted list, if the minimum is in one of the other
intervals.

This is mostly all handled using recursion which keeps with the nature of the examples in the problem statement. This is fine, and obviously works, but after having worked out the details of finding a solution it seems there is room to make this more, well, logical?

Preferring *reasoning over recursion* as a principle of Prolog development seems to be
reasonable (pun intended!). Here we might, for example, arrange the intervals in
Key-Value pairs stored in the Prolog database and make better use of built in
backtracking. But even that would include a little extra work as we would need to sort
the pairs by value and ultimately the same number of lines of code would be written as
shown here, it would just arguably be a bit more *Prological*.

Sometimes the best solution to a problem is only discovered after working on it for a bit using alternative means!

## References

posted at: 11:21 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-08-22

#### The Weekly Challenge 126 (Prolog Solutions)

## Part 1

*You are given a positive integer $N. Write a script to print count of numbers from 1 to
$N that donâ€™t contain digit 1.*

### Solution

```
:-initialization(main).
has_1(N):-
number_codes(N, Codes),
memberchk(49, Codes).
count_numbers_without_1(N, Count):-
count_numbers_without_1(N, 0, Count).
count_numbers_without_1(1, Count, Count).
count_numbers_without_1(N, CountAccum, Count):-
\+ has_1(N),
succ(CountAccum, C),
N0 is N - 1,
count_numbers_without_1(N0, C, Count).
count_numbers_without_1(N, CountAccum, Count):-
has_1(N),
N0 is N - 1,
count_numbers_without_1(N0, CountAccum, Count).
main:-
count_numbers_without_1(15, Count0),
write(Count0), nl,
count_numbers_without_1(25, Count1),
write(Count1), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
8
13
```

### Notes

The `count_numbers_without_1`

predicates recurse over the range of numbers and tally the
qualifying numbers at each step. `has_1/1`

converts the numeral to the list of associated
ascii codes and then we see if the ascii code for '1' (49) is present.

## Part 2

*You are given a rectangle with points marked with either x or *. Please consider the x as
a land mine. Write a script to print a rectangle with numbers and x as in the Minesweeper
game.*

### Solution

```
:-initialization(main).
rows(5).
columns(10).
grid([x, '*', '*', '*', x, '*', x, x, x, x,
'*', '*', '*', '*', '*', '*', '*', '*', '*', x,
'*', '*', '*', '*', x, '*', x, '*', x, '*',
'*', '*', '*', x, x, '*', '*', '*', '*', '*',
x, '*', '*', '*', x, '*', '*', '*', '*', x]).
write_grid([H|T]):-
write('\t'),
write_grid([H|T], 0).
write_grid([], _).
write_grid([H|T], Counter):-
columns(Columns),
succ(Counter, C),
M is C mod Columns,
M \== 0,
write(H),
write(' '),
write_grid(T, C).
write_grid([H|T], Counter):-
columns(Columns),
C is Counter + 1,
M is C mod Columns,
M == 0,
write(H),
((rows(Rows),
columns(Columns),
Cells is Rows * Columns,
C \== Cells,
nl,
write('\t'),
write_grid(T, C)
);
(
write_grid(T, C)
)).
minecount(List, MineCount):-
minecount(List, 0, MineCount).
minecount([], MineCount, MineCount).
minecount([H|T], MineCountPartial, MineCount):-
H == x,
MCP is MineCountPartial + 1,
minecount(T, MCP, MineCount).
minecount([H|T], MineCountPartial, MineCount):-
H \== x,
minecount(T, MineCountPartial, MineCount).
adjacent_bottomleft(Cell, Grid, AdjacentCell):-
columns(Columns),
Columns1 is Columns - 1,
C is Cell + Columns1,
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M0 \== 0, M0 \==1, M1 < M0);
(M0 == 0, M1 == Columns1)),
nth(C, Grid, AdjacentCell).
adjacent_bottomleft(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_left(Cell, Grid, AdjacentCell):-
columns(Columns),
Columns1 is Columns - 1,
C is Cell - 1,
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M0 \== 0, M0 \==1, M1 < M0);
(M0 == 0, M1 == Columns1)),
nth(C, Grid, AdjacentCell).
adjacent_left(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_topleft(Cell, Grid, AdjacentCell):-
columns(Columns),
Columns1 is Columns - 1,
C is Cell - (Columns + 1),
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M0 \== 0, M0 \==1, M1 < M0);
(M0 == 0, M1 == Columns1)),
nth(C, Grid, AdjacentCell).
adjacent_topleft(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_bottomright(Cell, Grid, AdjacentCell):-
columns(Columns),
Columns1 is Columns - 1,
C is Cell + (Columns + 1),
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M1 > M0, M0 \== 0);
(M1 == 0, M0 == Columns1)),
nth(C, Grid, AdjacentCell).
adjacent_bottomright(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_right(Cell, Grid, AdjacentCell):-
columns(Columns),
Columns1 is Columns - 1,
C is Cell + 1,
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M1 > M0, M0 \== 0);
(M1 == 0, M0 == Columns1)),
nth(C, Grid, AdjacentCell).
adjacent_right(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_topright(Cell, Grid, AdjacentCell):-
columns(Columns),
Columns1 is Columns - 1,
C is Cell - Columns1,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M1 > M0, M0 \== 0);
(M1 == 0, M0 == Columns1)),
nth(C, Grid, AdjacentCell).
adjacent_topright(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_up(Cell, Grid, AdjacentCell):-
columns(Columns),
C is Cell - Columns,
C > 0,
nth(C, Grid, AdjacentCell).
adjacent_up(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent_down(Cell, Grid, AdjacentCell):-
columns(Columns),
C is Cell + Columns,
C > 0,
nth(C, Grid, AdjacentCell).
adjacent_down(_, _, AdjacentCell):-
AdjacentCell = null.
adjacent(Cell, Grid, AdjacentCells):-
adjacent_left(Cell, Grid, AdjacentCellLeft),
adjacent_right(Cell, Grid, AdjacentCellRight),
adjacent_up(Cell, Grid, AdjacentCellUp),
adjacent_down(Cell, Grid, AdjacentCellDown),
adjacent_topleft(Cell, Grid, AdjacentCellTopLeft),
adjacent_topright(Cell, Grid, AdjacentCellTopRight),
adjacent_bottomleft(Cell, Grid, AdjacentCellBottomLeft),
adjacent_bottomright(Cell, Grid, AdjacentCellBottomRight),
AdjacentCells = [AdjacentCellLeft, AdjacentCellRight, AdjacentCellUp, AdjacentCellDown,
AdjacentCellTopLeft, AdjacentCellTopRight, AdjacentCellBottomLeft,
AdjacentCellBottomRight],!.
make_grid(Grid, NewGrid):-
make_grid(Grid, 1, [], NewGrid).
make_grid(Grid, Counter, NewGridPartial, NewGrid):-
nth(Counter, Grid, CurrentCell),
CurrentCell \== x,
adjacent(Counter, Grid, AdjacentCells),
minecount(AdjacentCells, MineCount),
append(NewGridPartial, [MineCount], NGP),
((rows(Rows),
columns(Columns),
Cells is Rows * Columns,
Counter == Cells,
!,
NewGrid = NGP
);
(succ(Counter, C),
make_grid(Grid, C, NGP, NewGrid))).
make_grid(Grid, Counter, NewGridPartial, NewGrid):-
nth(Counter, Grid, CurrentCell),
CurrentCell == x,
append(NewGridPartial, [CurrentCell], NGP),
((rows(Rows),
columns(Columns),
Cells is Rows * Columns,
Counter == Cells,
!,
NewGrid = NGP
);
(succ(Counter, C),
make_grid(Grid, C, NGP, NewGrid))).
main:-
grid(Grid),
write('Input:'), nl,
write_grid(Grid), nl,
make_grid(Grid, NewGrid),
write('Output:'), nl,
write_grid(NewGrid), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
Input:
x * * * x * x x x x
* * * * * * * * * x
* * * * x * x * x *
* * * x x * * * * *
x * * * x * * * * x
Output:
x 1 0 1 x 2 x x x x
1 1 0 2 2 4 3 5 5 x
0 0 1 3 x 3 x 2 x 2
1 1 1 x x 4 1 2 2 2
x 1 1 3 x 2 0 0 1 x
```

### Notes

For every cell C which does not contain a mine (x) we need to look at all eight adjacent cells and count the number of mines they contain. The number of mines in the adjacent cells is then set as C's label.

Obtaining the contents of the adjacent cells gets a little tedious. In fact, doing so is the majority of the code here. Care must be taken to make sure that we do not accidentally look for a cell which does not exist or is not actually adjacent. The logic for find the adjacent cells in this way is re-used from a coding challenge I did last year. The Advent of Code 2020 Day 11, in part, had a similar requirement.

After the logic of determining the contents of adjacent cells is worked out, the rest proceeds in a much less complicated way. The contents of all the adjacent cells are examined for mines and the cell labels are set appropriately.

## References

posted at: 17:38 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-08-01

#### The Weekly Challenge 123 (Prolog Solutions)

## Part 1

*You are given an integer n >= 1. Write a script to find the nth Ugly Number.*

### Solution

```
:-initialization(main).
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).
ugly(N, UglyNumber):-
ugly(N, 1, 1, _, UglyNumber).
ugly(1, _, _, _, 1).
ugly(N, _, N, UglyNumber, UglyNumber).
ugly(N, X, I, _, UglyNumber):-
prime_factors(X, Factors),
member(Factor, Factors),
(Factor == 2; Factor == 3; Factor == 5),
X0 is X + 1,
I0 is I + 1,
ugly(N, X0, I0, X, UglyNumber).
ugly(N, X, I, UglyNext, UglyNumber):-
X0 is X + 1,
ugly(N, X0, I, UglyNext, UglyNumber).
main:-
ugly(10, UglyNumber),
write(UglyNumber), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
12
```

### Notes

Here the first N ugly numbers are generated in a pretty routine way. Much of the code is related to computing the prime factors. Once that is out of way the rest of the code seems to be straightforward to follow: recursively counting up each Ugly Number until we reach the Nth one.

## Part 2

*You are given coordinates of four points. Write a script to find out if the given four
points form a square.*

### Solution

```
:-initialization(main).
dot_product(X0-Y0, X1-Y1, N):-
N0 is X0 * X1,
N is N0 + Y0 * Y1.
swap_key_value([], []).
swap_key_value([A-B|R], [B-A|S]):-
swap_key_value(R, S).
square(Points):-
setof(X, member(X-_, Points), Xs),
setof(Y, member(_-Y, Points), Ys),
length(Xs, LXs),
length(Ys, LYs),
keysort(Points, PointsByX),
swap_key_value(Points, Swapped),
keysort(Swapped, PointsByY0),
swap_key_value(PointsByY0, PointsByY),
last(PointsByY, Sx-Sy),
last(PointsByX, Tx-Ty),
nth(1, PointsByY, Ux-Uy),
nth(1, PointsByX, Vx-Vy),
SUx is Sx + Ux,
TVx is Tx + Vx,
SUy is Sy + Uy,
TVy is Ty + Vy,
SUym is Sy - Uy,
TVxm is Tx - Vx,
DVSTx is Sx - Tx,
DVSTy is Sy - Ty,
DVTUx is Tx - Ux,
DVTUy is Ty - Uy,
DVUVx is Ux - Vx,
DVUVy is Uy - Vy,
DVVSx is Vx - Sx,
DVVSy is Vy - Sy,
dot_product(DVSTx-DVSTy, DVTUx-DVTUy, DPSTU),
dot_product(DVTUx-DVTUy, DVUVx-DVUVy, DPTUV),
dot_product(DVUVx-DVUVy, DVVSx-DVVSy, DPUVS),
((LXs == 2, LYs == 2); (SUx == TVx, SUy == TVy, SUym == TVxm, DPSTU == 0, DPTUV == 0, DPUVS == 0)).
main:-
((square([10-20, 20-20, 20-10, 10-10]), write(1)); (write(0))),
nl,
((square([12-24, 16-10, 20-12, 18-16]), write(1)); (write(0))),
nl,
((square([-3-1, 4-2, -(9,-3), -(2,-4)]), write(1)); (write(0))),
nl,
((square([0-0, 2-1, -(3,-1), -(1,-2)]), write(1)); (write(0))),
nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
1
0
0
1
```

### Notes

This is most likely the most tedious Prolog code I have written in a long time! The actual logic of determining if the points determine a square is not so bad:

- Are there only two each of X and Y co-ordinates? Then that is enough to establish that we have a square.
- Otherwise, make sure the side lengths are all equivalent and that the angles between the sides are all 90 degrees.

The tedious part is just all the computation of the distance vectors, the sorting and arranging of the points, and so forth.

The points are represented as pairs. To
orient the points they are sorted by X (key) or Y (value). This is both done by the
builtin `keysort/2`

predicate, with keys and values swapped to facilitate the sorting
by value.

In the case of negative co-ordinates we use the alternative `(-)/2`

syntax. For example,
`-(3,-1)`

is used since `3--1`

is not valid syntactically.

In the example output we see that the first and last sets of points are both squares. The first is an example of a square with two unique X and Y co-ordinates. The third example is a rhombus and so is a good test to make sure the angles are being checked correctly.

## References

posted at: 16:58 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-07-25

#### The Weekly Challenge 122 (Prolog Solutions)

## Part 1

*You are given a stream of numbers, @N. Write a script to print the average of the stream
at every point.*

### Solution

```
:-initialization(main).
moving_average(N):-
moving_average(0, N, 1).
moving_average(Sum, N, I):-
I \== N,
Sum0 is Sum + (I * 10),
Average is Sum0 / I,
write(Average), nl,
I0 is I + 1,
moving_average(Sum0, N, I0).
moving_average(_, N, I):-
I == N.
main:-
moving_average(10),
halt.
```

### Sample Run

```
$ gplc prolog/ch-1.p
$ prolog/ch-1
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
```

### Notes

Typically when one thinks of a *stream* the idea is of a virtually endless source of data.
Or, at least, data which is handled as if this were the case. Here the "stream" is
simulated by a generated sequence of numbers. For each recursive call to
`moving_average/3`

we increase the simulated "stream" by 10 and compute the moving
average.

(The idea of a stream in Prolog is fairly specific. My preferred Prolog is Gnu Prolog, which has a very nice write up of the subject.)

## Part 2

*You are given a score $S. You can win basketball points e.g. 1 point, 2 points and 3
points. Write a script to find out the different ways you can score $S.*

### Solution

```
:-initialization(main).
points --> [].
points --> point, points.
point --> [0]; [1]; [2]; [3].
basketball_points(Points, Goal):-
length(Points, Goal),
phrase(points, Points),
sum_list(Points, Goal).
zero_remove([], []).
zero_remove([H|T], [ZR_H|ZR_T]):-
delete(H, 0, ZR_H),
zero_remove(T, ZR_T).
main:-
findall(Ps, basketball_points(Ps, 4), Points),
zero_remove(Points, PointsZR),
sort(PointsZR, PointsZR_Unique),
write(PointsZR_Unique), nl,
halt.
```

### Sample Run

```
$ gplc prolog/ch-2.p
$ prolog/ch-2
[[1,1,1,1],[1,1,2],[1,2,1],[1,3],[2,1,1],[2,2],[3,1]]
```

### Notes

This is almost exactly identical to a solution for Challenge 112. The only difference is that I changed the predicate and variable names to match the current problem statement!

## References

posted at: 18:09 by: Adam Russell | path: /prolog | permanent link to this entry