# RabbitFarm

### 2021-04-25

#### The Weekly Challenge 109 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution. Also, the challenge statements are given in a Perl context although for Prolog solutions some adjustments are made to account for the differing semantics between the two languages.

## Part 1

Write a script to display the first 20 Chowla Numbers.

### Solution

``````
:-initialization(main).

print_with_comma([H|[]]):-
write(H), nl.
print_with_comma([H|T]):-
write(H),
write(', '),
print_with_comma(T).

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].

chowla(ChowlaNumber):-
between(1, 20, N),
findall(F, factor(N, F), Fs),
flatten(Fs, Factors),
sum_list(Factors, ChowlaNumber).

main:-
findall(ChowlaNumber, chowla(ChowlaNumber), ChowlaNumbers),
print_with_comma(ChowlaNumbers).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
0, 0, 0, 2, 0, 5, 0, 6, 3, 7, 0, 15, 0, 9, 8, 14, 0, 20, 0, 21
``````

Gnu Prolog’s builtin FD solver makes quick work of this problem! To compute a Chowla Number we must compute a number’s unique factors and then take their sum. 1 and the number itself are excluded. So, in the case of primes the Chowla Number is 0. In the case of numbers where a factor is repeated (e.g. 4 = 2 * 2) the repeated factor is only included once.

## Part 2

Place the given unique numbers in the square box so that sum of numbers in each box is the same.

### Solution

``````
/*
You are given four squares as below with numbers named a,b,c,d,e,f,g.
to place the given unique numbers in the square box so that sum of
numbers in each box is the same.
(1)                    (3)
+--------------+      +--------------+
?              ?      ?              ?
?      a       ?      ?      e       ?
?              ? (2)  ?              ?  (4)
?          +---+------+---+      +---+---------+
?          ?   ?      ?   ?      ?   ?         ?
?          ? b ?      ? d ?      ? f ?         ?
?          ?   ?      ?   ?      ?   ?         ?
?          ?   ?      ?   ?      ?   ?         ?
+----------+---+      +---+------+---+         ?
?       c      ?      ?      g      ?
?              ?      ?             ?
?              ?      ?             ?
+--------------+      +-------------+
*/

:-initialization(main).

print_with_tab([H|[]]):-
write(H), nl.
print_with_tab([H|T]):-
write(H),
write('\t'),
print_with_tab(T).

print_values([]).
print_values([H|T]):-
print_with_tab(H),
print_values(T).

print_solutions(Solutions):-
print_with_tab([a, b, c, d, e, f, g]),
print_values(Solutions).

all_unique(_, []).
all_unique(L, [V|T]) :-
fd_exactly(1, L, V),
all_unique(L, T).

sums_in_squares_naive(Numbers, [A, B, C, D, E, F, G]):-
member(A, Numbers),
member(B, Numbers),
member(C, Numbers),
member(D, Numbers),
member(E, Numbers),
member(F, Numbers),
member(G, Numbers),
\+ (A == B; A == C; A == D; A == E; A == F; A == G),
\+ (B == A; B == C; B == D; B == E; B == F; B == G),
\+ (C == A; C == B; C == D; C == E; C == F; C == G),
\+ (D == A; D == B; D == C; D == E; D == F; D == G),
\+ (E == A; E == B; E == C; E == D; E == F; E == G),
\+ (F == A; F == B; F == C; F == D; F == E; F == G),
\+ (G == A; G == B; G == C; G == D; G == E; G == F),
Box1 is A + B,
Box2 is B + C + D,
Box3 is D + E + F,
Box4 is F + G,
Box1 == Box2,
Box2 == Box3,
Box3 == Box4.

sums_in_squares_fd(Numbers, [A, B, C, D, E, F, G]):-
fd_domain([A, B, C, D, E, F, G], Numbers),
all_unique([A, B, C, D, E, F, G], Numbers),
Box1 = A + B,
Box2 = B + C + D,
Box3 = D + E + F,
Box4 = F + G,
Box1 #= Box2,
Box2 #= Box3,
Box3 #= Box4,
fd_labeling([A, B, C, D, E, F, G]).

main:-
setof(S, sums_in_squares_fd([1,2,3,4,5,6,7], S), Squares),
print_solutions(Squares).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
a       b       c       d       e       f       g
3       7       2       1       5       4       6
4       5       3       1       6       2       7
4       7       1       3       2       6       5
5       6       2       3       1       7       4
6       4       1       5       2       3       7
6       4       5       1       2       7       3
7       2       6       1       3       5       4
7       3       2       5       1       4       6
``````

### Notes

The Perl solution to this problem used a builtin Prolog available as a Perl module on CPAN. The “naive” version of the solution is effectively the same code as what was used there. I repeated it here to make a couple of comparisons, but that perl module, AI::Prolog, is little better than a toy implementation. Here with Gnu Prolog we can see that with an FD solver this problem can be very quickly computed with a minimum of code.

Challenge 109