SWISH -- SWI-Prolog for SHaring

% start program with a query like either:
%   is_connected(64,A).
%   setof(A, is_connected(1,A), Output).

% connections work both ways (currently disabled)
is_connected(A,B) :- con_test(A,B,1).
%is_connected(A,B) :- con_test(B,A,1).

% recursion rules
% Depth prevents infinite loops
con_test(A, B, Depth) :-
    dif(A,B), Depth < 5, con(A,B),
    write(A), write(" is connected to "), writeln(B).

con_test(A, B, Depth) :-
    dif(A,B), Depth < 5,
    con_test(A,X,Depth+1), con_test(X,B,Depth+1).
%    write("--Therefore, "), write(B), write(" connects to "), write(A), write(" through "), writeln(X).

%---------------
% knowledgebase
%---------------

% Rule: Split up a two-digit number and sum its digits
% e.g. 26 -> 2 + 6 -> 8
con(A,B) :- A < 100, A > 9, X is A mod 10, Y is A div 10, B is X + Y,
    write("By summing digits, "). %, write(X), write(" and "), write(Y), write(", ").

% Rule: Split up a two-digit number and multiply its digits
% e.g. 26 -> 2 * 6 -> 12
% numbers with zeroes are ignored
con(A,B) :- A < 100, X is A mod 10, Y is A div 10, dif(X,0), dif(Y,0), B is X * Y,
    write("By multiplying digits, "). % , write(X), write(" and "), write(Y), write(", ").

% add text when using a fact
con(A,B) :- con_gem(A,B),
    write("By using gematria, ").

% Precomputed gematria facts
% e.g. ZERO -> 26+5+18+15 -> 64
con_gem(0,64).
con_gem(1,34).
con_gem(2,58).
con_gem(3,56).
con_gem(4,60).
con_gem(5,42).
con_gem(6,52).
con_gem(7,65).
con_gem(8,49).
con_gem(9,42).