SWISH -- SWI-Prolog for SHaring

int_words( N , _             ) :- \+ integer(N), !, fail.
int_words( N , [negative|Ws] ) :- N <  0, X is abs(N), number_phrase(X,Ws).
int_words( N ,           Ws  ) :- N >= 0,              number_phrase(N,Ws).

number_phrase(N,Ws) :-
    groups(N,[],Gs),
    length(Gs,P),
    groups_to_words(Gs,P,Ws).

groups_to_words( [G]    , _ , Ws ) :-
    group_name(G,Ws,[]).
groups_to_words( [G|Gs] , P , Ws ) :-
    period(P,Sfx),
    group_name(G,Ws,[Sfx|W1]),
    P1 is P-1,
    groups_to_words(Gs,P1,W1).

group_name( 0 , Ws , Ws ) :- !.
group_name( N , Ws , W1 ) :-
    div_rem(N,100,Q,R),
    hundreds(Q,Ws,W0),
    tens_and_ones(R,W0,W1).
                 
hundreds( 0 , Ws              , Ws ).
hundreds( N ,  [W,hundred|W0] , W0 ) :- N > 0, special(N,W).

tens_and_ones( N ,     [W|Ws] , Ws ) :- special(N,W), !.
tens_and_ones( N , [W1-W2|Ws] , Ws ) :-
    Ones is N rem 10,
    Tens is N - Ones,
    special(Tens,W1),
    special(Ones,W2).

groups( 0 , Gs , Gs ) .
groups( N , Ts , Gs ) :- N > 0, div_rem(N,1000,Q,R), groups(Q,[R|Ts],Gs).

div_rem(X,Y,Q,R) :-
    Q is X div Y,
    R is X rem Y.

period( 2, thousand    ).
period( 3, million     ).
period( 4, billion     ).
period( 5, trillion    ).
period( 6, quadrillion ).
period( 7, quintillion ).
period( 8, sextillion  ).

special(  0, zero       ).
special(  1, one        ).
special(  2, two        ).
special(  3, three      ).
special(  4, four       ).
special(  5, five       ).
special(  6, six        ).
special(  7, seven      ).
special(  8, eight      ).
special(  9, nine       ).
special( 10, ten        ).
special( 11, eleven     ).
special( 12, twelve     ).
special( 13, thirteen   ).
special( 14, fourteeen  ).
special( 15, fifteen    ).
special( 16, sixteen    ).
special( 17, seventeen  ). 
special( 18, eighteen   ).
special( 19, nineteen   ).
special( 20, twenty     ).
special( 30, thirty     ).
special( 40, forty      ).
special( 50, fifty      ).
special( 60, sixty      ).
special( 70, seventy    ).
special( 80, eighty     ).
special( 90, ninety     ).