/* $Id: queens.pl,v 321.2 2002-03-06 17:52:25-08 - - $ */

remove( Item, [ Item | Tail ], Tail ).

remove( Item, [ Head1 | Tail1 ], [ Head1 | Tail2 ] ) :-
	remove( Item, Tail1, Tail2 ).

threatens( Rank1, File1, Rank2, File2 ) :-
	Rankdiff is abs( Rank1 - Rank2 ),
	Filediff is abs( File1 - File2 ),
	Rankdiff =:= Filediff.

place( Rank, File, Rank, [], [ File ] ).

place( Rank1, File1, Rank2, [ File2 | Rest ], [ File2 | Gives ] ) :-
	not( threatens( Rank1, File1, Rank2, File2 )),
	Nextrank2 is Rank2 + 1,
	place( Rank1, File1, Nextrank2, Rest, Gives ).

queens( _, [], Sequence, Sequence ).

queens( Rank, Start, Partial, Answer ) :-
	remove( One, Start, Remaining ),
	place( Rank, One, 1, Partial, New ),
	Nextrank is Rank + 1,
	queens( Nextrank, Remaining, New, Answer ).

queens :-
	queens( 1, [1,2,3,4,5,6,7,8], [], Answer ),
	write( Answer ), nl,
	fail.

:- queens.