/* $Id: foxchicken.pl,v 321.2 2002-03-06 17:50:54-08 - - $ */

/*
* A farmer has with him a fox, a chicken, and a sack of grain.
* He comes to a river and sees a small boat.  He needs to bring
* all three of his things across the river, but the boat is so
* small that only one thing will fit in it with him.  He can not
* leave the fox and the chicken together or the fox will eat the
* chicken.  He can not leave the chicken and the grain together
* or the chicken will eat the grain.  The fox, however, does not
* eat grain.  How should the farmer proceed?
*/

/*
* FACTS AND CULLINARY HABITS:
*/

eats( fox, chicken ).
eats( chicken, grain ).
property( [ fox, chicken, grain ] ).
goal( other ).
start( first ).

chow_time( List ) :-
	member_of( Diner, List ),
	member_of( Dinner, List ),
	eats( Diner, Dinner ).

/*
* SET RELATIONS:
*/

member_of( H, [ H | _ ] ).
member_of( H, [ _ | T ] ) :- member_of( H, T ).

matches( [], [] ).
matches( [ H | T1 ], [ H | T2 ] ) :- matches( T1, T2 ).

remove( H, [ H | T ], T ).
remove( X, [ H | T ], [ H | U ] ) :- remove( X, T, U ).

insert( H, T, [ H | T ] ).

/*
* TRAVEL PLANS AND FREQUENT PADDLER MILES.
*/

travel :-
	start( From ),
	goal( To ),
	property( Property ),
	print_start( From, To, Property, [] ),
	move( From, To, Property, [], nothing ),
	print_done.

move( From, _, _, [], _ ) :-
	goal( From ).

move( From, To, This, That, _ ) :-
	start( To ),
	goal( From ),
	not( chow_time( This )),
	print_alone( From, To, This, That ),
	move( To, From, That, This, nothing ).

move( From, To, This, That, Just_took ) :-
	remove( What, This, This_later ),
	not( Just_took = What ),
	not( chow_time( This_later )),
	insert( What, That, That_later ),
	print_takes( What, From, To, This_later, That_later ),
	move( To, From, That_later, This_later, What ).

/*
* TRAVEL INSTRUCTIONS.
*/

print_start( From, To, This, That ) :-
	nl,
	write( 'The farmer is by a river and ' ),
	write( 'wants to take his cargo across.' ),
	nl,
	print_status( From, This ),
	print_status( To, That ).

print_done :-
	nl,
	write( 'Finally, the farmer is done!' ),
	nl.

print_takes( What, From, To, This, That ) :-
	nl,
	write( 'The farmer takes the ' ),
	write( What ),
	write( ' from the ' ),
	write( From ),
	write( ' side to the ' ),
	write( To ),
	write( ' side.' ),
	nl,
	print_status( From, This ),
	print_status( To, That ).

print_alone( From, To, This, That ) :-
	nl,
	write( 'The farmer travels alone from the ' ),
	write( From ),
	write( ' side to the ' ),
	write( To ),
	write( ' side.' ),
	nl,
	print_status( From, This ),
	print_status( To, That ).

print_status( Where, What ) :-
	tab( 10 ),
	write( 'On the ' ),
	write( Where ),
	write( ' side is ' ),
	print_list( nothing, '', What ).

print_list( Nothing, _, [] ) :-
	write( Nothing ),
	write( '.' ),
	nl.

print_list( _, Comma, [ H | T ] ) :-
	write( Comma ),
	write( 'the ' ),
	write( H ),
	print_list( '', ', ', T ).

/*
* TRACE ON COMMAND.
*/

trace_on :-
	trace( eats ),
	trace( property ),
	trace( goal ),
	trace( start ),
	trace( chow_time ),
	trace( member_of ),
	trace( matches ),
	trace( remove ),
	trace( insert ),
	trace( travel ),
	trace( move ).