🚀 go-pugleaf

The following code works, but I have not a clue how to generalise it. 
At least, I have not chanced upon a case where it does not give the best
answer.  The reason for the op/3 call will become clear in a minute,
although those over thirty will recognise it on sight:

:-  op(500, xf, '/-').

make(A/-,C) :-
	Sum is 12 * A,
	make(Sum,C).

make(A/B,C) :-
	Sum is A * 12 + B,
	make(Sum,C).

make(0,[]).

make(Sum,[Name|Rest]) :-
	Sixpences is Sum//6,			% If the number of sixpences
	coin(Name,Value),			% in the sum is an exact
	Value =< Sum,				% multiple of 4, then do not
	\+ (	0 is Sixpences mod 4,		% allocate a half crown.
		Value is 30			% No clue how to generalise
	   ),					% this to other currencies.
	Residue is Sum - Value,			% The LCM of 4 and 5 is 20 so
	make(Residue,Rest).			% 10/- is dispensed as a
						% ten shilling note, not as
						% five florins (wrong)
% Coins; must be in order of decreasing value.	

coin('ten shilling note',120).
coin('half-crown',30).
coin(florin,24).
coin(shilling,12).
coin(sixpence,6).
coin('threepenny bit',3).
coin(penny,1).

% Sample Run

This program understands the traditional way of writing `four shillings'
as 4/- and four shillings and elevenpence as 4/11. 

 ?- make(4/-,X).
     X=[florin,florin]

 ?- make(4/6,X).
     X=[half-crown,florin]

 ?- make(5/-,X).
     X=[half-crown,half-crown]

 ?- make(5/11,X).
     X=[half-crown,half-crown,sixpence,threepenny bit,penny,penny]


Ken Johnson

-- 
Son, all the pretty, intelligent, healthy     #    Ken Johnson, AIAI,
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somewhere else.        -- my dad  1906-1992   #    Fax 031-650 6513