60 lines
1.5 KiB
Prolog
60 lines
1.5 KiB
Prolog
:- module(random, [maybe/0, random/1, random_integer/3, set_random/1]).
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/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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To retain desirable declarative properties, predicates that internally
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use random numbers should be equipped with an argument that specifies
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the random seed. This makes everything completely reproducible.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
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:- use_module(library(error)).
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% succeeds with probability 0.5.
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maybe :- '$maybe'.
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% The higher the precision, the slower it gets.
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random_number_precision(64).
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random(R) :-
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var(R),
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random_number_precision(N),
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rnd(N, R).
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random_integer(Lower, Upper, R) :-
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var(R),
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( (var(Lower) ; var(Upper)) ->
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instantiation_error(random_integer/3)
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; \+ integer(Lower) ->
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type_error(integer, Lower, random_integer/3)
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; \+ integer(Upper) ->
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type_error(integer, Upper, random_integer/3)
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; Upper > Lower,
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random(R0),
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R is floor((Upper - Lower) * R0 + Lower)
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).
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rnd(N, R) :-
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rnd_(N, 0, R).
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rnd_(0, R, R) :- !.
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rnd_(N, R0, R) :-
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maybe,
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!,
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N1 is N - 1,
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rnd_(N1, R0, R).
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rnd_(N, R0, R) :-
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N1 is N - 1,
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R1 is R0 + 1.0 / 2.0 ^ N,
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rnd_(N1, R1, R).
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set_random(Seed) :-
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( nonvar(Seed) ->
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( Seed = seed(S) ->
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( var(S) -> instantiation_error(set_random/1)
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; integer(S) -> '$set_seed'(S)
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; type_error(integer, S, set_random/1)
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)
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)
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; instantiation_error(set_random/1)
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).
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