%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 1996-2011. All Rights Reserved.
%%
%% The contents of this file are subject to the Erlang Public License,
%% Version 1.1, (the "License"); you may not use this file except in
%% compliance with the License. You should have received a copy of the
%% Erlang Public License along with this software. If not, it can be
%% retrieved online at http://www.erlang.org/.
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and limitations
%% under the License.
%%
%% %CopyrightEnd%
%%
-module(random).
%% Reasonable random number generator.
%% The method is attributed to B. A. Wichmann and I. D. Hill
%% See "An efficient and portable pseudo-random number generator",
%% Journal of Applied Statistics. AS183. 1982. Also Byte March 1987.
-export([seed/0, seed/1, seed/3, uniform/0, uniform/1,
uniform_s/1, uniform_s/2, seed0/0]).
-define(PRIME1, 30269).
-define(PRIME2, 30307).
-define(PRIME3, 30323).
%%-----------------------------------------------------------------------
%% The type of the state
-type ran() :: {integer(), integer(), integer()}.
%%-----------------------------------------------------------------------
-spec seed0() -> ran().
seed0() ->
{3172, 9814, 20125}.
%% seed()
%% Seed random number generation with default values
-spec seed() -> ran().
seed() ->
case seed_put(seed0()) of
undefined -> seed0();
{_,_,_} = Tuple -> Tuple
end.
%% seed({A1, A2, A3})
%% Seed random number generation
-spec seed({A1, A2, A3}) -> 'undefined' | ran() when
A1 :: integer(),
A2 :: integer(),
A3 :: integer().
seed({A1, A2, A3}) ->
seed(A1, A2, A3).
%% seed(A1, A2, A3)
%% Seed random number generation
-spec seed(A1, A2, A3) -> 'undefined' | ran() when
A1 :: integer(),
A2 :: integer(),
A3 :: integer().
seed(A1, A2, A3) ->
seed_put({(abs(A1) rem (?PRIME1-1)) + 1, % Avoid seed numbers that are
(abs(A2) rem (?PRIME2-1)) + 1, % even divisors of the
(abs(A3) rem (?PRIME3-1)) + 1}). % corresponding primes.
-spec seed_put(ran()) -> 'undefined' | ran().
seed_put(Seed) ->
put(random_seed, Seed).
%% uniform()
%% Returns a random float between 0 and 1.
-spec uniform() -> float().
uniform() ->
{A1, A2, A3} = case get(random_seed) of
undefined -> seed0();
Tuple -> Tuple
end,
B1 = (A1*171) rem ?PRIME1,
B2 = (A2*172) rem ?PRIME2,
B3 = (A3*170) rem ?PRIME3,
put(random_seed, {B1,B2,B3}),
R = B1/?PRIME1 + B2/?PRIME2 + B3/?PRIME3,
R - trunc(R).
%% uniform(N) -> I
%% Given an integer N >= 1, uniform(N) returns a random integer
%% between 1 and N.
-spec uniform(N) -> pos_integer() when
N :: pos_integer().
uniform(N) when is_integer(N), N >= 1 ->
trunc(uniform() * N) + 1.
%%% Functional versions
%% uniform_s(State) -> {F, NewState}
%% Returns a random float between 0 and 1.
-spec uniform_s(State0) -> {float(), State1} when
State0 :: ran(),
State1 :: ran().
uniform_s({A1, A2, A3}) ->
B1 = (A1*171) rem ?PRIME1,
B2 = (A2*172) rem ?PRIME2,
B3 = (A3*170) rem ?PRIME3,
R = B1/?PRIME1 + B2/?PRIME2 + B3/?PRIME3,
{R - trunc(R), {B1,B2,B3}}.
%% uniform_s(N, State) -> {I, NewState}
%% Given an integer N >= 1, uniform(N) returns a random integer
%% between 1 and N.
-spec uniform_s(N, State0) -> {integer(), State1} when
N :: pos_integer(),
State0 :: ran(),
State1 :: ran().
uniform_s(N, State0) when is_integer(N), N >= 1 ->
{F, State1} = uniform_s(State0),
{trunc(F * N) + 1, State1}.
