quickrand

%-*-Mode:erlang;coding:utf-8;tab-width:4;c-basic-offset:4;indent-tabs-mode:()-*-
% ex: set ft=erlang fenc=utf-8 sts=4 ts=4 sw=4 et:
%%%
%%%------------------------------------------------------------------------
%%% @doc
%%% ==Quick Random Number Generation==
%%% @end
%%%
%%% BSD LICENSE
%%% 
%%% Copyright (c) 2012-2014, Michael Truog <mjtruog at gmail dot com>
%%% All rights reserved.
%%% 
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%%%       software must display the following acknowledgment:
%%%         This product includes software developed by Michael Truog
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%%% @author Michael Truog <mjtruog [at] gmail (dot) com>
%%% @copyright 2012-2014 Michael Truog
%%% @version 1.4.0 {@date} {@time}
%%%------------------------------------------------------------------------

-module(quickrand).
-author('mjtruog [at] gmail (dot) com').

%% external interface
-export([seed/0,
         uniform/1,
         strong_uniform/1,
         strong_float/0]).

%%%------------------------------------------------------------------------
%%% External interface functions
%%%------------------------------------------------------------------------

%%-------------------------------------------------------------------------
%% @doc
%% ===Seed random number generation.===
%% @end
%%-------------------------------------------------------------------------

-spec seed() ->
    'ok'.

seed() ->
    % to provide better seeding than erlang:now() or os:timestamp()
    <<B1:32/unsigned-integer,
      B2:32/unsigned-integer,
      B3:32/unsigned-integer,
      B4:32/unsigned-integer>> = try crypto:strong_rand_bytes(16)
    catch
        error:low_entropy ->
            error_logger:info_msg("quickrand: low_entropy!~n"),
            crypto:rand_bytes(16)
    end,
    random:seed(B1, B2, B3),
    random_wh06_int:seed(B1, B2, B3, B4),
    ok.

%%-------------------------------------------------------------------------
%% @doc
%% ===Quick uniform random number generation.===
%% Not meant for cryptographic purposes.
%% @end
%%-------------------------------------------------------------------------

-spec uniform(N :: 1..21267638781707063560975648195455661513) ->
    1..21267638781707063560975648195455661513.

uniform(N) when is_integer(N), N < 1 ->
    erlang:exit(badarg);

uniform(1) ->
    1;

uniform(N) when is_integer(N), N < 1000000 ->
    % os:timestamp/0 is currently the quickest source of uniform randomness
    {_, _, MicroSecs} = os:timestamp(),
    (MicroSecs rem N) + 1;

uniform(N) when is_integer(N), N =< 27817185604309 ->
    % 27817185604309 == 30269 * 30307 * 30323
    random:uniform(N);

uniform(N) when is_integer(N), N =< 21267638781707063560975648195455661513 ->
    % 21267638781707063560975648195455661513 ==
    %   2147483579 * 2147483543 * 2147483423 * 2147483123
    random_wh06_int:uniform(N).

%%-------------------------------------------------------------------------
%% @doc
%% ===Strong uniform random number generation.===
%% @end
%%-------------------------------------------------------------------------

-spec strong_uniform(N :: pos_integer()) ->
    pos_integer().

strong_uniform(N) when is_integer(N), N < 1 ->
    erlang:exit(badarg);

strong_uniform(1) ->
    1;

strong_uniform(N) when is_integer(N), N > 1 ->
    Bytes = erlang:byte_size(binary:encode_unsigned(N)),
    (binary:decode_unsigned(crypto:strong_rand_bytes(Bytes), big) rem N) + 1.

%%-------------------------------------------------------------------------
%% @doc
%% ===Return an Erlang floating point random number (double-precision).===
%% @end
%%-------------------------------------------------------------------------

-spec strong_float() ->
    float().  % return a floating point value between 0.0 and 1.0, inclusive

strong_float() ->
    % 53 bits maximum for double precision floating point representation
    Bytes = 7, % erlang:round(53.0 / 8), % bytes for random number
    MaxRand = 72057594037927935, % (2 ** (7 * 8)) - 1 % max random number
    binary:decode_unsigned(crypto:strong_rand_bytes(Bytes)) / MaxRand.