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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>random</title>
    <prepared>Joe Armstrong</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno>1</docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>1996-09-09</date>
    <rev>A</rev>
    <file>random.xml</file>
  </header>
  <module since="">random</module>
  <modulesummary>Pseudo-random number generation.</modulesummary>
  <description>
    <p>This module provides a random number generator. The method is attributed
      to B.A. Wichmann and I.D. Hill in 'An efficient and portable
      pseudo-random number generator', Journal of Applied
      Statistics. AS183. 1982. Also Byte March 1987.</p>

    <p>The algorithm is a modification of the version attributed
      to Richard A. O'Keefe in the standard Prolog library.</p>

    <p>Every time a random number is requested, a state is used to calculate
      it, and a new state is produced. The state can either be implicit (kept
      in the process dictionary) or be an explicit argument and return value.
      In this implementation, the state (the type <c>ran()</c>) consists of a
      tuple of three integers.</p>

    <note>
      <p>This random number generator is not cryptographically
        strong. If a strong cryptographic random number generator is
        needed, use one of functions in the
        <seealso marker="crypto:crypto"><c>crypto</c></seealso>
        module, for example, <seealso marker="crypto:crypto">
        <c>crypto:strong_rand_bytes/1</c></seealso>.</p>
    </note>

    <note>
      <p>The improved <seealso marker="rand"><c>rand</c></seealso>
        module is to be used instead of this module.</p>
    </note>
  </description>

  <datatypes>
    <datatype>
      <name name="ran"/>
      <desc><p>The state.</p></desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="seed" arity="0" since=""/>
      <fsummary>Seed random number generation with default values.</fsummary>
      <desc>
        <p>Seeds random number generation with default (fixed) values
          in the process dictionary and returns the old state.</p>
      </desc>
    </func>

    <func>
      <name name="seed" arity="1" since=""/>
      <fsummary>Seed random number generator.</fsummary>
      <desc>
        <p><c>seed({<anno>A1</anno>, <anno>A2</anno>, <anno>A3</anno>})</c>
          is equivalent to
          <c>seed(<anno>A1</anno>, <anno>A2</anno>, <anno>A3</anno>)</c>.</p>
      </desc>
    </func>

   <func>
      <name name="seed" arity="3" since=""/>
      <fsummary>Seed random number generator.</fsummary>
      <desc>
        <p>Seeds random number generation with integer values in the process
          dictionary and returns the old state.</p>
        <p>The following is an easy way of obtaining a unique value to seed
          with:</p>
        <code type="none">
random:seed(erlang:phash2([node()]),
            erlang:monotonic_time(),
            erlang:unique_integer())</code>
        <p>For details, see
          <seealso marker="erts:erlang#phash2/1">
          <c>erlang:phash2/1</c></seealso>,
          <seealso marker="erts:erlang#node/0">
          <c>erlang:node/0</c></seealso>,
          <seealso marker="erts:erlang#monotonic_time/0">
          <c>erlang:monotonic_time/0</c></seealso>, and
          <seealso marker="erts:erlang#unique_integer/0">
          <c>erlang:unique_integer/0</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="seed0" arity="0" since=""/>
      <fsummary>Return default state for random number generation.</fsummary>
      <desc>
        <p>Returns the default state.</p>
      </desc>
    </func>

    <func>
      <name name="uniform" arity="0" since=""/>
      <fsummary>Return a random float.</fsummary>
      <desc>
        <p>Returns a random float uniformly distributed between <c>0.0</c>
          and <c>1.0</c>, updating the state in the process dictionary.</p>
      </desc>
    </func>

    <func>
      <name name="uniform" arity="1" since=""/>
      <fsummary>Return a random integer.</fsummary>
      <desc>
        <p>Returns, for a specified integer <c><anno>N</anno> >= 1</c>,
          a random integer uniformly distributed between <c>1</c> and
          <c><anno>N</anno></c>, updating the state in the process
          dictionary.</p>
      </desc>
    </func>

    <func>
      <name name="uniform_s" arity="1" since=""/>
      <fsummary>Return a random float.</fsummary>
      <desc>
        <p>Returns, for a specified state, a random float uniformly
          distributed between <c>0.0</c> and <c>1.0</c>, and a new state.</p>
      </desc>
    </func>

    <func>
      <name name="uniform_s" arity="2" since=""/>
      <fsummary>Return a random integer.</fsummary>
      <desc>
        <p>Returns, for a specified integer <c><anno>N</anno> >= 1</c> and a
          state, a random integer uniformly distributed between <c>1</c> and
          <c><anno>N</anno></c>, and a new state.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Note</title>
    <p>Some of the functions use the process dictionary variable
      <c>random_seed</c> to remember the current seed.</p>

    <p>If a process calls
      <seealso marker="#uniform/0"><c>uniform/0</c></seealso> or
      <seealso marker="#uniform/1"><c>uniform/1</c></seealso>
      without setting a seed first,
      <seealso marker="#seed/0"><c>seed/0</c></seealso>
      is called automatically.</p>

    <p>The implementation changed in Erlang/OTP R15. Upgrading to R15 breaks
      applications that expect a specific output for a specified seed. The
      output is still deterministic number series, but different compared to
      releases older than R15. Seed <c>{0,0,0}</c> does, for example, no longer
      produce a flawed series of only zeros.</p>
  </section>
</erlref>

