From b52a2170d13fe7ff6e4e3b146b63d8f68c2aaadf Mon Sep 17 00:00:00 2001
From: Sverker Eriksson <sverker@erlang.org>
Date: Mon, 28 Mar 2011 16:38:57 +0200
Subject: Improve algorithm in module random.

Avoid seed values that are even divisors of the primes
and by that prevent getting seeds that are stuck on zero.

Example: random:seed(0,0,0) would produce a series of only zeros.
---
 lib/stdlib/doc/src/random.xml |  7 ++++++-
 lib/stdlib/src/random.erl     | 44 ++++++++++++++++++++++++-------------------
 2 files changed, 31 insertions(+), 20 deletions(-)

(limited to 'lib/stdlib')

diff --git a/lib/stdlib/doc/src/random.xml b/lib/stdlib/doc/src/random.xml
index dcc6d756e1..26f22db48e 100644
--- a/lib/stdlib/doc/src/random.xml
+++ b/lib/stdlib/doc/src/random.xml
@@ -4,7 +4,7 @@
 <erlref>
   <header>
     <copyright>
-      <year>1996</year><year>2009</year>
+      <year>1996</year><year>2011</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -146,6 +146,11 @@
       <c>random_seed</c> to remember the current seed.</p>
     <p>If a process calls <c>uniform/0</c> or <c>uniform/1</c> without
       setting a seed first, <c>seed/0</c> is called automatically.</p>
+    <p>The implementation changed in R15. Upgrading to R15 will break
+      applications that expect a specific output for a given seed. The output
+      is still deterministic number series, but different compared to releases
+      older than R15. The seed <c>{0,0,0}</c> will for example no longer
+      produce a flawed series of only zeros.</p>
   </section>
 </erlref>
 
diff --git a/lib/stdlib/src/random.erl b/lib/stdlib/src/random.erl
index d94722cb33..606edbb1fb 100644
--- a/lib/stdlib/src/random.erl
+++ b/lib/stdlib/src/random.erl
@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %% 
-%% Copyright Ericsson AB 1996-2009. All Rights Reserved.
+%% 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
@@ -26,6 +26,10 @@
 -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
 
@@ -44,7 +48,11 @@ seed0() ->
 -spec seed() -> ran().
 
 seed() ->
-    reseed(seed0()).
+    case seed_put(seed0()) of
+	undefined -> seed0();
+	{_,_,_} = Tuple -> Tuple
+    end.	
+
 
 %% seed({A1, A2, A3}) 
 %%  Seed random number generation 
@@ -60,17 +68,15 @@ seed({A1, A2, A3}) ->
 -spec seed(integer(), integer(), integer()) -> 'undefined' | ran().
 
 seed(A1, A2, A3) ->
-    put(random_seed, 
-	{abs(A1) rem 30269, abs(A2) rem 30307, abs(A3) rem 30323}).
+    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 reseed(ran()) -> ran().
-
-reseed({A1, A2, A3}) ->
-    case seed(A1, A2, A3) of
-	undefined -> seed0();
-	{_,_,_} = Tuple -> Tuple
-    end.	
+-spec seed_put(ran()) -> 'undefined' | ran().
+     
+seed_put(Seed) ->
+    put(random_seed, Seed).
 
 %% uniform()
 %%  Returns a random float between 0 and 1.
@@ -82,11 +88,11 @@ uniform() ->
 		       undefined -> seed0();
 		       Tuple -> Tuple
 		   end,
-    B1 = (A1*171) rem 30269,
-    B2 = (A2*172) rem 30307,
-    B3 = (A3*170) rem 30323,
+    B1 = (A1*171) rem ?PRIME1,
+    B2 = (A2*172) rem ?PRIME2,
+    B3 = (A3*170) rem ?PRIME3,
     put(random_seed, {B1,B2,B3}),
-    R = B1/30269 + B2/30307 + B3/30323,
+    R = B1/?PRIME1 + B2/?PRIME2 + B3/?PRIME3,
     R - trunc(R).
 
 %% uniform(N) -> I
@@ -107,10 +113,10 @@ uniform(N) when is_integer(N), N >= 1 ->
 -spec uniform_s(ran()) -> {float(), ran()}.
 
 uniform_s({A1, A2, A3}) ->
-    B1 = (A1*171) rem 30269,
-    B2 = (A2*172) rem 30307,
-    B3 = (A3*170) rem 30323,
-    R = B1/30269 + B2/30307 + B3/30323,
+    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}
-- 
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