Use a different algorithm with the low entropy source for field trials.

third_party/mt19937ar/mt19937ar.h

Index: third_party/mt19937ar/mt19937ar.h
===================================================================
--- third_party/mt19937ar/mt19937ar.h	(revision 0)
+++ third_party/mt19937ar/mt19937ar.h	(revision 0)
@@ -0,0 +1,65 @@
+/* 
+ A C-program for MT19937, with initialization improved 2002/1/26.
+ Coded by Takuji Nishimura and Makoto Matsumoto.
+ 
+ Before using, initialize the state by using init_genrand(seed)  
+ or init_by_array(init_key, key_length).
+ 
+ Copyright (C) 1997 - 2002, Makoto Matsumoto and Takuji Nishimura,
+ All rights reserved.                          
+ 
+ Redistribution and use in source and binary forms, with or without
+ modification, are permitted provided that the following conditions
+ are met:
+ 
+ 1. Redistributions of source code must retain the above copyright
+ notice, this list of conditions and the following disclaimer.
+ 
+ 2. Redistributions in binary form must reproduce the above copyright
+ notice, this list of conditions and the following disclaimer in the
+ documentation and/or other materials provided with the distribution.
+ 
+ 3. The names of its contributors may not be used to endorse or promote 
+ products derived from this software without specific prior written 
+ permission.
+ 
+ THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ 
+ 
+ Any feedback is very welcome.
+ http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html
+ email: m-mat @ math.sci.hiroshima-u.ac.jp (remove space)
+ */
+
+#ifndef THIRD_PARTY_MT19937AR_MT19937AR_H_
+#define THIRD_PARTY_MT19937AR_MT19937AR_H_
+
+#include <vector>
+
+#include "base/basictypes.h"
+

[SteveT, 2012/08/16 20:55:42]

BTW, do we have to do any special documentation given we've made edits to the
original code? (to get it to fit in as a class, and to remove globals, etc).

[Alexei Svitkine (slow), 2012/08/17 14:08:59]

I'm not sure. Let's see what open-source-third-party-reviews say.

[Ilya Sherman, 2012/08/17 17:53:20]

I think that edits are usually saved to a .patch file within the third_party
directory, plus a README file that specifies how to apply the patches.  That way
you can update to a newer version of the library and re-apply the patch so that
it plays nicely with Chrome code.

+class MersenneTwister {
+ public:
+  MersenneTwister();
+  ~MersenneTwister();
+
+  void init_genrand(uint32 s);
+  void init_by_array(uint32 init_key[], int key_length);
+  uint32 genrand_int32(void);
+
+ private:
+  std::vector<uint32> mt; /* the array for the state vector  */
+  int mti; /* mti==N+1 means mt[N] is not initialized */
+};
+
+#endif  // THIRD_PARTY_MT19937AR_MT19937AR_H_