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

third_party/mt19937ar/mt19937ar.cc

+/* 
+   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)
+*/
+
+#include "mt19937ar.h"
+
+/* Period parameters */  
+#define N 624
+#define M 397
+#define MATRIX_A 0x9908b0dfUL   /* constant vector a */
+#define UPPER_MASK 0x80000000UL /* most significant w-r bits */
+#define LOWER_MASK 0x7fffffffUL /* least significant r bits */
+
+MersenneTwister::MersenneTwister() : mt(N), mti(N+1) {
+}
+
+MersenneTwister::~MersenneTwister() {
+}
+
+/* initializes mt[N] with a seed */
+void MersenneTwister::init_genrand(uint32 s)
+{
+    mt[0]= s & 0xffffffffUL;
+    for (mti=1; mti<N; mti++) {
+        mt[mti] = 
+            (1812433253UL * (mt[mti-1] ^ (mt[mti-1] >> 30)) + mti); 
+        /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
+        /* In the previous versions, MSBs of the seed affect   */
+        /* only MSBs of the array mt[].                        */
+        /* 2002/01/09 modified by Makoto Matsumoto             */
+        mt[mti] &= 0xffffffffUL;
+        /* for >32 bit machines */
+    }
+}
+
+/* initialize by an array with array-length */
+/* init_key is the array for initializing keys */
+/* key_length is its length */
+/* slight change for C++, 2004/2/26 */
+void MersenneTwister::init_by_array(uint32 init_key[], int key_length)
+{
+    int i, j, k;
+    init_genrand(19650218UL);
+    i=1; j=0;
+    k = (N>key_length ? N : key_length);
+    for (; k; k--) {
+        mt[i] = (mt[i] ^ ((mt[i-1] ^ (mt[i-1] >> 30)) * 1664525UL))
+          + init_key[j] + j; /* non linear */
+        mt[i] &= 0xffffffffUL; /* for WORDSIZE > 32 machines */
+        i++; j++;
+        if (i>=N) { mt[0] = mt[N-1]; i=1; }
+        if (j>=key_length) j=0;
+    }
+    for (k=N-1; k; k--) {
+        mt[i] = (mt[i] ^ ((mt[i-1] ^ (mt[i-1] >> 30)) * 1566083941UL))
+          - i; /* non linear */
+        mt[i] &= 0xffffffffUL; /* for WORDSIZE > 32 machines */
+        i++;
+        if (i>=N) { mt[0] = mt[N-1]; i=1; }
+    }
+
+    mt[0] = 0x80000000UL; /* MSB is 1; assuring non-zero initial array */ 
+}
+
+/* generates a random number on [0,0xffffffff]-interval */
+uint32 MersenneTwister::genrand_int32(void)
+{
+    uint32 y;
+    static uint32 mag01[2]={0x0UL, MATRIX_A};
+    /* mag01[x] = x * MATRIX_A  for x=0,1 */
+
+    if (mti >= N) { /* generate N words at one time */
+        int kk;
+
+        if (mti == N+1)   /* if init_genrand() has not been called, */
+            init_genrand(5489UL); /* a default initial seed is used */
+
+        for (kk=0;kk<N-M;kk++) {
+            y = (mt[kk]&UPPER_MASK)|(mt[kk+1]&LOWER_MASK);
+            mt[kk] = mt[kk+M] ^ (y >> 1) ^ mag01[y & 0x1UL];
+        }
+        for (;kk<N-1;kk++) {
+            y = (mt[kk]&UPPER_MASK)|(mt[kk+1]&LOWER_MASK);
+            mt[kk] = mt[kk+(M-N)] ^ (y >> 1) ^ mag01[y & 0x1UL];
+        }
+        y = (mt[N-1]&UPPER_MASK)|(mt[0]&LOWER_MASK);
+        mt[N-1] = mt[M-1] ^ (y >> 1) ^ mag01[y & 0x1UL];
+
+        mti = 0;
+    }
+  
+    y = mt[mti++];
+
+    /* Tempering */
+    y ^= (y >> 11);
+    y ^= (y << 7) & 0x9d2c5680UL;
+    y ^= (y << 15) & 0xefc60000UL;
+    y ^= (y >> 18);
+
+    return y;
+}