Add one-time randomization support for FieldTrial, and the ability to

base/rand_util.cc

-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/rand_util.h"
 
 #include <math.h>
 
 #include <limits>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
 
 namespace base {
 
 int RandInt(int min, int max) {
   DCHECK(min <= max);
 
   uint64 range = static_cast<uint64>(max) - min + 1;
   int result = min + static_cast<int>(base::RandGenerator(range));
   DCHECK(result >= min && result <= max);
   return result;
 }
 
 double RandDouble() {
+  return BitsToRandDouble(base::RandUint64());
+}
+
+double BitsToRandDouble(uint64 bits) {
   // We try to get maximum precision by masking out as many bits as will fit
   // in the target type's mantissa, and raising it to an appropriate power to
   // produce output in the range [0, 1).  For IEEE 754 doubles, the mantissa
   // is expected to accommodate 53 bits.

[jar (doing other things), 2011/04/21 01:03:50]

Why don't you just do a conversion to float (which will take as many bits as it
can!), and then divide by 2^64?

The only risk you' would then have is that the result *could* == 1.0, but that
has probability around 1 in 2^54.... so you can then map that outline *anywhere*
constant and not (significantly) impact the randomness (for any tests we can
do!!).

[Jói, 2011/04/21 19:50:33]

As discussed, this is just extracting most of the existing body of RandDouble()
so that its implementation of converting 8 bytes to a random float in the range
[0.0, 1.0) can be reused.

 
   COMPILE_ASSERT(std::numeric_limits<double>::radix == 2, otherwise_use_scalbn);
   static const int kBits = std::numeric_limits<double>::digits;
-  uint64 random_bits = base::RandUint64() & ((GG_UINT64_C(1) << kBits) - 1);
+  uint64 random_bits = bits & ((GG_UINT64_C(1) << kBits) - 1);
   double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
   DCHECK(result >= 0.0 && result < 1.0);
   return result;
 }
 
 uint64 RandGenerator(uint64 max) {
   DCHECK_GT(max, 0ULL);
   return base::RandUint64() % max;
 }
 
 }  // namespace base