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

base/rand_util.cc

 // 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"
 #include "base/string_util.h"
 
 namespace base {
 
 int RandInt(int min, int max) {
   DCHECK_LE(min, max);
 
   uint64 range = static_cast<uint64>(max) - min + 1;
   int result = min + static_cast<int>(base::RandGenerator(range));
   DCHECK_GE(result, min);
   DCHECK_LE(result, max);
   return result;
 }
 
 double RandDouble() {
+  return BitsToOpenEndedUnitInterval(base::RandUint64());
+}
+
+double BitsToOpenEndedUnitInterval(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.
 
   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_GE(result, 0.0);
   DCHECK_LT(result, 1.0);
   return result;
 }
 
 uint64 RandGenerator(uint64 max) {
   DCHECK_GT(max, 0ULL);
   return base::RandUint64() % max;
 }
 
 void RandBytes(void* output, size_t output_length) {
   uint64 random_int;
   size_t random_int_size = sizeof(random_int);
   for (size_t i = 0; i < output_length; i += random_int_size) {
     random_int = base::RandUint64();
     size_t copy_count = std::min(output_length - i, random_int_size);
     memcpy(((uint8*)output) + i, &random_int, copy_count);
   }
 }
 
 std::string RandBytesAsString(size_t length) {
   std::string result;
   RandBytes(WriteInto(&result, length + 1), length);
   return result;
 }
 
 }  // namespace base