Fix off-by-one error in PRNG.

webrtc/test/random.cc

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/test/random.h"
 
 #include <math.h>
 
 #include "webrtc/base/checks.h"
 
 namespace webrtc {
 
 namespace test {
 
 Random::Random(uint32_t seed) : a_(0x531FDB97 ^ seed), b_(0x6420ECA8 + seed) {
 }
 
 float Random::Rand() {
-  const float kScale = 1.0f / 0xffffffff;
-  float result = kScale * b_;
+  const double kScale = 1.0f / (static_cast<uint64_t>(1) << 32);
+  double result = kScale * b_;
   a_ ^= b_;
   b_ += a_;
-  return result;
+  return static_cast<float>(result);
 }
 
 int Random::Rand(int low, int high) {
   RTC_DCHECK(low <= high);
   float uniform = Rand() * (high - low + 1) + low;
   return static_cast<int>(uniform);
 }
 
 int Random::Gaussian(int mean, int standard_deviation) {
   // Creating a Normal distribution variable from two independent uniform
   // variables based on the Box-Muller transform, which is defined on the
   // interval (0, 1], hence the mask+add below.
   const double kPi = 3.14159265358979323846;
   const double kScale = 1.0 / 0x80000000ul;
   double u1 = kScale * ((a_ & 0x7ffffffful) + 1);
   double u2 = kScale * ((b_ & 0x7ffffffful) + 1);
   a_ ^= b_;
   b_ += a_;
   return static_cast<int>(
       mean + standard_deviation * sqrt(-2 * log(u1)) * cos(2 * kPi * u2));
 }
 
 int Random::Exponential(float lambda) {
   float uniform = Rand();
   return static_cast<int>(-log(uniform) / lambda);
 }
 }  // namespace test
 }  // namespace webrtc