Reland "Move webrtc/{base => rtc_base}" (https://codereview.webrtc.org/2877023002)

Reland the base->rtc_base without adding stub headers (will be
done in follow-up CL). This preserves git blame history of all files.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Change-Id: Iea3bb6f3f67b8374c96337b63e8f5aa3e6181012
Reviewed-on: https://chromium-review.googlesource.com/554611
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18821}

webrtc/rtc_base/random.cc

@@ -0,0 +1,86 @@
+/*
+ *  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/base/random.h"
+
+#include <math.h>
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+Random::Random(uint64_t seed) {
+  RTC_DCHECK(seed != 0x0ull);
+  state_ = seed;
+}
+
+uint32_t Random::Rand(uint32_t t) {
+  // Casting the output to 32 bits will give an almost uniform number.
+  // Pr[x=0] = (2^32-1) / (2^64-1)
+  // Pr[x=k] = 2^32 / (2^64-1) for k!=0
+  // Uniform would be Pr[x=k] = 2^32 / 2^64 for all 32-bit integers k.
+  uint32_t x = NextOutput();
+  // If x / 2^32 is uniform on [0,1), then x / 2^32 * (t+1) is uniform on
+  // the interval [0,t+1), so the integer part is uniform on [0,t].
+  uint64_t result = x * (static_cast<uint64_t>(t) + 1);
+  result >>= 32;
+  return result;
+}
+
+uint32_t Random::Rand(uint32_t low, uint32_t high) {
+  RTC_DCHECK(low <= high);
+  return Rand(high - low) + low;
+}
+
+int32_t Random::Rand(int32_t low, int32_t high) {
+  RTC_DCHECK(low <= high);
+  // We rely on subtraction (and addition) to be the same for signed and
+  // unsigned numbers in two-complement representation. Thus, although
+  // high - low might be negative as an int, it is the correct difference
+  // when interpreted as an unsigned.
+  return Rand(high - low) + low;
+}
+
+template <>
+float Random::Rand<float>() {
+  double result = NextOutput() - 1;
+  result = result / 0xFFFFFFFFFFFFFFFEull;
+  return static_cast<float>(result);
+}
+
+template <>
+double Random::Rand<double>() {
+  double result = NextOutput() - 1;
+  result = result / 0xFFFFFFFFFFFFFFFEull;
+  return result;
+}
+
+template <>
+bool Random::Rand<bool>() {
+  return Rand(0, 1) == 1;
+}
+
+double Random::Gaussian(double mean, double 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]. Note that we rely on NextOutput to generate integers
+  // in the range [1, 2^64-1]. Normally this behavior is a bit frustrating,
+  // but here it is exactly what we need.
+  const double kPi = 3.14159265358979323846;
+  double u1 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull;
+  double u2 = static_cast<double>(NextOutput()) / 0xFFFFFFFFFFFFFFFFull;
+  return mean + standard_deviation * sqrt(-2 * log(u1)) * cos(2 * kPi * u2);
+}
+
+double Random::Exponential(double lambda) {
+  double uniform = Rand<double>();
+  return -log(uniform) / lambda;
+}
+
+}  // namespace webrtc