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.h

@@ -0,0 +1,93 @@
+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_RTC_BASE_RANDOM_H_
+#define WEBRTC_RTC_BASE_RANDOM_H_
+
+#include <limits>
+
+#include "webrtc/typedefs.h"
+#include "webrtc/base/constructormagic.h"
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+class Random {
+ public:
+  // TODO(tommi): Change this so that the seed can be initialized internally,
+  // e.g. by offering two ways of constructing or offer a static method that
+  // returns a seed that's suitable for initialization.
+  // The problem now is that callers are calling clock_->TimeInMicroseconds()
+  // which calls TickTime::Now().Ticks(), which can return a very low value on
+  // Mac and can result in a seed of 0 after conversion to microseconds.
+  // Besides the quality of the random seed being poor, this also requires
+  // the client to take on extra dependencies to generate a seed.
+  // If we go for a static seed generator in Random, we can use something from
+  // webrtc/base and make sure that it works the same way across platforms.
+  // See also discussion here: https://codereview.webrtc.org/1623543002/
+  explicit Random(uint64_t seed);
+
+  // Return pseudo-random integer of the specified type.
+  // We need to limit the size to 32 bits to keep the output close to uniform.
+  template <typename T>
+  T Rand() {
+    static_assert(std::numeric_limits<T>::is_integer &&
+                      std::numeric_limits<T>::radix == 2 &&
+                      std::numeric_limits<T>::digits <= 32,
+                  "Rand is only supported for built-in integer types that are "
+                  "32 bits or smaller.");
+    return static_cast<T>(NextOutput());
+  }
+
+  // Uniformly distributed pseudo-random number in the interval [0, t].
+  uint32_t Rand(uint32_t t);
+
+  // Uniformly distributed pseudo-random number in the interval [low, high].
+  uint32_t Rand(uint32_t low, uint32_t high);
+
+  // Uniformly distributed pseudo-random number in the interval [low, high].
+  int32_t Rand(int32_t low, int32_t high);
+
+  // Normal Distribution.
+  double Gaussian(double mean, double standard_deviation);
+
+  // Exponential Distribution.
+  double Exponential(double lambda);
+
+ private:
+  // Outputs a nonzero 64-bit random number.
+  uint64_t NextOutput() {
+    state_ ^= state_ >> 12;
+    state_ ^= state_ << 25;
+    state_ ^= state_ >> 27;
+    RTC_DCHECK(state_ != 0x0ULL);
+    return state_ * 2685821657736338717ull;
+  }
+
+  uint64_t state_;
+
+  RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(Random);
+};
+
+// Return pseudo-random number in the interval [0.0, 1.0).
+template <>
+float Random::Rand<float>();
+
+// Return pseudo-random number in the interval [0.0, 1.0).
+template <>
+double Random::Rand<double>();
+
+// Return pseudo-random boolean value.
+template <>
+bool Random::Rand<bool>();
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_RTC_BASE_RANDOM_H_