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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* The xorshift128+ pseudo-random number generator. */
+
+#ifndef mozilla_XorShift128Plus_h
+#define mozilla_XorShift128Plus_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/FloatingPoint.h"
+
+#include <inttypes.h>
+
+namespace mozilla {
+namespace non_crypto {
+
+/*
+ * A stream of pseudo-random numbers generated using the xorshift+ technique
+ * described here:
+ *
+ * Vigna, Sebastiano (2014). "Further scramblings of Marsaglia's xorshift
+ * generators". arXiv:1404.0390 (http://arxiv.org/abs/1404.0390)
+ *
+ * That paper says:
+ *
+ *     In particular, we propose a tightly coded xorshift128+ generator that
+ *     does not fail systematically any test from the BigCrush suite of TestU01
+ *     (even reversed) and generates 64 pseudorandom bits in 1.10 ns on an
+ *     Intel(R) Core(TM) i7-4770 CPU @3.40GHz (Haswell). It is the fastest
+ *     generator we are aware of with such empirical statistical properties.
+ *
+ * The stream of numbers produced by this method repeats every 2**128 - 1 calls
+ * (i.e. never, for all practical purposes). Zero appears 2**64 - 1 times in
+ * this period; all other numbers appear 2**64 times. Additionally, each *bit*
+ * in the produced numbers repeats every 2**128 - 1 calls.
+ *
+ * This generator is not suitable as a cryptographically secure random number
+ * generator.
+ */
+class XorShift128PlusRNG {
+  uint64_t mState[2];
+
+ public:
+  /*
+   * Construct a xorshift128+ pseudo-random number stream using |aInitial0| and
+   * |aInitial1| as the initial state.  These MUST NOT both be zero.
+   *
+   * If the initial states contain many zeros, for a few iterations you'll see
+   * many zeroes in the generated numbers.  It's suggested to seed a SplitMix64
+   * generator <http://xorshift.di.unimi.it/splitmix64.c> and use its first two
+   * outputs to seed xorshift128+.
+   */
+  XorShift128PlusRNG(uint64_t aInitial0, uint64_t aInitial1) {
+    setState(aInitial0, aInitial1);
+  }
+
+  /**
+   * Return a pseudo-random 64-bit number.
+   */
+  uint64_t next() {
+    /*
+     * The offsetOfState*() methods below are provided so that exceedingly-rare
+     * callers that want to observe or poke at RNG state in C++ type-system-
+     * ignoring means can do so. Don't change the next() or nextDouble()
+     * algorithms without altering code that uses offsetOfState*()!
+     */
+    uint64_t s1 = mState[0];
+    const uint64_t s0 = mState[1];
+    mState[0] = s0;
+    s1 ^= s1 << 23;
+    mState[1] = s1 ^ s0 ^ (s1 >> 17) ^ (s0 >> 26);
+    return mState[1] + s0;
+  }
+
+  /*
+   * Return a pseudo-random floating-point value in the range [0, 1). More
+   * precisely, choose an integer in the range [0, 2**53) and divide it by
+   * 2**53. Given the 2**128 - 1 period noted above, the produced doubles are
+   * all but uniformly distributed in this range.
+   */
+  double nextDouble() {
+    /*
+     * Because the IEEE 64-bit floating point format stores the leading '1' bit
+     * of the mantissa implicitly, it effectively represents a mantissa in the
+     * range [0, 2**53) in only 52 bits. FloatingPoint<double>::kExponentShift
+     * is the width of the bitfield in the in-memory format, so we must add one
+     * to get the mantissa's range.
+     */
+    static constexpr int kMantissaBits =
+      mozilla::FloatingPoint<double>::kExponentShift + 1;
+    uint64_t mantissa = next() & ((UINT64_C(1) << kMantissaBits) - 1);
+    return double(mantissa) / (UINT64_C(1) << kMantissaBits);
+  }
+
+  /*
+   * Set the stream's current state to |aState0| and |aState1|. These must not
+   * both be zero; ideally, they should have an almost even mix of zero and one
+   * bits.
+   */
+  void setState(uint64_t aState0, uint64_t aState1) {
+    MOZ_ASSERT(aState0 || aState1);
+    mState[0] = aState0;
+    mState[1] = aState1;
+  }
+
+  static size_t offsetOfState0() {
+    return offsetof(XorShift128PlusRNG, mState[0]);
+  }
+  static size_t offsetOfState1() {
+    return offsetof(XorShift128PlusRNG, mState[1]);
+  }
+};
+
+} // namespace non_crypto
+} // namespace mozilla
+
+#endif // mozilla_XorShift128Plus_h