1 files changed, 101 insertions, 0 deletions
diff --git a/mfbt/tests/TestXorShift128PlusRNG.cpp b/mfbt/tests/TestXorShift128PlusRNG.cpp
new file mode 100644
index 0000000000..12b3c547ac
--- /dev/null
+++ b/mfbt/tests/TestXorShift128PlusRNG.cpp
@@ -0,0 +1,101 @@
+/* -*- 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/. */
+
+#include <math.h>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/XorShift128PlusRNG.h"
+
+using mozilla::non_crypto::XorShift128PlusRNG;
+
+static void TestDumbSequence() {
+  XorShift128PlusRNG rng(1, 4);
+
+  // Calculated by hand following the algorithm given in the paper. The upper
+  // bits are mostly zero because we started with a poor seed; once it has run
+  // for a while, we'll get an even mix of ones and zeros in all 64 bits.
+  MOZ_RELEASE_ASSERT(rng.next() == 0x800049);
+  MOZ_RELEASE_ASSERT(rng.next() == 0x3000186);
+  MOZ_RELEASE_ASSERT(rng.next() == 0x400003001145);
+
+  // Using ldexp here lets us write out the mantissa in hex, so we can compare
+  // them with the results generated by hand.
+  MOZ_RELEASE_ASSERT(rng.nextDouble() ==
+                     ldexp(static_cast<double>(0x1400003105049), -53));
+  MOZ_RELEASE_ASSERT(rng.nextDouble() ==
+                     ldexp(static_cast<double>(0x2000802e49146), -53));
+  MOZ_RELEASE_ASSERT(rng.nextDouble() ==
+                     ldexp(static_cast<double>(0x248300468544d), -53));
+}
+
+static size_t Population(uint64_t n) {
+  size_t pop = 0;
+
+  while (n > 0) {
+    n &= n - 1;  // Clear the rightmost 1-bit in n.
+    pop++;
+  }
+
+  return pop;
+}
+
+static void TestPopulation() {
+  XorShift128PlusRNG rng(698079309544035222ULL, 6012389156611637584ULL);
+
+  // Give it some time to warm up; it should tend towards more
+  // even distributions of zeros and ones.
+  for (size_t i = 0; i < 40; i++) rng.next();
+
+  for (size_t i = 0; i < 40; i++) {
+    size_t pop = Population(rng.next());
+    MOZ_RELEASE_ASSERT(24 <= pop && pop <= 40);
+  }
+}
+
+static void TestSetState() {
+  static const uint64_t seed[2] = {1795644156779822404ULL,
+                                   14162896116325912595ULL};
+  XorShift128PlusRNG rng(seed[0], seed[1]);
+
+  const size_t n = 10;
+  uint64_t log[n];
+
+  for (size_t i = 0; i < n; i++) log[i] = rng.next();
+
+  rng.setState(seed[0], seed[1]);
+
+  for (size_t i = 0; i < n; i++) MOZ_RELEASE_ASSERT(log[i] == rng.next());
+}
+
+static void TestDoubleDistribution() {
+  XorShift128PlusRNG rng(0xa207aaede6859736, 0xaca6ca5060804791);
+
+  const size_t n = 100;
+  size_t bins[n];
+  mozilla::PodArrayZero(bins);
+
+  // This entire file runs in 0.006s on my laptop. Generating
+  // more numbers lets us put tighter bounds on the bins.
+  for (size_t i = 0; i < 100000; i++) {
+    double d = rng.nextDouble();
+    MOZ_RELEASE_ASSERT(0.0 <= d && d < 1.0);
+    bins[(int)(d * n)]++;
+  }
+
+  for (size_t i = 0; i < n; i++) {
+    MOZ_RELEASE_ASSERT(900 <= bins[i] && bins[i] <= 1100);
+  }
+}
+
+int main() {
+  TestDumbSequence();
+  TestPopulation();
+  TestSetState();
+  TestDoubleDistribution();
+
+  return 0;
+}