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-rw-r--r--mfbt/tests/TestXorShift128PlusRNG.cpp115
1 files changed, 115 insertions, 0 deletions
diff --git a/mfbt/tests/TestXorShift128PlusRNG.cpp b/mfbt/tests/TestXorShift128PlusRNG.cpp
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+++ b/mfbt/tests/TestXorShift128PlusRNG.cpp
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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/. */
+
+#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;
+}