CSE2410 Fall 2014 Bounce

// Copyright 2013 the V8 project authors. All rights reserved.

// Redistribution and use in source and binary forms, with or without

// modification, are permitted provided that the following conditions are

// met:

//

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//       notice, this list of conditions and the following disclaimer.

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//       with the distribution.

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//

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS

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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

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#include "utils/random-number-generator.h"

#include <cstdio>

#include <cstdlib>

#include "flags.h"

#include "platform/mutex.h"

#include "platform/time.h"

#include "utils.h"

namespace v8 {

namespace internal {

static LazyMutex entropy_mutex = LAZY_MUTEX_INITIALIZER;

static RandomNumberGenerator::EntropySource entropy_source = NULL;

// static

void RandomNumberGenerator::SetEntropySource(EntropySource source) {

  LockGuard<Mutex> lock_guard(entropy_mutex.Pointer());

  entropy_source = source;

}

RandomNumberGenerator::RandomNumberGenerator() {

  // Check --random-seed flag first.

  if (FLAG_random_seed != 0) {

    SetSeed(FLAG_random_seed);

    return;

  }

  // Check if embedder supplied an entropy source.

  { LockGuard<Mutex> lock_guard(entropy_mutex.Pointer());

    if (entropy_source != NULL) {

      int64_t seed;

      if (entropy_source(reinterpret_cast<unsigned char*>(&seed),

                         sizeof(seed))) {

        SetSeed(seed);

        return;

      }

    }

  }

#if V8_OS_CYGWIN || V8_OS_WIN

  // Use rand_s() to gather entropy on Windows. See:

  // https://code.google.com/p/v8/issues/detail?id=2905

  unsigned first_half, second_half;

  errno_t result = rand_s(&first_half);

  ASSERT_EQ(0, result);

  result = rand_s(&second_half);

  ASSERT_EQ(0, result);

  SetSeed((static_cast<int64_t>(first_half) << 32) + second_half);

#else

  // Gather entropy from /dev/urandom if available.

  FILE* fp = fopen("/dev/urandom", "rb");

  if (fp != NULL) {

    int64_t seed;

    size_t n = fread(&seed, sizeof(seed), 1, fp);

    fclose(fp);

    if (n == 1) {

      SetSeed(seed);

      return;

    }

  }

  // We cannot assume that random() or rand() were seeded

  // properly, so instead of relying on random() or rand(),

  // we just seed our PRNG using timing data as fallback.

  // This is weak entropy, but it's sufficient, because

  // it is the responsibility of the embedder to install

  // an entropy source using v8::V8::SetEntropySource(),

  // which provides reasonable entropy, see:

  // https://code.google.com/p/v8/issues/detail?id=2905

  int64_t seed = Time::NowFromSystemTime().ToInternalValue() << 24;

  seed ^= TimeTicks::HighResolutionNow().ToInternalValue() << 16;

  seed ^= TimeTicks::Now().ToInternalValue() << 8;

  SetSeed(seed);

#endif  // V8_OS_CYGWIN || V8_OS_WIN

}

int RandomNumberGenerator::NextInt(int max) {

  ASSERT_LE(0, max);

  // Fast path if max is a power of 2.

  if (IsPowerOf2(max)) {

    return static_cast<int>((max * static_cast<int64_t>(Next(31))) >> 31);

  }

  while (true) {

    int rnd = Next(31);

    int val = rnd % max;

    if (rnd - val + (max - 1) >= 0) {

      return val;

    }

  }

}

double RandomNumberGenerator::NextDouble() {

  return ((static_cast<int64_t>(Next(26)) << 27) + Next(27)) /

      static_cast<double>(static_cast<int64_t>(1) << 53);

}

void RandomNumberGenerator::NextBytes(void* buffer, size_t buflen) {

  for (size_t n = 0; n < buflen; ++n) {

    static_cast<uint8_t*>(buffer)[n] = static_cast<uint8_t>(Next(8));

  }

}

int RandomNumberGenerator::Next(int bits) {

  ASSERT_LT(0, bits);

  ASSERT_GE(32, bits);

  int64_t seed = (seed_ * kMultiplier + kAddend) & kMask;

  seed_ = seed;

  return static_cast<int>(seed >> (48 - bits));

}

void RandomNumberGenerator::SetSeed(int64_t seed) {

  seed_ = (seed ^ kMultiplier) & kMask;

}

} }  // namespace v8::internal