Create variations target that depends on base.

chrome/common/metrics/entropy_provider.cc

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "chrome/common/metrics/entropy_provider.h"
-
-#include <algorithm>
-#include <limits>
-#include <vector>
-
-#include "base/logging.h"
-#include "base/rand_util.h"
-#include "base/sha1.h"
-#include "base/sys_byteorder.h"
-#include "chrome/common/metrics/metrics_util.h"
-
-namespace metrics {
-
-namespace internal {
-
-SeededRandGenerator::SeededRandGenerator(uint32 seed) {
-  mersenne_twister_.init_genrand(seed);
-}
-
-SeededRandGenerator::~SeededRandGenerator() {
-}
-
-uint32 SeededRandGenerator::operator()(uint32 range) {
-  // Based on base::RandGenerator().
-  DCHECK_GT(range, 0u);
-
-  // We must discard random results above this number, as they would
-  // make the random generator non-uniform (consider e.g. if
-  // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
-  // as likely as a result of 3 or 4).
-  uint32 max_acceptable_value =
-      (std::numeric_limits<uint32>::max() / range) * range - 1;
-
-  uint32 value;
-  do {
-    value = mersenne_twister_.genrand_int32();
-  } while (value > max_acceptable_value);
-
-  return value % range;
-}
-
-void PermuteMappingUsingRandomizationSeed(uint32 randomization_seed,
-                                          std::vector<uint16>* mapping) {
-  for (size_t i = 0; i < mapping->size(); ++i)
-    (*mapping)[i] = static_cast<uint16>(i);
-
-  SeededRandGenerator generator(randomization_seed);
-  std::random_shuffle(mapping->begin(), mapping->end(), generator);
-}
-
-}  // namespace internal
-
-SHA1EntropyProvider::SHA1EntropyProvider(const std::string& entropy_source)
-    : entropy_source_(entropy_source) {
-}
-
-SHA1EntropyProvider::~SHA1EntropyProvider() {
-}
-
-double SHA1EntropyProvider::GetEntropyForTrial(
-    const std::string& trial_name,
-    uint32 randomization_seed) const {
-  // Given enough input entropy, SHA-1 will produce a uniformly random spread
-  // in its output space. In this case, the input entropy that is used is the
-  // combination of the original |entropy_source_| and the |trial_name|.
-  //
-  // Note: If |entropy_source_| has very low entropy, such as 13 bits or less,
-  // it has been observed that this method does not result in a uniform
-  // distribution given the same |trial_name|. When using such a low entropy
-  // source, PermutedEntropyProvider should be used instead.
-  std::string input(entropy_source_ + trial_name);
-  unsigned char sha1_hash[base::kSHA1Length];
-  base::SHA1HashBytes(reinterpret_cast<const unsigned char*>(input.c_str()),
-                      input.size(),
-                      sha1_hash);
-
-  uint64 bits;
-  COMPILE_ASSERT(sizeof(bits) < sizeof(sha1_hash), need_more_data);
-  memcpy(&bits, sha1_hash, sizeof(bits));
-  bits = base::ByteSwapToLE64(bits);
-
-  return base::BitsToOpenEndedUnitInterval(bits);
-}
-
-PermutedEntropyProvider::PermutedEntropyProvider(
-    uint16 low_entropy_source,
-    size_t low_entropy_source_max)
-    : low_entropy_source_(low_entropy_source),
-      low_entropy_source_max_(low_entropy_source_max) {
-  DCHECK_LT(low_entropy_source, low_entropy_source_max);
-  DCHECK_LE(low_entropy_source_max, std::numeric_limits<uint16>::max());
-}
-
-PermutedEntropyProvider::~PermutedEntropyProvider() {
-}
-
-double PermutedEntropyProvider::GetEntropyForTrial(
-    const std::string& trial_name,
-    uint32 randomization_seed) const {
-  if (randomization_seed == 0)
-    randomization_seed = HashName(trial_name);
-
-  return GetPermutedValue(randomization_seed) /
-         static_cast<double>(low_entropy_source_max_);
-}
-
-uint16 PermutedEntropyProvider::GetPermutedValue(
-    uint32 randomization_seed) const {
-  std::vector<uint16> mapping(low_entropy_source_max_);
-  internal::PermuteMappingUsingRandomizationSeed(randomization_seed, &mapping);
-  return mapping[low_entropy_source_];
-}
-
-}  // namespace metrics