bucket_ranges.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 "base/metrics/bucket_ranges.h"

#include <cmath>

#include "base/logging.h"

namespace base {

// Static table of checksums for all possible 8 bit bytes.
const uint32_t kCrcTable[256] = {
    0x0,         0x77073096L, 0xee0e612cL, 0x990951baL, 0x76dc419L,
    0x706af48fL, 0xe963a535L, 0x9e6495a3L, 0xedb8832L,  0x79dcb8a4L,
    0xe0d5e91eL, 0x97d2d988L, 0x9b64c2bL,  0x7eb17cbdL, 0xe7b82d07L,
    0x90bf1d91L, 0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
    0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 0x136c9856L,
    0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 0x14015c4fL, 0x63066cd9L,
    0xfa0f3d63L, 0x8d080df5L, 0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L,
    0xa2677172L, 0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
    0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 0x32d86ce3L,
    0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 0x26d930acL, 0x51de003aL,
    0xc8d75180L, 0xbfd06116L, 0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L,
    0xb8bda50fL, 0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
    0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 0x76dc4190L,
    0x1db7106L,  0x98d220bcL, 0xefd5102aL, 0x71b18589L, 0x6b6b51fL,
    0x9fbfe4a5L, 0xe8b8d433L, 0x7807c9a2L, 0xf00f934L,  0x9609a88eL,
    0xe10e9818L, 0x7f6a0dbbL, 0x86d3d2dL,  0x91646c97L, 0xe6635c01L,
    0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 0x6c0695edL,
    0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 0x65b0d9c6L, 0x12b7e950L,
    0x8bbeb8eaL, 0xfcb9887cL, 0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L,
    0xfbd44c65L, 0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
    0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 0x4369e96aL,
    0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 0x44042d73L, 0x33031de5L,
    0xaa0a4c5fL, 0xdd0d7cc9L, 0x5005713cL, 0x270241aaL, 0xbe0b1010L,
    0xc90c2086L, 0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
    0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 0x59b33d17L,
    0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 0xedb88320L, 0x9abfb3b6L,
    0x3b6e20cL,  0x74b1d29aL, 0xead54739L, 0x9dd277afL, 0x4db2615L,
    0x73dc1683L, 0xe3630b12L, 0x94643b84L, 0xd6d6a3eL,  0x7a6a5aa8L,
    0xe40ecf0bL, 0x9309ff9dL, 0xa00ae27L,  0x7d079eb1L, 0xf00f9344L,
    0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 0xf762575dL, 0x806567cbL,
    0x196c3671L, 0x6e6b06e7L, 0xfed41b76L, 0x89d32be0L, 0x10da7a5aL,
    0x67dd4accL, 0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
    0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 0xd1bb67f1L,
    0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 0xd80d2bdaL, 0xaf0a1b4cL,
    0x36034af6L, 0x41047a60L, 0xdf60efc3L, 0xa867df55L, 0x316e8eefL,
    0x4669be79L, 0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
    0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 0xc5ba3bbeL,
    0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 0xc2d7ffa7L, 0xb5d0cf31L,
    0x2cd99e8bL, 0x5bdeae1dL, 0x9b64c2b0L, 0xec63f226L, 0x756aa39cL,
    0x26d930aL,  0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x5005713L,
    0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0xcb61b38L,  0x92d28e9bL,
    0xe5d5be0dL, 0x7cdcefb7L, 0xbdbdf21L,  0x86d3d2d4L, 0xf1d4e242L,
    0x68ddb3f8L, 0x1fda836eL, 0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L,
    0x18b74777L, 0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
    0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 0xa00ae278L,
    0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 0xa7672661L, 0xd06016f7L,
    0x4969474dL, 0x3e6e77dbL, 0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L,
    0x37d83bf0L, 0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
    0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 0xbad03605L,
    0xcdd70693L, 0x54de5729L, 0x23d967bfL, 0xb3667a2eL, 0xc4614ab8L,
    0x5d681b02L, 0x2a6f2b94L, 0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL,
    0x2d02ef8dL,
};

// We generate the CRC-32 using the low order bits to select whether to XOR in
// the reversed polynomial 0xedb88320L.  This is nice and simple, and allows us
// to keep the quotient in a uint32_t.  Since we're not concerned about the
// nature of corruptions (i.e., we don't care about bit sequencing, since we are
// handling memory changes, which are more grotesque) so we don't bother to get
// the CRC correct for big-endian vs little-ending calculations.  All we need is
// a nice hash, that tends to depend on all the bits of the sample, with very
// little chance of changes in one place impacting changes in another place.
static uint32_t Crc32(uint32_t sum, HistogramBase::Sample value) {
  // TODO(jar): Switch to false and watch stats.
  const bool kUseRealCrc = true;

  if (kUseRealCrc) {
    union {
      HistogramBase::Sample range;
      unsigned char bytes[sizeof(HistogramBase::Sample)];
    } converter;
    converter.range = value;
    for (size_t i = 0; i < sizeof(converter); ++i)
      sum = kCrcTable[(sum & 0xff) ^ converter.bytes[i]] ^ (sum >> 8);
  } else {
    // Use hash techniques provided in ReallyFastHash, except we don't care
    // about "avalanching" (which would worsten the hash, and add collisions),
    // and we don't care about edge cases since we have an even number of bytes.
    union {
      HistogramBase::Sample range;
      uint16_t ints[sizeof(HistogramBase::Sample) / 2];
    } converter;
    DCHECK_EQ(sizeof(HistogramBase::Sample), sizeof(converter));
    converter.range = value;
    sum += converter.ints[0];
    sum = (sum << 16) ^ sum ^ (static_cast<uint32_t>(converter.ints[1]) << 11);
    sum += sum >> 11;
  }
  return sum;
}

BucketRanges::BucketRanges(size_t num_ranges)
    : ranges_(num_ranges, 0),
      checksum_(0) {}

BucketRanges::~BucketRanges() {}

void BucketRanges::set_range(size_t i, HistogramBase::Sample value) {
  DCHECK_LT(i, ranges_.size());
  CHECK_GE(value, 0);
  ranges_[i] = value;
}

uint32_t BucketRanges::CalculateChecksum() const {
  // Seed checksum.
  uint32_t checksum = static_cast<uint32_t>(ranges_.size());

  for (size_t index = 0; index < ranges_.size(); ++index)
    checksum = Crc32(checksum, ranges_[index]);
  return checksum;
}

bool BucketRanges::HasValidChecksum() const {
  return CalculateChecksum() == checksum_;
}

void BucketRanges::ResetChecksum() {
  checksum_ = CalculateChecksum();
}

bool BucketRanges::Equals(const BucketRanges* other) const {
  if (checksum_ != other->checksum_)
    return false;
  if (ranges_.size() != other->ranges_.size())
    return false;
  for (size_t index = 0; index < ranges_.size(); ++index) {
    if (ranges_[index] != other->ranges_[index])
      return false;
  }
  return true;
}

}  // namespace base