base/metrics/metrics_hashes.cc

// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/metrics/metrics_hashes.h"

#include <string.h>

#include <string_view>

#include "base/check_op.h"
#include "base/containers/span.h"
#include "base/hash/md5.h"
#include "base/hash/sha1.h"
#include "base/numerics/byte_conversions.h"

namespace base {
namespace {

// Converts the 8-byte prefix of an MD5 hash into a uint64_t value.
inline uint64_t DigestToUInt64(const MD5Digest& digest) {
  return U64FromBigEndian(span(digest.a).first<8u>());
}

// Converts the 4-byte prefix of an MD5 hash into a uint32_t value.
inline uint32_t DigestToUInt32(const MD5Digest& digest) {
  return U32FromBigEndian(span(digest.a).first<4u>());
}

}  // namespace

uint64_t HashMetricName(std::string_view name) {
  // Corresponding Python code for quick look up:
  //
  //   import struct
  //   import hashlib
  //   struct.unpack('>Q', hashlib.md5(name.encode('utf-8')).digest()[:8])[0]
  //
  MD5Digest digest;
  MD5Sum(as_byte_span(name), &digest);
  return DigestToUInt64(digest);
}

uint32_t HashMetricNameAs32Bits(std::string_view name) {
  MD5Digest digest;
  MD5Sum(as_byte_span(name), &digest);
  return DigestToUInt32(digest);
}

uint32_t ParseMetricHashTo32Bits(uint64_t hash) {
  return (hash >> 32);
}

uint32_t HashFieldTrialName(std::string_view name) {
  // SHA-1 is designed to produce a uniformly random spread in its output space,
  // even for nearly-identical inputs.
  SHA1Digest sha1_hash = SHA1Hash(as_byte_span(name));
  return U32FromLittleEndian(span(sha1_hash).first<4u>());
}

}  // namespace base