formats/png/crc.cc

// Copyright 2026 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

#include "formats/png/crc.h"

#include <array>
#include <cstdint>

#include "absl/strings/string_view.h"

namespace credentio {
namespace {

// Precomputed CRC-32 table (IEEE 802.3 polynomial: 0xedb88320)
constexpr std::array<uint32_t, 256> MakeCrcTable() {
  std::array<uint32_t, 256> table = {};
  for (uint32_t i = 0; i < 256; ++i) {
    uint32_t c = i;
    for (int j = 0; j < 8; ++j) {
      if (c & 1) {
        c = 0xedb88320L ^ (c >> 1);
      } else {
        c = c >> 1;
      }
    }
    table[i] = c;
  }
  return table;
}

static constexpr std::array<uint32_t, 256> kCrcTable = MakeCrcTable();

uint32_t UpdateCrc(uint32_t crc, absl::string_view data) {
  uint32_t c = crc;
  for (char byte : data) {
    c = kCrcTable[(c ^ static_cast<uint8_t>(byte)) & 0xff] ^ (c >> 8);
  }
  return c;
}

}  // namespace

uint32_t PngChunkCrc(absl::string_view type, absl::string_view data) {
  uint32_t crc = 0xffffffffL;
  crc = UpdateCrc(crc, type);
  crc = UpdateCrc(crc, data);
  return crc ^ 0xffffffffL;
}

}  // namespace credentio