/*
 * Copyright 2024 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
 *
 *     http://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.
 */

#pragma once

#include <stdint.h>
#include <string.h>

//! \file
//! Calculate the legacy checksum of data.
//!
//! The calculation is somewhat like a CRC with the CRC-32 polynomial, but with
//! the data bytes reordered oddly. The checksum is calculated 32 bits at a
//! time, little-endian, MSB-first. The legacy checksum of bytes A B C D E F G H
//! is equal to the CRC-32 of bytes D C B A H G F E (xor 0xFFFFFFFF). When the
//! data being checksummmed is not a multiple of four bytes in length, the
//! remainder bytes are zero-padded and byte-swapped(!) before being checksummed
//! like the previous full words. For example, the legacy checksum of bytes
//! 1 2 3 4 5 6 is equal to the checksum of bytes 1 2 3 4 6 5 0 0, which is
//! equivalent to the CRC-32 of bytes 4 3 2 1 0 0 5 6 (xor 0xFFFFFFFF).
//!
//! The legacy checksum should not be used except when required for
//! backwards-compatibility purposes.

typedef struct LegacyChecksum {
  uint32_t reg;
  uint8_t accumulator[3];
  uint8_t accumulated_length;
} LegacyChecksum;

void legacy_defective_checksum_init(LegacyChecksum *checksum);

void legacy_defective_checksum_update(
    LegacyChecksum * restrict checksum,
    const void * restrict data, size_t length);

uint32_t legacy_defective_checksum_finish(LegacyChecksum *checksum);

//! Convenience wrapper to checksum memory in one shot.
uint32_t legacy_defective_checksum_memory(const void * restrict data,
                                          size_t length);