Aaru.Checksums.Native/crc16_ccitt.c

/*
 * This file is part of the Aaru Data Preservation Suite.
 * Copyright (c) 2019-2025 Natalia Portillo.
 *
 * This library is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of the
 * License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, see <http://www.gnu.org/licenses/>.
 */

#include <stdint.h>
#include <stdlib.h>

#include "library.h"
#include "crc16_ccitt.h"

/**
 * @brief Initializes the CRC-16 checksum algorithm with the CCITT polynomial.
 *
 * This function initializes the state variables required for the CRC-16
 * checksum algorithm using the CCITT polynomial. It prepares the algorithm
 * to calculate the checksum for a new data set.
 *
 * @return Pointer to a structure containing the checksum state.
 */
AARU_EXPORT crc16_ccitt_ctx *AARU_CALL crc16_ccitt_init(void)
{
    crc16_ccitt_ctx *ctx = (crc16_ccitt_ctx *)malloc(sizeof(crc16_ccitt_ctx));

    if(!ctx) return NULL;

    ctx->crc = 0x0000;

    return ctx;
}

/**
 * @brief Updates the CRC-16 checksum with new data.
 *
 * This function updates the CRC-16 checksum.
 * The checksum is updated for the given data by using the CCITT polynomial.
 * The algorithm continues the checksum calculation from the previous state,
 * so it can be used to update the checksum with new data as it is read.
 *
 * @param ctx Pointer to the CRC-16 context structure.
 * @param data Pointer to the input data buffer.
 * @param len The length of the input data buffer.
 *
 * @returns 0 on success, -1 on error.
 */
AARU_EXPORT int AARU_CALL crc16_ccitt_update(crc16_ccitt_ctx *ctx, const uint8_t *data, uint32_t len)
{
    // Unroll according to Intel slicing by uint8_t
    // http://www.intel.com/technology/comms/perfnet/download/CRC_generators.pdf
    // http://sourceforge.net/projects/slicing-by-8/

    if(!ctx || !data) return -1;

    uint16_t       crc;
    const uint8_t *current_char     = data;
    const size_t   unroll           = 4;
    const size_t   bytes_at_once    = 8 * unroll;
    uintptr_t      unaligned_length = (4 - (((uintptr_t)current_char) & 3)) & 3;

    crc = ctx->crc;

    while((len != 0) && (unaligned_length != 0))
    {
        crc = (crc << 8) ^ crc16_ccitt_table[0][(crc >> 8) ^ *current_char++];
        len--;
        unaligned_length--;
    }

    while(len >= bytes_at_once)
    {
        size_t unrolling;
        for(unrolling = 0; unrolling < unroll; unrolling++)
        {
            crc = crc16_ccitt_table[7][current_char[0] ^ (crc >> 8)] ^
                  crc16_ccitt_table[6][current_char[1] ^ (crc & 0xFF)] ^ crc16_ccitt_table[5][current_char[2]] ^
                  crc16_ccitt_table[4][current_char[3]] ^ crc16_ccitt_table[3][current_char[4]] ^
                  crc16_ccitt_table[2][current_char[5]] ^ crc16_ccitt_table[1][current_char[6]] ^
                  crc16_ccitt_table[0][current_char[7]];

            current_char += 8;
        }

        len -= bytes_at_once;
    }

    while(len-- != 0) crc = (crc << 8) ^ crc16_ccitt_table[0][(crc >> 8) ^ *current_char++];

    ctx->crc = crc;
    return 0;
}

/**
 * @brief Finalizes the calculation of the CRC-16 checksum.
 *
 * This function finalizes the calculation of the CRC-16 checksum and returns
 * its value.
 *
 * @param[in] ctx Pointer to the CRC-16 context structure.
 * @param[out] checksum Pointer to a 16-bit unsigned integer to store the checksum value.
 *
 * @returns 0 on success, -1 on error.
 */
AARU_EXPORT int AARU_CALL crc16_ccitt_final(crc16_ccitt_ctx *ctx, uint16_t *crc)
{
    if(!ctx) return -1;

    *crc = ~ctx->crc;

    return 0;
}

/**
 * @brief Frees the resources allocated for the CRC-16 checksum context.
 *
 * This function should be called to release the memory used by the CRC-16 checksum
 * context structure after it is no longer needed.
 *
 * @param ctx The CRC-16 checksum context structure, to be freed.
 */
AARU_EXPORT void AARU_CALL crc16_ccitt_free(crc16_ccitt_ctx *ctx)
{
    if(ctx) free(ctx);
}