crc.cpp

/*
 * 
 * Copyright (c) 2015. Lukas Dresel, Julius Lohmann, Benedikt Lorch, Burkhard Ringlein, Andreas Rupp, Yuriy Sulima, Carola Touchy
 * 
 * This file is part of GarbageCollector. 
 * 
 *	 GarbageCollector is free software: you can redistribute it and/or modify
 * 	 it under the terms of the GNU General Public License as published by
 * 	 the Free Software Foundation, either version 3 of the License, or
 * 	 (at your option) any later version.
 * 
 * 	 GarbageCollector 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 General Public License for more details.
 * 
 * 	 You should have received a copy of the GNU General Public License
 * 	 along with GarbageCollector.  If not, see <http://www.gnu.org/licenses/>.
 *
 */ 


#include "crc.h"

/* Table of CRCs of all 8-bit messages. */
uint32_t crc_table[256];

/* Flag: has the table been computed? Initially false. */
int crc_table_computed = 0;

/* Make the table for a fast CRC. */
void make_crc_table(void)
{
    uint32_t c;
    int n, k;

    for (n = 0; n < 256; n++) 
    {
        c = (uint32_t) n;
        for (k = 0; k < 8; k++) 
        {
            if (c & 1)
                c = 0xedb88320L ^ (c >> 1);
            else
                c = c >> 1;
        }
        crc_table[n] = c;
    }
    crc_table_computed = 1;
}

/* Update a running CRC with the bytes buf[0..len-1]--the CRC
  should be initialized to all 1's, and the transmitted value
  is the 1's complement of the final running CRC (see the
  crc() routine below)). */

uint32_t update_crc(uint32_t crc, unsigned char *buf, uint64_t len)
{
    uint32_t c = crc;
    uint64_t n;

    if (!crc_table_computed)
    {
        make_crc_table();
    }
    for (n = 0; n < len; n++) 
    {
        c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
    }
    
    return c;
}

uint32_t start_crc()
{
    return 0xffffffffL;
}
uint32_t end_crc(uint32_t crc)
{
    return(crc ^ 0xffffffffL);
}

/* Return the CRC of the bytes buf[0..len-1]. */
uint32_t crc(unsigned char *buf, int len)
{
    uint32_t crc = start_crc();
    crc = update_crc(crc, buf, len);
    return end_crc(crc);
}