/*
*  C Implementation: crc32
*
*  code from http://www.w3.org/TR/PNG/#D-CRCAppendix
*
*/
#include "internal.h"

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

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

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

    for (n = 0; n < 256; n++) {
        c = (unsigned long) 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). */

static unsigned long update_crc(unsigned long crc, unsigned char *buf, int len)
{
    unsigned long c = crc;
    int 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;
}

/* Return the CRC of the bytes buf[0..len-1]. */
unsigned long mpc_crc32(unsigned char *buf, int len)
{
    return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}
