/*
 * CRC code taken from https://www.rfc-editor.org/rfc/rfc1952#section-8
 *
 * Copyright (c) 1996 L. Peter Deutsch
 *
 * Permission is granted to copy and distribute this document for any
 * purpose and without charge, including translations into other
 * languages and incorporation into compilations, provided that the
 * copyright notice and this notice are preserved, and that any
 * substantive changes or deletions from the original are clearly
 * marked.
 */

#include <stdlib.h>
#include <stdbool.h>

/* Table of CRCs of all 8-bit messages. */
unsigned long 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) {
    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] and return
the updated crc. The crc should be initialized to zero. Pre- and
post-conditioning (one's complement) is performed within this
function so it shouldn't be done by the caller. Usage example:

 unsigned long crc = 0L;

 while (read_buffer(buffer, length) != EOF) {
   crc = update_crc(crc, buffer, length);
 }
 if (crc != original_crc) error();
*/
unsigned long update_crc(unsigned long crc, unsigned char *buf, int len) {
    unsigned long c = crc ^ 0xffffffffL;
    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 ^ 0xffffffffL;
}

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