/*
 * This is an implementation of the CRC-24Q cyclic redundancy checksum
 * used by Qualcomm, RTCM104V3, and PGP 6.5.1. According to the RTCM104V3
 * standard, it uses the error polynomial
 *
 *    x^24+ x^23+ x^18+ x^17+ x^14+ x^11+ x^10+ x^7+ x^6+ x^5+ x^4+ x^3+ x+1
 *
 * This corresponds to a mask of 0x1864CFB.  For a primer on CRC theory, 
 * including detailed discussion of how and why the error polynomial is
 * expressed by this mask, see <http://www.ross.net/crc/>.
 *
 * 1) It detects all single bit errors per 24-bit code word.
 * 2) It detects all double bit error combinations in a code word.
 * 3) It detects any odd number of errors.
 * 4) It detects any burst error for which the length of the burst is less than
 *    or equal to 24 bits.
 * 5) It detects most large error bursts with length greater than 24 bits;
 *    the odds of a false positive are at most 2^-23.
 *
 * This hash should not be considered cryptographically secure, but it
 * is extremely good at detecting noise errors.
 *
 * Note that this version has a seed of 0 wired in.  The RTCM104V3 standard
 * requires this.
 *
 * This file is Copyright (c) 2008,2010 by the GPSD project
 * BSD terms apply: see the file COPYING in the distribution root for details.
 */
#include <stdbool.h>

#include "crc24q.h"

#if 0
/*
 * The crc24q code table below can be regenerated with the following code:
 */
#include <stdio.h>
#include <stdlib.h>

unsigned table[25];

#define CRCSEED	0		/* could be NZ to detect leading zeros */
#define CRCPOLY	0x1864CFB	/* encodes all info about the polynomial */ 

static void
crc_init(unsigned table[256])
{
    unsigned i, j;
    unsigned h;

    table[0] = CRCSEED;
    table[1] = h = CRCPOLY;

    for (i = 2; i < 256; i *= 2) {
	if ((h <<= 1) & 0x1000000)
	    h ^= CRCPOLY;
	for (j = 0; j < i; j++)
	    table[i+j] = table[j] ^ h;
    }
}

int main(int argc, char *argv[])
{
    int i;

    crc_init(table);

    for (i = 0; i < 256; i++) {
	printf("0x%08X, ", table[i]);
	if ((i % 4) == 3)
	    putchar('\n');
    }

    exit(0);
}
#endif

static const unsigned crc24q[256] = {
    0x00000000, 0x01864CFB, 0x028AD50D, 0x030C99F6, 
    0x0493E6E1, 0x0515AA1A, 0x061933EC, 0x079F7F17, 
    0x08A18139, 0x0927CDC2, 0x0A2B5434, 0x0BAD18CF, 
    0x0C3267D8, 0x0DB42B23, 0x0EB8B2D5, 0x0F3EFE2E, 
    0x10C54E89, 0x11430272, 0x124F9B84, 0x13C9D77F, 
    0x1456A868, 0x15D0E493, 0x16DC7D65, 0x175A319E, 
    0x1864CFB0, 0x19E2834B, 0x1AEE1ABD, 0x1B685646, 
    0x1CF72951, 0x1D7165AA, 0x1E7DFC5C, 0x1FFBB0A7, 
    0x200CD1E9, 0x218A9D12, 0x228604E4, 0x2300481F, 
    0x249F3708, 0x25197BF3, 0x2615E205, 0x2793AEFE, 
    0x28AD50D0, 0x292B1C2B, 0x2A2785DD, 0x2BA1C926, 
    0x2C3EB631, 0x2DB8FACA, 0x2EB4633C, 0x2F322FC7, 
    0x30C99F60, 0x314FD39B, 0x32434A6D, 0x33C50696, 
    0x345A7981, 0x35DC357A, 0x36D0AC8C, 0x3756E077, 
    0x38681E59, 0x39EE52A2, 0x3AE2CB54, 0x3B6487AF, 
    0x3CFBF8B8, 0x3D7DB443, 0x3E712DB5, 0x3FF7614E, 
    0x4019A3D2, 0x419FEF29, 0x429376DF, 0x43153A24, 
    0x448A4533, 0x450C09C8, 0x4600903E, 0x4786DCC5, 
    0x48B822EB, 0x493E6E10, 0x4A32F7E6, 0x4BB4BB1D, 
    0x4C2BC40A, 0x4DAD88F1, 0x4EA11107, 0x4F275DFC, 
    0x50DCED5B, 0x515AA1A0, 0x52563856, 0x53D074AD, 
    0x544F0BBA, 0x55C94741, 0x56C5DEB7, 0x5743924C, 
    0x587D6C62, 0x59FB2099, 0x5AF7B96F, 0x5B71F594, 
    0x5CEE8A83, 0x5D68C678, 0x5E645F8E, 0x5FE21375, 
    0x6015723B, 0x61933EC0, 0x629FA736, 0x6319EBCD, 
    0x648694DA, 0x6500D821, 0x660C41D7, 0x678A0D2C, 
    0x68B4F302, 0x6932BFF9, 0x6A3E260F, 0x6BB86AF4, 
    0x6C2715E3, 0x6DA15918, 0x6EADC0EE, 0x6F2B8C15, 
    0x70D03CB2, 0x71567049, 0x725AE9BF, 0x73DCA544, 
    0x7443DA53, 0x75C596A8, 0x76C90F5E, 0x774F43A5, 
    0x7871BD8B, 0x79F7F170, 0x7AFB6886, 0x7B7D247D, 
    0x7CE25B6A, 0x7D641791, 0x7E688E67, 0x7FEEC29C, 
    0x803347A4, 0x81B50B5F, 0x82B992A9, 0x833FDE52, 
    0x84A0A145, 0x8526EDBE, 0x862A7448, 0x87AC38B3, 
    0x8892C69D, 0x89148A66, 0x8A181390, 0x8B9E5F6B, 
    0x8C01207C, 0x8D876C87, 0x8E8BF571, 0x8F0DB98A, 
    0x90F6092D, 0x917045D6, 0x927CDC20, 0x93FA90DB, 
    0x9465EFCC, 0x95E3A337, 0x96EF3AC1, 0x9769763A, 
    0x98578814, 0x99D1C4EF, 0x9ADD5D19, 0x9B5B11E2, 
    0x9CC46EF5, 0x9D42220E, 0x9E4EBBF8, 0x9FC8F703, 
    0xA03F964D, 0xA1B9DAB6, 0xA2B54340, 0xA3330FBB, 
    0xA4AC70AC, 0xA52A3C57, 0xA626A5A1, 0xA7A0E95A, 
    0xA89E1774, 0xA9185B8F, 0xAA14C279, 0xAB928E82, 
    0xAC0DF195, 0xAD8BBD6E, 0xAE872498, 0xAF016863, 
    0xB0FAD8C4, 0xB17C943F, 0xB2700DC9, 0xB3F64132, 
    0xB4693E25, 0xB5EF72DE, 0xB6E3EB28, 0xB765A7D3, 
    0xB85B59FD, 0xB9DD1506, 0xBAD18CF0, 0xBB57C00B, 
    0xBCC8BF1C, 0xBD4EF3E7, 0xBE426A11, 0xBFC426EA, 
    0xC02AE476, 0xC1ACA88D, 0xC2A0317B, 0xC3267D80, 
    0xC4B90297, 0xC53F4E6C, 0xC633D79A, 0xC7B59B61, 
    0xC88B654F, 0xC90D29B4, 0xCA01B042, 0xCB87FCB9, 
    0xCC1883AE, 0xCD9ECF55, 0xCE9256A3, 0xCF141A58, 
    0xD0EFAAFF, 0xD169E604, 0xD2657FF2, 0xD3E33309, 
    0xD47C4C1E, 0xD5FA00E5, 0xD6F69913, 0xD770D5E8, 
    0xD84E2BC6, 0xD9C8673D, 0xDAC4FECB, 0xDB42B230, 
    0xDCDDCD27, 0xDD5B81DC, 0xDE57182A, 0xDFD154D1, 
    0xE026359F, 0xE1A07964, 0xE2ACE092, 0xE32AAC69, 
    0xE4B5D37E, 0xE5339F85, 0xE63F0673, 0xE7B94A88, 
    0xE887B4A6, 0xE901F85D, 0xEA0D61AB, 0xEB8B2D50, 
    0xEC145247, 0xED921EBC, 0xEE9E874A, 0xEF18CBB1, 
    0xF0E37B16, 0xF16537ED, 0xF269AE1B, 0xF3EFE2E0, 
    0xF4709DF7, 0xF5F6D10C, 0xF6FA48FA, 0xF77C0401, 
    0xF842FA2F, 0xF9C4B6D4, 0xFAC82F22, 0xFB4E63D9, 
    0xFCD11CCE, 0xFD575035, 0xFE5BC9C3, 0xFFDD8538, 
};

unsigned crc24q_hash(unsigned char *data, int len)
{
    int i;
    unsigned crc = 0;

    for (i = 0; i < len; i++) {
	crc = (crc << 8) ^ crc24q[data[i] ^ (unsigned char)(crc>>16)];
    }

    crc = (crc & 0x00ffffff);

    return crc;
}

#define LO(x)	(unsigned char)((x) & 0xff)
#define MID(x)	(unsigned char)(((x) >> 8) & 0xff)
#define HI(x)	(unsigned char)(((x) >> 16) & 0xff)

void crc24q_sign(unsigned char *data, int len)
{
    unsigned crc = crc24q_hash(data, len);

    data[len]   = HI(crc);
    data[len+1] = MID(crc);
    data[len+2] = LO(crc);
}

bool crc24q_check(unsigned char *data, int len)
{
    unsigned crc = crc24q_hash(data, len-3);

    return (((data[len-3] == HI(crc)) &&
	     (data[len-2] == MID(crc)) &&
	     (data[len-1] == LO(crc))));
}