/*
 * 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) 2008-2018 by the GPSD project
 * SPDX-License-Identifier: BSD-2-clause
 */
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

#include "crc24q.h"

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

#define CRCSEED	0		/* could be NZ to detect leading zeros */
#define CRCPOLY	0x1864CFBu	/* encodes all info about the polynomial */

static void crc_init(unsigned int 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(EXIT_SUCCESS);
}
#endif

static const int unsigned crc24q[256] = {
    0x00000000u, 0x01864CFBu, 0x028AD50Du, 0x030C99F6u,
    0x0493E6E1u, 0x0515AA1Au, 0x061933ECu, 0x079F7F17u,
    0x08A18139u, 0x0927CDC2u, 0x0A2B5434u, 0x0BAD18CFu,
    0x0C3267D8u, 0x0DB42B23u, 0x0EB8B2D5u, 0x0F3EFE2Eu,
    0x10C54E89u, 0x11430272u, 0x124F9B84u, 0x13C9D77Fu,
    0x1456A868u, 0x15D0E493u, 0x16DC7D65u, 0x175A319Eu,
    0x1864CFB0u, 0x19E2834Bu, 0x1AEE1ABDu, 0x1B685646u,
    0x1CF72951u, 0x1D7165AAu, 0x1E7DFC5Cu, 0x1FFBB0A7u,
    0x200CD1E9u, 0x218A9D12u, 0x228604E4u, 0x2300481Fu,
    0x249F3708u, 0x25197BF3u, 0x2615E205u, 0x2793AEFEu,
    0x28AD50D0u, 0x292B1C2Bu, 0x2A2785DDu, 0x2BA1C926u,
    0x2C3EB631u, 0x2DB8FACAu, 0x2EB4633Cu, 0x2F322FC7u,
    0x30C99F60u, 0x314FD39Bu, 0x32434A6Du, 0x33C50696u,
    0x345A7981u, 0x35DC357Au, 0x36D0AC8Cu, 0x3756E077u,
    0x38681E59u, 0x39EE52A2u, 0x3AE2CB54u, 0x3B6487AFu,
    0x3CFBF8B8u, 0x3D7DB443u, 0x3E712DB5u, 0x3FF7614Eu,
    0x4019A3D2u, 0x419FEF29u, 0x429376DFu, 0x43153A24u,
    0x448A4533u, 0x450C09C8u, 0x4600903Eu, 0x4786DCC5u,
    0x48B822EBu, 0x493E6E10u, 0x4A32F7E6u, 0x4BB4BB1Du,
    0x4C2BC40Au, 0x4DAD88F1u, 0x4EA11107u, 0x4F275DFCu,
    0x50DCED5Bu, 0x515AA1A0u, 0x52563856u, 0x53D074ADu,
    0x544F0BBAu, 0x55C94741u, 0x56C5DEB7u, 0x5743924Cu,
    0x587D6C62u, 0x59FB2099u, 0x5AF7B96Fu, 0x5B71F594u,
    0x5CEE8A83u, 0x5D68C678u, 0x5E645F8Eu, 0x5FE21375u,
    0x6015723Bu, 0x61933EC0u, 0x629FA736u, 0x6319EBCDu,
    0x648694DAu, 0x6500D821u, 0x660C41D7u, 0x678A0D2Cu,
    0x68B4F302u, 0x6932BFF9u, 0x6A3E260Fu, 0x6BB86AF4u,
    0x6C2715E3u, 0x6DA15918u, 0x6EADC0EEu, 0x6F2B8C15u,
    0x70D03CB2u, 0x71567049u, 0x725AE9BFu, 0x73DCA544u,
    0x7443DA53u, 0x75C596A8u, 0x76C90F5Eu, 0x774F43A5u,
    0x7871BD8Bu, 0x79F7F170u, 0x7AFB6886u, 0x7B7D247Du,
    0x7CE25B6Au, 0x7D641791u, 0x7E688E67u, 0x7FEEC29Cu,
    0x803347A4u, 0x81B50B5Fu, 0x82B992A9u, 0x833FDE52u,
    0x84A0A145u, 0x8526EDBEu, 0x862A7448u, 0x87AC38B3u,
    0x8892C69Du, 0x89148A66u, 0x8A181390u, 0x8B9E5F6Bu,
    0x8C01207Cu, 0x8D876C87u, 0x8E8BF571u, 0x8F0DB98Au,
    0x90F6092Du, 0x917045D6u, 0x927CDC20u, 0x93FA90DBu,
    0x9465EFCCu, 0x95E3A337u, 0x96EF3AC1u, 0x9769763Au,
    0x98578814u, 0x99D1C4EFu, 0x9ADD5D19u, 0x9B5B11E2u,
    0x9CC46EF5u, 0x9D42220Eu, 0x9E4EBBF8u, 0x9FC8F703u,
    0xA03F964Du, 0xA1B9DAB6u, 0xA2B54340u, 0xA3330FBBu,
    0xA4AC70ACu, 0xA52A3C57u, 0xA626A5A1u, 0xA7A0E95Au,
    0xA89E1774u, 0xA9185B8Fu, 0xAA14C279u, 0xAB928E82u,
    0xAC0DF195u, 0xAD8BBD6Eu, 0xAE872498u, 0xAF016863u,
    0xB0FAD8C4u, 0xB17C943Fu, 0xB2700DC9u, 0xB3F64132u,
    0xB4693E25u, 0xB5EF72DEu, 0xB6E3EB28u, 0xB765A7D3u,
    0xB85B59FDu, 0xB9DD1506u, 0xBAD18CF0u, 0xBB57C00Bu,
    0xBCC8BF1Cu, 0xBD4EF3E7u, 0xBE426A11u, 0xBFC426EAu,
    0xC02AE476u, 0xC1ACA88Du, 0xC2A0317Bu, 0xC3267D80u,
    0xC4B90297u, 0xC53F4E6Cu, 0xC633D79Au, 0xC7B59B61u,
    0xC88B654Fu, 0xC90D29B4u, 0xCA01B042u, 0xCB87FCB9u,
    0xCC1883AEu, 0xCD9ECF55u, 0xCE9256A3u, 0xCF141A58u,
    0xD0EFAAFFu, 0xD169E604u, 0xD2657FF2u, 0xD3E33309u,
    0xD47C4C1Eu, 0xD5FA00E5u, 0xD6F69913u, 0xD770D5E8u,
    0xD84E2BC6u, 0xD9C8673Du, 0xDAC4FECBu, 0xDB42B230u,
    0xDCDDCD27u, 0xDD5B81DCu, 0xDE57182Au, 0xDFD154D1u,
    0xE026359Fu, 0xE1A07964u, 0xE2ACE092u, 0xE32AAC69u,
    0xE4B5D37Eu, 0xE5339F85u, 0xE63F0673u, 0xE7B94A88u,
    0xE887B4A6u, 0xE901F85Du, 0xEA0D61ABu, 0xEB8B2D50u,
    0xEC145247u, 0xED921EBCu, 0xEE9E874Au, 0xEF18CBB1u,
    0xF0E37B16u, 0xF16537EDu, 0xF269AE1Bu, 0xF3EFE2E0u,
    0xF4709DF7u, 0xF5F6D10Cu, 0xF6FA48FAu, 0xF77C0401u,
    0xF842FA2Fu, 0xF9C4B6D4u, 0xFAC82F22u, 0xFB4E63D9u,
    0xFCD11CCEu, 0xFD575035u, 0xFE5BC9C3u, 0xFFDD8538u,
};

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)

#ifdef __UNUSED__
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);
}
#endif /* __UNUSED__ */

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))));
}