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/*
* This is an implementation of the CRC-24Q cyclic redundancy checksum
* used by Qualcomm, Nokia, and RTCM104V3. According to the RTCM104V3
* standard,
*
* 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 implementation by Eric Steven Raymond is Copyright (c) 2008
* for the GPSD project, and is released under BSD terms.
*/
#include <stdbool.h>
#include "crc24q.h"
static const unsigned crc24q[256] = {
0x00000000, 0x00D6A776, 0x00F64557, 0x0020E221, 0x00B78115,
0x00612663, 0x0041C442, 0x00976334, 0x00340991, 0x00E2AEE7,
0x00C24CC6, 0x0014EBB0, 0x00838884, 0x00552FF2, 0x0075CDD3,
0x00A36AA5, 0x00681322, 0x00BEB454, 0x009E5675, 0x0048F103,
0x00DF9237, 0x00093541, 0x0029D760, 0x00FF7016, 0x005C1AB3,
0x008ABDC5, 0x00AA5FE4, 0x007CF892, 0x00EB9BA6, 0x003D3CD0,
0x001DDEF1, 0x00CB7987, 0x00D02644, 0x00068132, 0x00266313,
0x00F0C465, 0x0067A751, 0x00B10027, 0x0091E206, 0x00474570,
0x00E42FD5, 0x003288A3, 0x00126A82, 0x00C4CDF4, 0x0053AEC0,
0x008509B6, 0x00A5EB97, 0x00734CE1, 0x00B83566, 0x006E9210,
0x004E7031, 0x0098D747, 0x000FB473, 0x00D91305, 0x00F9F124,
0x002F5652, 0x008C3CF7, 0x005A9B81, 0x007A79A0, 0x00ACDED6,
0x003BBDE2, 0x00ED1A94, 0x00CDF8B5, 0x001B5FC3, 0x00FB4733,
0x002DE045, 0x000D0264, 0x00DBA512, 0x004CC626, 0x009A6150,
0x00BA8371, 0x006C2407, 0x00CF4EA2, 0x0019E9D4, 0x00390BF5,
0x00EFAC83, 0x0078CFB7, 0x00AE68C1, 0x008E8AE0, 0x00582D96,
0x00935411, 0x0045F367, 0x00651146, 0x00B3B630, 0x0024D504,
0x00F27272, 0x00D29053, 0x00043725, 0x00A75D80, 0x0071FAF6,
0x005118D7, 0x0087BFA1, 0x0010DC95, 0x00C67BE3, 0x00E699C2,
0x00303EB4, 0x002B6177, 0x00FDC601, 0x00DD2420, 0x000B8356,
0x009CE062, 0x004A4714, 0x006AA535, 0x00BC0243, 0x001F68E6,
0x00C9CF90, 0x00E92DB1, 0x003F8AC7, 0x00A8E9F3, 0x007E4E85,
0x005EACA4, 0x00880BD2, 0x00437255, 0x0095D523, 0x00B53702,
0x00639074, 0x00F4F340, 0x00225436, 0x0002B617, 0x00D41161,
0x00777BC4, 0x00A1DCB2, 0x00813E93, 0x005799E5, 0x00C0FAD1,
0x00165DA7, 0x0036BF86, 0x00E018F0, 0x00AD85DD, 0x007B22AB,
0x005BC08A, 0x008D67FC, 0x001A04C8, 0x00CCA3BE, 0x00EC419F,
0x003AE6E9, 0x00998C4C, 0x004F2B3A, 0x006FC91B, 0x00B96E6D,
0x002E0D59, 0x00F8AA2F, 0x00D8480E, 0x000EEF78, 0x00C596FF,
0x00133189, 0x0033D3A8, 0x00E574DE, 0x007217EA, 0x00A4B09C,
0x008452BD, 0x0052F5CB, 0x00F19F6E, 0x00273818, 0x0007DA39,
0x00D17D4F, 0x00461E7B, 0x0090B90D, 0x00B05B2C, 0x0066FC5A,
0x007DA399, 0x00AB04EF, 0x008BE6CE, 0x005D41B8, 0x00CA228C,
0x001C85FA, 0x003C67DB, 0x00EAC0AD, 0x0049AA08, 0x009F0D7E,
0x00BFEF5F, 0x00694829, 0x00FE2B1D, 0x00288C6B, 0x00086E4A,
0x00DEC93C, 0x0015B0BB, 0x00C317CD, 0x00E3F5EC, 0x0035529A,
0x00A231AE, 0x007496D8, 0x005474F9, 0x0082D38F, 0x0021B92A,
0x00F71E5C, 0x00D7FC7D, 0x00015B0B, 0x0096383F, 0x00409F49,
0x00607D68, 0x00B6DA1E, 0x0056C2EE, 0x00806598, 0x00A087B9,
0x007620CF, 0x00E143FB, 0x0037E48D, 0x001706AC, 0x00C1A1DA,
0x0062CB7F, 0x00B46C09, 0x00948E28, 0x0042295E, 0x00D54A6A,
0x0003ED1C, 0x00230F3D, 0x00F5A84B, 0x003ED1CC, 0x00E876BA,
0x00C8949B, 0x001E33ED, 0x008950D9, 0x005FF7AF, 0x007F158E,
0x00A9B2F8, 0x000AD85D, 0x00DC7F2B, 0x00FC9D0A, 0x002A3A7C,
0x00BD5948, 0x006BFE3E, 0x004B1C1F, 0x009DBB69, 0x0086E4AA,
0x005043DC, 0x0070A1FD, 0x00A6068B, 0x003165BF, 0x00E7C2C9,
0x00C720E8, 0x0011879E, 0x00B2ED3B, 0x00644A4D, 0x0044A86C,
0x00920F1A, 0x00056C2E, 0x00D3CB58, 0x00F32979, 0x00258E0F,
0x00EEF788, 0x003850FE, 0x0018B2DF, 0x00CE15A9, 0x0059769D,
0x008FD1EB, 0x00AF33CA, 0x007994BC, 0x00DAFE19, 0x000C596F,
0x002CBB4E, 0x00FA1C38, 0x006D7F0C, 0x00BBD87A, 0x009B3A5B,
0x004D9D2D
};
unsigned crc24q_hash(unsigned char *data, int len)
{
int i;
unsigned crc = 0xffffff;
for (i = 0; i < len; i++) {
crc = (crc << 8) ^ crc24q[data[i] ^ (unsigned char)(crc>>16)];
}
crc = (crc & 0x00ffffff);
return crc;
}
#define LO(x) ((x) & 0x0000ffff)
#define MID(x) (((x) >> 8) & 0x0000ffff)
#define HI(x) (((x) >> 16) & 0x0000ffff)
void crc24q_sign(unsigned char *data, int len)
{
unsigned crc = crc24q_hash(data, len);
data[len] = LO(crc);
data[len+1] = MID(crc);
data[len+2] = HI(crc);
}
bool crc24q_check(unsigned char *data, int len)
{
unsigned crc = crc24q_hash(data, len);
return (((data[len-3] == LO(crc)) &&
(data[len-2] == MID(crc)) &&
(data[len-1] == HI(crc))));
}
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