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static const uint8_t crc4_tab[] = {
0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
};
uint8_t crc4(uint8_t c, uint64_t x, int bits)
{
int i;
/* mask off anything above the top bit */
x &= (1ull << bits) - 1;
/* Align to 4-bits */
bits = (bits + 3) & ~0x3;
/* Calculate crc4 over four-bit nibbles, starting at the MSbit */
for (i = bits - 4; i >= 0; i -= 4)
c = crc4_tab[c ^ ((x >> i) & 0xf)];
return c;
}