OK, after some more trial and error, it seems the magic parameters are width=4 poly=0x3 init=0x0 refin=false refout=false xorout=0x0 , which matches nothing in the CRC catalog. This seems to work: // https://github.com/madler/crcany // width=4 poly=0x3 init=0x0 refin=false refout=false xorout=0x0 uint8_t crc4x_bit(uint8_t crc, void const *mem, size_t len) { unsigned char const *data = mem; if (data == NULL) return 0; crc >= 4; return crc; } static uint8_t const table_byte[256] = { 0x00, 0x30, 0x60, 0x50, 0xc0, 0xf0, 0xa0, 0x90, 0xb0, 0x80, 0xd0, 0xe0, 0x70, 0x40, 0x10, 0x20, 0x50, 0x60, 0x30, 0x00, 0x90, 0xa0, 0xf0, 0xc0, 0xe0, 0xd0, 0x80, 0xb0, 0x20, 0x10, 0x40, 0x70, 0xa0, 0x90, 0xc0, 0xf0, 0x60, 0x50, 0x00, 0x30, 0x10, 0x20, 0x70, 0x40, 0xd0, 0xe0, 0xb0, 0x80, 0xf0, 0xc0, 0x90, 0xa0, 0x30, 0x00, 0x50, 0x60, 0x40, 0x70, 0x20, 0x10, 0x80, 0xb0, 0xe0, 0xd0, 0x70, 0x40, 0x10, 0x20, 0xb0, 0x80, 0xd0, 0xe0, 0xc0, 0xf0, 0xa0, 0x90, 0x00, 0x30, 0x60, 0x50, 0x20, 0x10, 0x40, 0x70, 0xe0, 0xd0, 0x80, 0xb0, 0x90, 0xa0, 0xf0, 0xc0, 0x50, 0x60, 0x30, 0x00, 0xd0, 0xe0, 0xb0, 0x80, 0x10, 0x20, 0x70, 0x40, 0x60, 0x50, 0x00, 0x30, 0xa0, 0x90, 0xc0, 0xf0, 0x80, 0xb0, 0xe0, 0xd0, 0x40, 0x70, 0x20, 0x10, 0x30, 0x00, 0x50, 0x60, 0xf0, 0xc0, 0x90, 0xa0, 0xe0, 0xd0, 0x80, 0xb0, 0x20, 0x10, 0x40, 0x70, 0x50, 0x60, 0x30, 0x00, 0x90, 0xa0, 0xf0, 0xc0, 0xb0, 0x80, 0xd0, 0xe0, 0x70, 0x40, 0x10, 0x20, 0x00, 0x30, 0x60, 0x50, 0xc0, 0xf0, 0xa0, 0x90, 0x40, 0x70, 0x20, 0x10, 0x80, 0xb0, 0xe0, 0xd0, 0xf0, 0xc0, 0x90, 0xa0, 0x30, 0x00, 0x50, 0x60, 0x10, 0x20, 0x70, 0x40, 0xd0, 0xe0, 0xb0, 0x80, 0xa0, 0x90, 0xc0, 0xf0, 0x60, 0x50, 0x00, 0x30, 0x90, 0xa0, 0xf0, 0xc0, 0x50, 0x60, 0x30, 0x00, 0x20, 0x10, 0x40, 0x70, 0xe0, 0xd0, 0x80, 0xb0, 0xc0, 0xf0, 0xa0, 0x90, 0x00, 0x30, 0x60, 0x50, 0x70, 0x40, 0x10, 0x20, 0xb0, 0x80, 0xd0, 0xe0, 0x30, 0x00, 0x50, 0x60, 0xf0, 0xc0, 0x90, 0xa0, 0x80, 0xb0, 0xe0, 0xd0, 0x40, 0x70, 0x20, 0x10, 0x60, 0x50, 0x00, 0x30, 0xa0, 0x90, 0xc0, 0xf0, 0xd0, 0xe0, 0xb0, 0x80, 0x10, 0x20, 0x70, 0x40 }; __attribute__((optimize("O3"))) uint8_t crc4x_byte(uint8_t crc, void const *mem, size_t len) { unsigned char const *data = mem; if (data == NULL) return 0; crc >= 4; return crc; }
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