| rjw | 1f88458 | 2022-01-06 17:20:42 +0800 | [diff] [blame^] | 1 | /* |
| 2 | * Computes the CRC for transmitted and received data using |
| 3 | * the CCITT 16bit algorithm (X^16 + X^12 + X^5 + 1) with |
| 4 | * a 0xFFFF initialization vector. |
| 5 | */ |
| 6 | |
| 7 | #define CRC16_INIT_VALUE 0xFFFF |
| 8 | |
| 9 | /* |
| 10 | * Computes an updated version of the CRC from existing CRC. |
| 11 | * crc: the previous values of the CRC |
| 12 | * buf: the data on which to apply the checksum |
| 13 | * length: the number of bytes of data in 'buf' to be calculated. |
| 14 | */ |
| 15 | unsigned short update_crc16(unsigned short crc, const unsigned char *buf, |
| 16 | unsigned int length) |
| 17 | { |
| 18 | unsigned int i; |
| 19 | for (i = 0; i < length; i++) { |
| 20 | crc = (unsigned char) (crc >> 8) | (crc << 8); |
| 21 | crc ^= buf[i]; |
| 22 | crc ^= (unsigned char) (crc & 0xff) >> 4; |
| 23 | crc ^= (crc << 8) << 4; |
| 24 | crc ^= ((crc & 0xff) << 4) << 1; |
| 25 | } |
| 26 | return crc; |
| 27 | } |
| 28 | |
| 29 | /* |
| 30 | * Computes a CRC, starting with an initialization value. |
| 31 | * buf: the data on which to apply the checksum |
| 32 | * length: the number of bytes of data in 'buf' to be calculated. |
| 33 | */ |
| 34 | unsigned short crc16(const unsigned char *buf, unsigned int length) |
| 35 | { |
| 36 | unsigned short crc = CRC16_INIT_VALUE; |
| 37 | return update_crc16(crc, buf, length); |
| 38 | } |