crc16.c

#include <stdio.h>
#include <string.h>
#include <stdint.h>

static uint16_t *crc16_table()
{
  static uint16_t crc_table[256];
  static _Bool generate_table = 1;
  if (generate_table) {
    const uint16_t p = 0xA001;
    for (int i = 0; i < 256; ++i) {
      uint16_t crc = 0;
      uint16_t c = i;
      for (int j = 0; j < 8; ++j) {
        crc = (crc >> 1) ^ (((crc ^ c) & 1) ? p : 0);
        c >>= 1;
      }
      crc_table[i] = crc;
    }
    generate_table = 0;
  }
  return crc_table;
}

uint16_t crc16(void *data, size_t size)
{
  uint16_t *crc_table = crc16_table();
  uint8_t *data_u8 = data;
  uint16_t crc = 0x0000;
  for (size_t i = 0; i < size; ++i)
    crc = (crc >> 8) ^ crc_table[(crc ^ data_u8[i]) & 0x00FF];
  return crc;
}

int main()
{
  char s[256];
  gets(s);
  printf("0x%04X\n", crc16(s, strlen(s)));
}