#include <stdint.h>


uint16_t crc16_ccitt(char* data, unsigned int data_len) {
    uint16_t crc = 0xFFFF;

    if (data_len == 0)
        return 0;

    for (unsigned int i = 0; i < data_len; ++i) {
		uint16_t dbyte = data[i];
        crc ^= dbyte << 8;
		
		for (unsigned char j = 0; j < 8; ++j) {
			uint16_t mix = crc & 0x8000;
			crc = (crc << 1);
			if (mix)
				crc = crc ^ 0x1021;
		}
	}

    return crc;
}