CRC 在線校驗地址：https://www.lammertbies.nl/comm/info/crc-calculation.html

本文采用 16位的 CRC 校驗碼，并采用 CRC-CCITT 多項式。以下為源碼：

.h

@interface NSData (VJCRC16)

-(NSData*)crc16 ;

@end

.m

#import "NSData+VJCRC16.h"

#define PLOY 0X1021

@implementation NSData (VJCRC16)

- (NSData*)crc16 {
    const uint8_t *byte = (const uint8_t *)self.bytes;
    uint16_t length = (uint16_t)self.length;
    uint16_t res =  gen_crc16(byte, length);
    
    NSData *val = [NSData dataWithBytes:&res length:sizeof(res)];
    
    return val;
}

uint16_t gen_crc16(const uint8_t *data, uint16_t size) {
    uint16_t crc = 0;
    uint8_t i;
    for (; size > 0; size--) {
        crc = crc ^ (*data++ <<8);
        for (i = 0; i < 8; i++) {
            if (crc & 0X8000) {
                crc = (crc << 1) ^ PLOY;
            }else {
                crc <<= 1;
            }
        }
        crc &= 0XFFFF;
    }
    return crc;
}

@end