学步园

/*
 * A8h reflected is 15h, i.e. 10101000 <--> 00010101
*/
int reflect(int data, int len)
{
	int ref = 0;

	for (int i = 0; i < len; i++) {
		if (data & 0x1) {
			ref |= (1 << ((len - 1) - i));
		}
		data = (data >> 1);
	}

	return ref;
}

/*
* Function to calculate the CRC32
*/
unsigned int calculate_crc32(unsigned char *buffer, int len)
{
	int byte_length = 8;	/*length of unit (i.e. byte) */
	int msb = 0;
	int polynomial = 0x104C11DB7;	/* IEEE 32bit polynomial */
	unsigned int regs = 0xFFFFFFFF;	/* init to all ones */
	int regs_mask = 0xFFFFFFFF;	/* ensure only 32 bit answer */
	int regs_msb = 0;
	unsigned int reflected_regs;

	for (int i = 0; i < len; i++) {
		int data_byte = buffer[i];
		data_byte = reflect(data_byte, 8);
		for (int j = 0; j < byte_length; j++) {
			msb = data_byte >> (byte_length - 1);	/* get MSB */
			msb &= 1;	/* ensure just 1 bit */
			regs_msb = (regs >> 31) & 1;	/* MSB of regs */
			regs = regs << 1;	/* shift regs for CRC-CCITT */
			if (regs_msb ^ msb) {	/* MSB is a 1 */
				regs = regs ^ polynomial;	/* XOR with generator poly */
			}
			regs = regs & regs_mask;	/* Mask off excess upper bits */
			data_byte <<= 1;	/* get to next bit */
		}
	}
	regs = regs & regs_mask;
	reflected_regs = reflect(regs, 32) ^ 0xFFFFFFFF;

	return reflected_regs;
}