png_crc.cpp

本源码是用于实现在PNG格式的图片中的CRC的算法。

/* Table of CRCs of all 8-bit messages. */
unsigned long crc_table[256];

/* Flag: has the table been computed? Initially false. */
int crc_table_computed = 0;

/* Make the table for a fast CRC. */
void make_crc_table(void)
{
	unsigned long c;
	int n, k;
	for (n=0; n<256; n++) 
	{
		c = (unsigned long) n;
		for (k=0; k<8; k++) 
		{
			if (c & 1)
				c = 0xedb88320L ^ (c >> 1);
			else
				c = c >> 1;
		}
		crc_table[n] = c;
	}
	crc_table_computed = 1;
}

/* Update a running CRC with the bytes buf[0..len-1]--the CRC
should be initialized to all 1's, and the transmitted value
is the 1's complement of the final running CRC (see the
crc() routine below)). */
   
unsigned long update_crc(unsigned long crc, unsigned char *buf,
                        int len)
{
	unsigned long c = crc;
	int n;
	if (!crc_table_computed)
		make_crc_table(); //如果crc表还没有生成则生成
	
	for (n=0; n<len; n++) 
	{
		c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
	}
	return c;
}

/* Return the CRC of the bytes buf[0..len-1]. */
unsigned long crc(unsigned char *buf, int len)
{
 return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
}

//要CRC的数据
unsigned char data[43] = {
	0x50, 0x4C, 0x54, 0x45, 0xFF, 0xFF, 0x00, 0xFF, 0xED, 0x00, 0xFF, 0xC1, 0x00, 0xFF, 0x99, 0x00, 
	0xFF, 0x66, 0x00, 0xFF, 0x3B, 0x00, 0xFF, 0x0F, 0x00, 0xE2, 0x00, 0x15, 0xB7, 0x00, 0x34, 0x8B, 
	0x00, 0x54, 0x60, 0x00, 0x73, 0x33, 0x00, 0x99, 0x09, 0x00, 0xB2
};


void main()
{
	//开始crc效验
	unsigned long  val = crc(data, sizeof(data));
}