Despite the terse beauty of the line


//crc算法
r=0;
while (len--)
r = ((r << 8) ¦ *++) ^ t[(r >> 24) & 0xFF];

those optimizing hackers couldn't leave it alone. The trouble, you see, is that this loop operates upon the AUGMENTED message and in order to use this code, you have to append W/8 zero bytes to the end of the message before pointing p at it. Depending on the run-time environment, this may or may not be a problem; if the block of data was handed to us by some other code, it could be a BIG problem. One alternative is simply to append the following line after the above loop, once for each zero byte:


//为什么是w/4不是w/8呢？
for (i=0; i<W/4; i++)
r = (r << 8) ^ t[(r >> 24) & 0xFF];

This looks like a sane enough solution to me. However, at the further expense of clarity (which, you must admit, is already a pretty scare commodity in this code) we can reorganize this small loop further so as to avoid the need to either augment the message with zero bytes, or to explicitly process zero bytes at the end as above. To explain the optimization, we return to the processing diagram given earlier.



3 2 1 0 Bytes
+----+----+----+----+
+-----<¦ ¦ ¦ ¦ ¦ <----- Augmented message
¦ +----+----+----+----+
¦ ^
¦ ¦
¦ XOR
¦ ¦
¦ 0+----+----+----+----+
v +----+----+----+----+
¦ +----+----+----+----+
¦ +----+----+----+----+
¦ +----+----+----+----+
¦ +----+----+----+----+
¦ +----+----+----+----+
+----->+----+----+----+----+
+----+----+----+----+
+----+----+----+----+
+----+----+----+----+
+----+----+----+----+
255+----+----+----+----+

Algorithm


1Shift the register left by one byte, reading in a new message byte.
2Use the top byte just rotated out of the register to index the table of 256 32-bit values.
3XOR the table value into the register.
4Goto 1 iff more augmented message bytes