Implementation of CRC Checksum

"For a long time no More blog:: >_<::"

Computer network class learned the basic principle of CRC School (Jiao) test, but the lesson of that little thing only enough to do the exam question = =, so want to try to write a file CRC check the small program

First, review the most basic principles of CRC calibration:

Core idea: In fact, and hash very much like ... It's mod.

Set M is the source file, G is the number of checks used (specified by the CRC standard)

First, m passes the checksum to N to satisfy the N mod g==0 (where mod is the modulo two not carry division of the textbook)

Then send n over the network and receive the packet Q in destination

Destination after receiving also check: if the Q mod g==0, the documentation is correct, otherwise the error

Of course, this method will also exist in the case of missed (such as n=q+k*g when it was accepted, but in fact is wrong).

Choosing the right G can reduce the missed rate (just like when a hash is used to select a large prime number).

The basic CRC algorithm is this:

Set G with R bit

STEP1: r-1 0 at the end of M, becomes XRM

STEP2: Modulo two non-carry division (in fact, the vertical in addition to the Mrs. Fremont to replace the XOR)

[XrM] mod [G], take remainder remainder

STEP3: Make N=xrm-remainder,

The resulting n is a well-calibrated packet. Send it straight out.

In the process above, remainder is the CRC check code

Where modulo two does not carry a detailed process of division:

Program implementation:

Current bit =15→0 in for data
The highest bit of tr=register
Register shift left One
Inserts the current bit in data into the lowest bit of the register
If TR is 1:
Register=register XOR Poly

1 #Load The register with zero bits.2 #Augment the message by appending W zero bits to the end of it.3 #While (more message bits)4 #Begin5 #Shift The register left by one bit, reading the next bit of the6 #augmented message into register bit position 0.7 #If (a 1 bit popped out of the register during step 3)8 #Register = Register XOR Poly.9 #EndTen #The register now contains the remainder. One  Adata=0x35b -Poly=0x13 -Dbit=16 thePbit=5 -regi=0x0 -  -data=data<< (pbit-1) +Dpoint=dbit-1 -  while(dpoint>=0): +Tr= (regi>> (pbit-2)) &0x1#MSB in Register ARegi=regi<<1 attmp= (Data>>dpoint) &0x1 -regi+=tmp -     if(tr==1): -regi=regi^Poly -Dpoint=dpoint-1 -  in Print(Regi)

View Code

The above method is the most bare method ... A bit at a bit to handle.

But if you do it in practice, it's too slow. =

Implementation of CRC Checksum