How do parity files work.?

Question

I understand how one parity file can fix any one error, but how does parity work when there are multiple pieces missing, and you have mulitple par files. What is the math behind it?

Par Files that are usually used with newsgroups, can fix any one file. I understand how one parity can fix one missing piece, but what I dont understand is how 2 PAR files can fix 2 missing pieces etc. Does anyone know the algorithmn that is used?

Answer 1

I'm not sure what you mean by a parity file, but I can tell you how parity is calculated. Let's say you have some data you want to transmit or store. To use parity, you divide the data into small blocks (most commonly 8 bits long) and designate one bit as a parity bit. The parity bit is usually one way (like 0) if the number of 1's in the block is even, and the other way (in this case 1) if the number of 1's is odd. It is so unlikely that an error will appear on two bits that it almost guarantees that you will know that there is an error. I imagine that similar algorithms for parity checking may be created for more complex data.

I'm not sure, but I guess that the parity you describe works in a similar way and the appropriate programs analyze the data in context to determine the best corrections for the errors it finds.

UPDATE: Here's what you're looking for on wikipedia:
http://en.wikipedia.org/wiki/Parchive
At the bottom there are links to resources that describe the full specifications of the algorithm.