NAME

Algorithm::FEC - Forward Error Correction using Vandermonde Matrices

SYNOPSIS

use Algorithm::FEC;

DESCRIPTION

This module is an interface to the fec library by Luigi Rizzo et al., see the file README.fec in the distribution for more details.

This library implements a simple (encoded_blocks,data_blocks) erasure code based on Vandermonde matrices. The encoder takes data_blocks blocks of size block_size each, and is able to produce up to encoded_blocks different encoded blocks, numbered from 0 to encoded_blocks-1, such that any subset of data_blocks members permits reconstruction of the original data.

Allowed values for data_blocks and encoded_blocks must obey the following equation:

data_blocks <= encoded_blocks <= MAXBLOCKS

Where MAXBLOCKS=256 for the fast implementation and MAXBLOCKS=65536 for the slow implementation (the implementation is chosen automatically).

$fec = new Algorithm::FEC $data_blocks, $encoded_blocks, $blocksize

Creates a new Algorithm::FEC object with the given parameters.

$fec->set_encode_blocks ([array_of_blocks])

Sets the data blocks used for the encoding. Each member of the array can either be:

• a string of size blocksize exactly.

  This is useful for small files (encoding entirely in memory).

• a filehandle of a file of size blocksize exactly.

  This is useful when the amount of data is large and resides in single files.

• a reference to an array containing a filehandle and, optionally, an offset into that file.

  This is useful if the amount of data is large and resides in a single file. Needless to say, all parts must not overlap and must fit into the file.

If your data is not of the required size (i.e. a multiple of blocksize bytes), then you must pad it (e.g. with zero bytes) on encoding (and you should truncate it after decoding). Otherwise, this library croaks.

Future versions might instead load the short segment into memory or extend your scalar (this might enable nice tricks, like $fec-copy (..., my $x)> :). Mail me if you want this to happen.

If called without arguments, the internal storage associated with the blocks is freed again.

$block = $fec->encode ($block_index)

Creates a single encoded block of index block_index, which must be between 0 and encoded_blocks-1 (inclusive). The blocks from 0 to data_blocks-1 are simply copies of the original data blocks.

The encoded block is returned as a perl scalar (so the blocks should fit into memory. If this is a problem for you mail me and I'll make it a file.

$fec->set_decode_blocks ([array_of_blocks], [array_of_indices])

Prepares to decode data_blocks of blocks (see set_encode_blocks for the array_of_blocks parameter).

Since these are not usually the original data blocks, an array of indices (ranging from 0 to encoded_blocks-1) must be supplied as the second arrayref.

Both arrays must have exactly data_blocks entries.

This method also reorders the blocks and index array in place (if necessary) to reflect the order the blocks will have in the decoded result.

The index array represents the decoded ordering, in that the n-th entry in the indices array corresponds to the n-th data block of the decoded result. The value stored in the n-th place in the array will contain the index of the encoded data block.

Input blocks with indices less than data_blocks will be moved to their final position (block k to position k), while the gaps between them will be filled with check blocks. The decoding process will not modify the already decoded data blocks, but will modify the check blocks.

That is, if you call this function with indices = [4,3,1], with data_blocks = 3, then this array will be returned: [0,2,1]. This means that input block 0 corresponds to file block 0, input block 1 to file block 2 and input block 2 to data block 1.

You can just iterate over this array and write out the corresponding data block (although this is inefficient):

for my $i (0 .. $#idx)
   if ($idx[$i] != $i) # need we move this block?
      copy encoded block $idx[$i] to position $i
   }
}

The copy method can be helpful here.

This method destroys the block array as set up by set_encode_blocks.

$fec->shuffle ([array_of_blocks], [array_of_indices])

The same same as set_decode_blocks, with the exception that the blocks are not actually set for decoding.

This method is not normally used, but if you want to move blocks around after reordering and before decoding, then calling shuffle followed by set_decode_blocks incurs lower overhead than calling set_decode_blocks twice, as files are not mmapped etc.

$fec->decode

Decode the blocks set by a prior call to set_decode_blocks.

This method destroys the block array as set up by set_decode_blocks.

$fec->copy ($srcblock, $dstblock)

Utility function that simply copies one block (specified like in set_encode_blocks) into another. This, btw., destroys the blocks set by set_*_blocks.

COMPATIBILITY

The way this module works is compatible with the way freenet (http://freenet.sf.net) encodes files. Comaptibility to other file formats or networks is not known, please tell me if you find more examples.

SEE ALSO

Net::FCP. And the author, who might be happy to receive mail from any user, just to see that this rather rarely-used module is actually being used (except for freenet ;)

BUGS

* too complicated.
* largely untested, please change this.
* file descriptors are not supported, but should be.
* utility functions for files should be provided.
* 16 bit version not tested

AUTHOR

Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de