Algorithm::FEC - Forward Error Correction using Vandermonde Matrices
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 (
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
encoded_blocks-1, such that any subset of
data_blocks members permits reconstruction of the original data.
Allowed values for
encoded_blocks must obey the following equation:
data_blocks <= encoded_blocks <= MAXBLOCKS
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
This is useful for small files (encoding entirely in memory).
a filehandle of a file of size
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
blocksizebytes), 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
encoded_blocks-1(inclusive). The blocks from
data_blocks-1are 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_blocksof blocks (see
Since these are not usually the original data blocks, an array of indices (ranging from
encoded_blocks-1) must be supplied as the second arrayref.
Both arrays must have exactly
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_blockswill 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
0corresponds to file block
0, input block
1to file block
2and input block
2to data block
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 } }
copymethod can be helpful here.
This method destroys the block array as set up by
- $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
set_decode_blocksincurs lower overhead than calling
set_decode_blockstwice, as files are not mmapped etc.
Decode the blocks set by a prior call to
This method destroys the block array as set up by
- $fec->copy ($srcblock, $dstblock)
Utility function that simply copies one block (specified like in
set_encode_blocks) into another. This, btw., destroys the blocks set by
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.
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 ;)
* 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
Marc Lehmann <email@example.com> http://home.schmorp.de