Algorithm-FEC/FEC.pm

=head1 NAME

Algorithm::FEC - Forward Error Correction using Vandermonde Matrices

=head1 SYNOPSIS

 use Algorithm::FEC;

=head1 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 (C<encoded_blocks>,C<data_blocks>)
erasure code based on Vandermonde matrices.  The encoder takes
C<data_blocks> blocks of size C<block_size> each, and is able to produce
up to C<encoded_blocks> different encoded blocks, numbered from C<0>
to C<encoded_blocks-1>, such that any subset of C<data_blocks> members
permits reconstruction of the original data.

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

   data_blocks <= encoded_blocks <= MAXBLOCKS

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

=over 4

=cut

package Algorithm::FEC;

require XSLoader;

no warnings;

$VERSION = 0.4;

XSLoader::load Algorithm::FEC, $VERSION;

=item $fec = new data_blocks, encoded_blocks, blocksize

=item $fec->set_encode_blocks ([array_of_blocks])

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

=over 4

=item * a string of size C<blocksize> C<exactly>.

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

=item * a filehandle of a file of size C<blocksize> C<exactly>.

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

=item * 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.

=back

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

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

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

Creates a single encoded block of index C<block_index>, which must be
between C<0> and C<encoded_blocks-1> (inclusive). The blocks from C<0> to
C<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.

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

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

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

Both arrays must have exactly C<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.

Both arrays must have exactly C<data_blocks> entries.

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 C<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 C<indices = [4,3,1]>, with
C<data_blocks = 3>, then this array will be returned: C<[0,2,1]>. This
means that input block C<0> corresponds to file block C<0>, input block
C<1> to file block C<2> and input block C<2> to data block C<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 C<copy> method can be helpful here.

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

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

The same same as C<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 reodering and before decoding, then calling Cshuffle>
followed by C<set_decode_blocks> incurs lower overhead than calling
C<set_decode_blocks> twice, as files are not mmapped etc.

=item $fec->decode

Decode the blocks set by a prior call to C<set_decode_blocks>.

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

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

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

=item COMPATIBILITY

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

=head1 SEE ALSO

L<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 ;)

=head1 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

=head1 AUTHOR

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

=cut

1;

Revision:	1.7
Committed:	Sat Sep 13 21:58:36 2003 UTC (23 years ago) by root
Branch:	MAIN
Changes since 1.6:	+27 -17 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3	root	1.3	Algorithm::FEC - Forward Error Correction using Vandermonde Matrices
4	root	1.1
5			=head1 SYNOPSIS
6
7	root	1.3	use Algorithm::FEC;
8	root	1.1
9			=head1 DESCRIPTION
10
11			This module is an interface to the fec library by Luigi Rizzo et al., see
12			the file README.fec in the distribution for more details.
13
14	root	1.7	This library implements a simple (C<encoded_blocks>,C<data_blocks>)
15	root	1.1	erasure code based on Vandermonde matrices. The encoder takes
16	root	1.7	C<data_blocks> blocks of size C<block_size> each, and is able to produce
17			up to C<encoded_blocks> different encoded blocks, numbered from C<0>
18			to C<encoded_blocks-1>, such that any subset of C<data_blocks> members
19	root	1.1	permits reconstruction of the original data.
20
21	root	1.7	Allowed values for C<data_blocks> and C<encoded_blocks> must obey the
22	root	1.1	following equation:
23
24	root	1.7	data_blocks <= encoded_blocks <= MAXBLOCKS
25	root	1.1
26			Where C<MAXBLOCKS=256> for the fast implementation and C<MAXBLOCKS=65536>
27			for the slow implementation (the implementation is chosen automatically).
28
29			=over 4
30
31			=cut
32
33	root	1.3	package Algorithm::FEC;
34	root	1.1
35			require XSLoader;
36
37			no warnings;
38
39	root	1.6	$VERSION = 0.4;
40	root	1.1
41	root	1.3	XSLoader::load Algorithm::FEC, $VERSION;
42	root	1.1
43	root	1.7	=item $fec = new data_blocks, encoded_blocks, blocksize
44	root	1.1
45	root	1.6	=item $fec->set_encode_blocks ([array_of_blocks])
46	root	1.1
47			Sets the data blocks used for the encoding. Each member of the array can either be:
48
49			=over 4
50
51			=item * a string of size C<blocksize> C<exactly>.
52
53			This is useful for small files (encoding entirely in memory).
54
55			=item * a filehandle of a file of size C<blocksize> C<exactly>.
56
57			This is useful when the amount of data is large and resides in single files.
58
59			=item * a reference to an array containing a filehandle and, optionally, an offset into that file.
60
61			This is useful if the amount of data is large and resides in a single
62			file. Needless to say, all parts must not overlap and must fit into the
63			file.
64
65			=back
66
67			If your data is not of the required size (i.e. a multiple of C<blocksize>
68			bytes), then you must pad it (e.g. with zero bytes) on encoding, and
69			truncate it after decoding.
70
71			If called without arguments, the internal storage associated with the
72			blocks is freed again.
73
74			=item $block = $fec->encode (block_index)
75
76	root	1.7	Creates a single encoded block of index C<block_index>, which must be
77			between C<0> and C<encoded_blocks-1> (inclusive). The blocks from C<0> to
78			C<data_blocks-1> are simply copies of the original data blocks.
79	root	1.1
80			The encoded block is returned as a perl scalar (so the blocks should fit
81			into memory. If this is a problem for you mail me and I'll make it a file.
82
83	root	1.6	=item $fec->set_decode_blocks ([array_of_blocks], [array_of_indices])
84	root	1.1
85	root	1.7	Prepares to decode C<data_blocks> of blocks (see C<set_encode_blocks> for
86	root	1.6	the C<array_of_blocks> parameter).
87	root	1.1
88			Since these are not necessarily the original data blocks, an array of
89	root	1.7	indices (ranging from C<0> to C<encoded_blocks-1>) must be supplied as
90	root	1.1	the second arrayref.
91
92	root	1.7	Both arrays must have exactly C<data_blocks> entries.
93	root	1.1
94	root	1.6	This method also reorders the blocks and index array in place (if
95			necessary) to reflect the order the blocks will have in the decoded
96			result.
97
98	root	1.7	Both arrays must have exactly C<data_blocks> entries.
99	root	1.6
100			The index array represents the decoded ordering, in that the n-th entry
101			in the indices array corresponds to the n-th data block of the decoded
102			result. The value stored in the n-th place in the array will contain the
103			index of the encoded data block.
104
105	root	1.7	Input blocks with indices less than C<data_blocks> will be moved to their
106	root	1.6	final position (block k to position k), while the gaps between them will
107			be filled with check blocks. The decoding process will not modify the
108			already decoded data blocks, but will modify the check blocks.
109	root	1.1
110			That is, if you call this function with C<indices = [4,3,1]>, with
111	root	1.7	C<data_blocks = 3>, then this array will be returned: C<[0,2,1]>. This
112	root	1.4	means that input block C<0> corresponds to file block C<0>, input block
113			C<1> to file block C<2> and input block C<2> to data block C<1>.
114	root	1.1
115			You can just iterate over this array and write out the corresponding data
116	root	1.4	block (although this is inefficient):
117
118	root	1.6	for my $i (0 .. $#idx)
119			if ($idx[$i] != $i) # need we move this block?
120	root	1.7	copy encoded block $idx[$i] to position $i
121	root	1.6	}
122	root	1.4	}
123	root	1.1
124	root	1.6	The C<copy> method can be helpful here.
125	root	1.1
126	root	1.6	This method destroys the block array as set up by C<set_encode_blocks>.
127	root	1.7
128			=item $fec->shuffle ([array_of_blocks], [array_of_indices])
129
130			The same same as C<set_decode_blocks>, with the exception that the blocks
131			are not actually set for decoding.
132
133			This method is not normally used, but if you want to move blocks
134			around after reodering and before decoding, then calling Cshuffle>
135			followed by C<set_decode_blocks> incurs lower overhead than calling
136			C<set_decode_blocks> twice, as files are not mmapped etc.
137	root	1.6
138			=item $fec->decode
139
140			Decode the blocks set by a prior call to C<set_decode_blocks>.
141
142			This method destroys the block array as set up by C<set_decode_blocks>.
143	root	1.1
144	root	1.2	=item $fec->copy ($srcblock, $dstblock)
145	root	1.1
146			Utility function that simply copies one block (specified like in
147	root	1.6	C<set_encode_blocks>) into another. This, btw., destroys the blocks set by
148			C<set_*_blocks>.
149	root	1.1
150			=item COMPATIBILITY
151
152			The way this module works is compatible with the way freenet
153			(L<http://freenet.sf.net>) encodes files. Comaptibility to other file
154			formats or networks is not know, please tell me if you find more examples.
155
156			=head1 SEE ALSO
157
158			L<Net::FCP>. And the author, who might be happy to receive mail from any
159			user, just to see that this rather rarely-used module is actually being
160			used (except for freenet ;)
161
162			=head1 BUGS
163
164	root	1.6	* too complicated.
165	root	1.1	* largely untested, please change this.
166			* file descriptors are not supported, but should be.
167			* utility functions for files should be provided.
168			* 16 bit version not tested
169
170			=head1 AUTHOR
171
172			Marc Lehmann <pcg@goof.com>
173			http://home.schmorp.de
174
175			=cut
176
177			1;
178