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 = 0.5;

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 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 C<$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.

=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>.

=back

=head1 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:	1.8
Committed:	Mon Jun 21 13:07:52 2004 UTC (22 years, 2 months ago) by root
Branch:	MAIN
Changes since 1.7:	+10 -4 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	1.8	$VERSION = 0.5;
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 $fec->set_encode_blocks ([array_of_blocks])
46	root	1.1
47			Sets the data blocks used for the encoding. Each member of the array can either be:
48
49			=over 4
50
51			=item * a string of size C<blocksize> C<exactly>.
52
53			This is useful for small files (encoding entirely in memory).
54
55			=item * a filehandle of a file of size C<blocksize> C<exactly>.
56
57			This is useful when the amount of data is large and resides in single files.
58
59			=item * a reference to an array containing a filehandle and, optionally, an offset into that file.
60
61			This is useful if the amount of data is large and resides in a single
62			file. Needless to say, all parts must not overlap and must fit into the
63			file.
64
65			=back
66
67			If your data is not of the required size (i.e. a multiple of C<blocksize>
68	root	1.8	bytes), then you must pad it (e.g. with zero bytes) on encoding (and you
69			should truncate it after decoding). Otherwise, this library croaks.
70
71			Future versions might instead load the short segment into memory or extend
72			your scalar (this might enable nice tricks, like C<$fec->copy (..., my
73			$x)> :). Mail me if you want this to happen.
74	root	1.1
75			If called without arguments, the internal storage associated with the
76			blocks is freed again.
77
78			=item $block = $fec->encode (block_index)
79
80	root	1.7	Creates a single encoded block of index C<block_index>, which must be
81			between C<0> and C<encoded_blocks-1> (inclusive). The blocks from C<0> to
82			C<data_blocks-1> are simply copies of the original data blocks.
83	root	1.1
84			The encoded block is returned as a perl scalar (so the blocks should fit
85			into memory. If this is a problem for you mail me and I'll make it a file.
86
87	root	1.6	=item $fec->set_decode_blocks ([array_of_blocks], [array_of_indices])
88	root	1.1
89	root	1.7	Prepares to decode C<data_blocks> of blocks (see C<set_encode_blocks> for
90	root	1.6	the C<array_of_blocks> parameter).
91	root	1.1
92			Since these are not necessarily the original data blocks, an array of
93	root	1.7	indices (ranging from C<0> to C<encoded_blocks-1>) must be supplied as
94	root	1.1	the second arrayref.
95
96	root	1.7	Both arrays must have exactly C<data_blocks> entries.
97	root	1.1
98	root	1.6	This method also reorders the blocks and index array in place (if
99			necessary) to reflect the order the blocks will have in the decoded
100			result.
101
102	root	1.7	Both arrays must have exactly C<data_blocks> entries.
103	root	1.6
104			The index array represents the decoded ordering, in that the n-th entry
105			in the indices array corresponds to the n-th data block of the decoded
106			result. The value stored in the n-th place in the array will contain the
107			index of the encoded data block.
108
109	root	1.7	Input blocks with indices less than C<data_blocks> will be moved to their
110	root	1.6	final position (block k to position k), while the gaps between them will
111			be filled with check blocks. The decoding process will not modify the
112			already decoded data blocks, but will modify the check blocks.
113	root	1.1
114			That is, if you call this function with C<indices = [4,3,1]>, with
115	root	1.7	C<data_blocks = 3>, then this array will be returned: C<[0,2,1]>. This
116	root	1.4	means that input block C<0> corresponds to file block C<0>, input block
117			C<1> to file block C<2> and input block C<2> to data block C<1>.
118	root	1.1
119			You can just iterate over this array and write out the corresponding data
120	root	1.4	block (although this is inefficient):
121
122	root	1.6	for my $i (0 .. $#idx)
123			if ($idx[$i] != $i) # need we move this block?
124	root	1.7	copy encoded block $idx[$i] to position $i
125	root	1.6	}
126	root	1.4	}
127	root	1.1
128	root	1.6	The C<copy> method can be helpful here.
129	root	1.1
130	root	1.6	This method destroys the block array as set up by C<set_encode_blocks>.
131	root	1.7
132			=item $fec->shuffle ([array_of_blocks], [array_of_indices])
133
134			The same same as C<set_decode_blocks>, with the exception that the blocks
135			are not actually set for decoding.
136
137			This method is not normally used, but if you want to move blocks
138			around after reodering and before decoding, then calling Cshuffle>
139			followed by C<set_decode_blocks> incurs lower overhead than calling
140			C<set_decode_blocks> twice, as files are not mmapped etc.
141	root	1.6
142			=item $fec->decode
143
144			Decode the blocks set by a prior call to C<set_decode_blocks>.
145
146			This method destroys the block array as set up by C<set_decode_blocks>.
147	root	1.1
148	root	1.2	=item $fec->copy ($srcblock, $dstblock)
149	root	1.1
150			Utility function that simply copies one block (specified like in
151	root	1.6	C<set_encode_blocks>) into another. This, btw., destroys the blocks set by
152			C<set_*_blocks>.
153	root	1.1
154	root	1.8	=back
155
156			=head1 COMPATIBILITY
157	root	1.1
158			The way this module works is compatible with the way freenet
159			(L<http://freenet.sf.net>) encodes files. Comaptibility to other file
160			formats or networks is not know, please tell me if you find more examples.
161
162			=head1 SEE ALSO
163
164			L<Net::FCP>. And the author, who might be happy to receive mail from any
165			user, just to see that this rather rarely-used module is actually being
166			used (except for freenet ;)
167
168			=head1 BUGS
169
170	root	1.6	* too complicated.
171	root	1.1	* largely untested, please change this.
172			* file descriptors are not supported, but should be.
173			* utility functions for files should be provided.
174			* 16 bit version not tested
175
176			=head1 AUTHOR
177
178			Marc Lehmann <pcg@goof.com>
179			http://home.schmorp.de
180
181			=cut
182
183			1;
184