NAME
Encode::Mapper - Intuitive, yet efficient mappings for Encode
REVISION
$Revision: 417 $ $Date: 2007-09-24 15:03:33 +0200 (Mon, 24 Sep 2007) $
SYNOPSIS
use Encode::Mapper; ############################################# Enjoy the ride ^^
use Encode::Mapper ':others', ':silent'; # syntactic sugar for compiler options ..
Encode::Mapper->options ( # .. equivalent, see more in the text
'others' => sub { shift },
'silent' => 1,
);
Encode::Mapper->options ( # .. resetting, but not to use 'use' !!!
'others' => undef,
'silent' => 0
);
## Types of rules for mapping the data and controlling the engine's configuration #####
@rules = (
'x', 'y', # single 'x' be 'y', unless greediness prefers ..
'xx', 'Y', # .. double 'x' be 'Y' or other rules
'uc(x)x', sub { 'sorry ;)' }, # if 'x' follows 'uc(x)', be sorry, else ..
'uc(x)', [ '', 'X' ], # .. alias this *engine-initial* string
'xuc(x)', [ '', 'xX' ], # likewise, alias for the 'x' prefix
'Xxx', [ sub { $i++; '' }, 'X' ], # count the still married 'x'
);
## Constructors of the engine, i.e. one Encode::Mapper instance #######################
$mapper = Encode::Mapper->compile( @rules ); # engine constructor
$mapper = Encode::Mapper->new( @rules ); # equivalent alias
## Elementary performance of the engine ###############################################
@source = ( 'x', 'xx', 'xxuc(x)', 'xxx', '', 'xx' ); # distribution of the data ..
$source = join '', @source; # .. is ignored in this sense
@result = ($mapper->process(@source), $mapper->recover()); # the mapping procedure
@result = ($mapper->process($source), $mapper->recover()); # completely equivalent
$result = join '', map { ref $_ eq 'CODE' ? $_->() : $_ } @result;
# maps 'xxxxxuc(x)xxxxx' into ( 'Y', 'Y', '', 'y', CODE(...), CODE(...), 'y' ), ..
# .. then converts it into 'YYyy', setting $i == 2
@follow = $mapper->compute(@source); # follow the engine's computation over @source
$dumper = $mapper->dumper(); # returns the engine as a Data::Dumper object
## Module's higher API implemented for convenience ####################################
$encoder = [ $mapper, Encode::Mapper->compile( ... ), ... ]; # reference to mappers
$result = Encode::Mapper->encode($source, $encoder, 'utf8'); # encode down to 'utf8'
$decoder = [ $mapper, Encode::Mapper->compile( ... ), ... ]; # reference to mappers
$result = Encode::Mapper->decode($source, $decoder, 'utf8'); # decode up from 'utf8'
ABSTRACT
Encode::Mapper serves for intuitive, yet efficient construction of mappings for Encode.
The module finds direct application in Encode::Arabic. It provides an object-oriented
programming interface to convert data consistently, follow the engine's computation,
dump the engine using Data::Dumper, etc.
DESCRIPTION
It looks like the author of the extension ... ;) preferred giving formal
and terse examples to writing English. Please, see Encode::Arabic where
Encode::Mapper is used for solving complex real-world problems.
INTRO AND RULE TYPES
The module's core is an algoritm which, from the rules given by the
user, builds a finite-state transducer, i.e. an engine performing greedy
search in the input stream and producing output data and side effects
relevant to the results of the search. Transducers may be linked one
with another, thus forming multi-level devices suitable for nontrivial
encoding/decoding tasks.
The rules declare which input sequences of bytes to search for, and what
to do upon their occurence. If the left-hand side (LHS) of a rule is the
longest left-most string out of those applicable on the input, the
righ-hand side (RHS) of the rule is evaluated. The RHS defines the
corresponding output string, and possibly controls the engine as if the
extra text were prepended before the rest of the input:
$A => $X # $A .. literal string
# $X .. literal string or subroutine reference
$A => [$X, $Y] # $Y .. literal string for which 'length $Y < length $A'
The order of the rules does not matter, except when several rules with
the same LHS are stated. In such a case, redefinition warning is usually
issued before overriding the RHS.
LOW-LEVEL METHODS
compile (*$class,* @rules)
compile (*$class,* $opts, @rules)
The constructor of an Encode::Mapper instance. The first argument is
the name of the class, the rest is the list of rules ... LHS odd
elements, RHS even elements, unless the first element is a reference
to an array or a hash, which then becomes $opts.
If $opts is recognized, it is used to modify the compiler "options"
locally for the engine being constructed. If an option is not
overridden, its global setting holds.
The compilation algorithm, and the search algorithm itself, were
inspired by Aho-Corasick and Boyer-Moore algorithms, and by the
studies of finite automata with the restart operation. The engine is
implemented in the classical sense, using hashes for the transition
function for instance. We expect to improve this to Perl code
evaluation, if the speed-up is significant.
It is to explore the way Perl's regular expressions would cope with
the task, i.e. verify our initial doubts which prevented us from
trying. Since Encode::Mapper's functionality is much richer than
pure search, simulating it completely might be resource-expensive
and non-elegant. Therefore, experiment reports are welcome.
new (*$class,* @list)
Name alias to the "compile" constructor.
process (*$obj,* @list)
Process the input list with the engine. There is no resetting within
the call of the method. Internally, the text in the list is "split"
into bytes, and there is just no need for the user to "join"
his/hers strings or lines of data. Note the unveiled properties of
the Encode::Mapper class as well:
sub process ($@) { # returns the list of search results performed by Mapper
my $obj = shift @_;
my (@returns, $phrase, $token, $q);
use bytes; # ensures splitting into one-byte tokens
$q = $obj->{'current'};
foreach $phrase (@_) {
foreach $token (split //, $phrase) {
until (defined $obj->{'tree'}[$q]->{$token}) {
push @returns, @{$obj->{'bell'}[$q]};
$q = $obj->{'skip'}[$q];
}
$q = $obj->{'tree'}[$q]->{$token};
}
}
$obj->{'current'} = $q;
return @returns;
}
recover (*$obj,* $r, $q)
Since the search algorithm is greedy and the engine does not know
when the end of the data comes, there must be a method to tell.
Normally, "recover" is called on the object without the other two
optional parameters setting the initial and the final state,
respectively.
sub recover ($;$$) { # returns the 'in-progress' search result and resets Mapper
my ($obj, $r, $q) = @_;
my (@returns);
$q = $obj->{'current'} unless defined $q;
until ($q == 0) {
push @returns, @{$obj->{'bell'}[$q]};
$q = $obj->{'skip'}[$q];
}
$obj->{'current'} = defined $r ? $r : 0;
return @returns;
}
compute (*$obj,* @list)
Tracks down the computation over the list of data, resetting the
engine before and after to its initial state. Developers might like
this ;)
local $\ = "\n"; local $, = ":\t"; # just define the display
foreach $result ($mapper->compute($source)) { # follow the computation
print "Token" , $result->[0];
print "Source" , $result->[1];
print "Output" , join " + ", @{$result->[2]};
print "Target" , $result->[3];
print "Bell" , join ", ", @{$result->[4]};
print "Skip" , $result->[5];
}
dumper (*$obj,* $ref)
The individual instances of Encode::Mapper can be stored as
revertible data structures. For minimalistic reasons, dumping needs
to include explicit short-identifier references to the empty array
and the empty hash of the engine. For details, see Data::Dumper.
sub dumper ($;$) {
my ($obj, $ref) = @_;
$ref = ['L', 'H', 'mapper'] unless defined $ref;
require Data::Dumper;
return Data::Dumper->new([$obj->{'null'}{'list'}, $obj->{'null'}{'hash'}, $obj], $ref);
}
describe (*$obj,* $ref)
Describes the Encode::Mapper object and returns a hash of the
characteristics. If $ref is defined, the information is also
"print"ed into the $referenced stream, or to "STDERR" if $ref is not
a filehandle.
HIGH-LEVEL METHODS
In the Encode world, one can work with different encodings and is also
provided a function for telling if the data are in Perl's internal utf8
format or not. In the Encode::Mapper business, one is encouraged to
compile different mappers and stack them on top of each other, getting
an easy-to-work-with filtering device.
In combination, this module offers the following "encode" and "decode"
methods. In their prototypes, $encoder/$decoder represent merely a
reference to an array of mappers, although mathematics might do more
than that in future implementations ;)
Currently, the mappers involved are not reset with "recover" before the
computation. See the "--join" option for more comments on the code:
foreach $mapper (@{$_[2]}) { # either $encoder or $decoder
$join = defined $mapper->{'join'} ? $mapper->{'join'} :
defined $option{'join'} ? $option{'join'} : "";
$text = join $join, map {
UNIVERSAL::isa($_, 'CODE') ? $_->() : $_
} $mapper->process($text), $mapper->recover();
}
encode (*$class,* $text, $encoder, $enc)
If $enc is defined, the $text is encoded into that encoding, using
Encode. Then, the $encoder's engines are applied in series on the
data. The returned text should have the utf8 flag off.
decode (*$class,* $text, $decoder, $enc)
The $text is run through the sequence of engines in $decoder. If the
result does not have the utf8 flag on, decoding from $enc is further
performed by Encode. If $enc is not defined, utf8 is assumed.
OPTIONS AND EXPORT
The language the Encode::Mapper engine works on is not given exclusively
by the rules passed as parameters to the "compile" or "new" constructor
methods. The nature of the compilation is influenced by the current
setting of the following options:
--complement
This option accepts a reference to an array declaring rules which
are to complement the rules of the constructor. Redefinition
warnings are issued only if you redefine within the option's list,
not when a rule happens to be overridden during compilation.
--override
Overrides the rules of the constructor. Redefinition warnings are
issued, though. You might, for example, want to preserve all XML
markup in the data you are going to process through your
encoders/decoders:
'override' => [ # override rules of these LHS .. there's no other tricks ^^
( # combinations of '<' and '>' with the other bytes
map {
my $x = chr $_;
"<" . $x, [ "<" . $x, ">" ], # propagate the '>' sign implying ..
">" . $x, [ $x, ">" ], # .. preservation of the bytes
} 0x00..0x3B, 0x3D, 0x3F..0xFF
),
">>", ">", # stop the whole process ..
"<>", "<>", # .. do not even start it
"><", [ "<", ">" ], # rather than nested '<' and '>', ..
"<<", [ "<<", ">" ],
">\\<", [ "<", ">" ], # .. prefer these escape sequences
">\\\\", [ "\\", ">" ],
">\\>", [ ">", ">" ],
">", ">", # singular symbols may migrate right ..
"<", "<", # .. or preserve the rest of the data
]
--others
If defined, this option controls how to deal with 'others', i.e.
bytes of input for which there is no rule, by defining rules for
them. In case this option gets a code reference, the referenced
subroutine will be called with the 'other' LHS parameter to get the
rule's RHS. Otherwise, a defined scalar value will become the RHS of
each 'other' LHS.
To preserve the 'other' bytes, you can use
'others' => sub { shift } # preserve every non-treated byte
the effect of which is similar to including the "map" to the
"--complement" rules:
'complement' => [ ( map { ( chr $_ ) x 2 } 0x00..0xFF ), ... ] # ... is your rules
You may of course wish to return undefined values if there are any
non-treated bytes in the input. In order for the "undef" to be a
correct RHS, you have to protect it once more by the "sub" like
this:
'others' => sub { sub { undef } }
--silent
Setting it to a true value will prevent any warnings issued during
the engine's compilation, mostly reflecting an incorrect or dubious
use of a rule.
--join
This option enables less memory-requiring representation of the
engines. If this option is defined when the constructor is called,
the setting is stored in the instance internally. Any lists of
literal RHS which are to be emitted simultaneously from the engine
are joined into a string with the option's value, empty lists turn
into empty strings. If an engine was compiled with this option
defined, the value will be used to join output of "encode" and
"decode", too. If not, either the current value of the option or the
empty string will help instead.
The keywords of options can be in mixed case and/or start with any
number of dashes, and the next element in the list is taken as the
option's value. There are special keywords, however, beginning with a
colon and not gulping down the next element:
:others
Equivalent to the code "'others' => sub { shift }" explained above.
:silent
Equivalent to "'silent' => 1", or rather to the maximum silence if
more degrees of it are introduced in the future.
:join
Equivalent to 'join' => ''. Use this option if you are going to dump
and load the new engine often, and if you do not miss the
list-supporting uniformity of "process" and "recover".
Compiler options are associated with package names in the
%Encode::Mapper::options variable, and confined to them. While "options"
and "import" perform the setting with respect to the caller package,
accessing the hash directly is neither recommended, nor restricted.
There is a nice compile-time invocation of "import" with the "use""
Encode::Mapper LIST" idiom, which you might prefer to explicit method
calls. Local modification of the package's global setting that applies
just to the engine being constructed is done by supplying the options as
an extra parameter to "compile".
use Data::Dump 'dump'; # pretty data printing is below
$Encode::Mapper::options{'ByForce'} = { qw ':others - silent errors' };
package ByMethod; # import called at compile time
# no warnings, 'silent' is true
Encode::Mapper->options('complement' => [ 'X', 'Y' ], 'others' => 'X');
use Encode::Mapper 'silent' => 299_792_458;
package main; # import called at compile time
# 'non-existent' may exist once
print dump %Encode::Mapper::options;
use Encode::Mapper ':others', ':silent', 'non-existent', 'one';
# (
# "ByMethod",
# { complement => ["X", "Y"], others => "X", silent => 299_792_458 },
# "ByForce",
# { ":others" => "-", silent => "errors" },
# "main",
# { "non-existent" => "one", others => sub { "???" }, silent => 1 },
# )
options (*$class,* @list)
If $class is defined, enforces the options in the list globally for
the calling package. The return value of this method is the state of
the options before the proposed changes were set. If $class is
undefined, nothing is set, only the canonized forms of the declared
keywords and their values are returned.
import (*$class,* @list)
This module does not export any symbols. This method just calls
"options", provided there are some elements in the list.
SEE ALSO
There are related theoretical studies which the implementation may have
touched. You might be interested in Aho-Corasick and Boyer-Moore
algorithms as well as in finite automata with the restart operation.
Encode, Encode::Arabic, Data::Dumper
Encode Arabic: Exercise in Functional Parsing
<http://ufal.mff.cuni.cz/padt/online/2006/06/encode-arabic.html>
AUTHOR
Otakar Smrz, <http://ufal.mff.cuni.cz/~smrz/>
eval { 'E<lt>' . ( join '.', qw 'otakar smrz' ) . "\x40" . ( join '.', qw 'mff cuni cz' ) . 'E<gt>' }
Perl is also designed to make the easy jobs not that easy ;)
COPYRIGHT AND LICENSE
Copyright 2003-2007 by Otakar Smrz
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.