NAME¶
Encode::Mapper - Rewrite rules compiler and interpreter
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 algorithm 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 "<otakar-smrz users.sf.net>",
<
http://otakar-smrz.users.sf.net/>
COPYRIGHT AND LICENSE¶
Copyright (C) 2003-2014 Otakar Smrz
This library is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.