perl/lib/DBM_Filter.pm

package DBM_Filter ;

use strict;
use warnings;
our $VERSION = '0.01';

package Tie::Hash ;

use strict;
use warnings;

use Carp;


our %LayerStack = ();
our %origDESTROY = ();

our %Filters = map { $_, undef } qw(
            Fetch_Key
            Fetch_Value
            Store_Key
            Store_Value
        );

our %Options = map { $_, 1 } qw(
            fetch
            store
        );

#sub Filter_Enable
#{
#}
#
#sub Filter_Disable
#{
#}

sub Filtered
{
    my $this = shift;
    return defined $LayerStack{$this} ;
}

sub Filter_Pop
{
    my $this = shift;
    my $stack = $LayerStack{$this} || return undef ;
    my $filter = pop @{ $stack };

    # remove the filter hooks if this is the last filter to pop
    if ( @{ $stack } == 0 ) {
        $this->filter_store_key  ( undef );
        $this->filter_store_value( undef );
        $this->filter_fetch_key  ( undef );
        $this->filter_fetch_value( undef );
        delete $LayerStack{$this};
    }

    return $filter;
}

sub Filter_Key_Push
{
    &_do_Filter_Push;
}

sub Filter_Value_Push
{
    &_do_Filter_Push;
}


sub Filter_Push
{
    &_do_Filter_Push;
}

sub _do_Filter_Push
{
    my $this = shift;
    my %callbacks = ();
    my $caller = (caller(1))[3];
    $caller =~ s/^.*:://;
 
    croak "$caller: no parameters present" unless @_ ;

    if ( ! $Options{lc $_[0]} ) {
        my $class = shift;
        my @params = @_;

        # if $class already contains "::", don't prefix "DBM_Filter::"
        $class = "DBM_Filter::$class" unless $class =~ /::/;
    
        # does the "DBM_Filter::$class" exist?
        if ( ! defined %{ "${class}::"} ) {
            # Nope, so try to load it.
            eval " require $class ; " ;
            croak "$caller: Cannot Load DBM Filter '$class': $@" if $@;
        }
    
        no strict 'refs';
        my $fetch  = *{ "${class}::Fetch"  }{CODE};
        my $store  = *{ "${class}::Store"  }{CODE};
        my $filter = *{ "${class}::Filter" }{CODE};
        use strict 'refs';

        my $count = defined($filter) + defined($store) + defined($fetch) ;

        if ( $count == 0 )
          { croak "$caller: No methods (Filter, Fetch or Store) found in class '$class'" }
        elsif ( $count == 1 && ! defined $filter) {
           my $need = defined($fetch) ? 'Store' : 'Fetch';
           croak "$caller: Missing method '$need' in class '$class'" ;
        }
        elsif ( $count >= 2 && defined $filter)
          { croak "$caller: Can't mix Filter with Store and Fetch in class '$class'" }

        if (defined $filter) {
            my $callbacks = &{ $filter }(@params);
            croak "$caller: '${class}::Filter' did not return a hash reference" 
                unless ref $callbacks && ref $callbacks eq 'HASH';
            %callbacks = %{ $callbacks } ;
        }
        else {
            $callbacks{Fetch} = $fetch;
            $callbacks{Store} = $store;
        }
    }
    else {
        croak "$caller: not even params" unless @_ % 2 == 0;
        %callbacks = @_;
    }
    
    my %filters = %Filters ;
    my @got = ();
    while (my ($k, $v) = each %callbacks )
    {
        my $key = $k;
        $k = lc $k;
        if ($k eq 'fetch') {
            push @got, 'Fetch';
            if ($caller eq 'Filter_Push')
              { $filters{Fetch_Key} = $filters{Fetch_Value} = $v }
            elsif ($caller eq 'Filter_Key_Push')
              { $filters{Fetch_Key} = $v }
            elsif ($caller eq 'Filter_Value_Push')
              { $filters{Fetch_Value} = $v }
        }
        elsif ($k eq 'store') {
            push @got, 'Store';
            if ($caller eq 'Filter_Push')
              { $filters{Store_Key} = $filters{Store_Value} = $v }
            elsif ($caller eq 'Filter_Key_Push')
              { $filters{Store_Key} = $v }
            elsif ($caller eq 'Filter_Value_Push')
              { $filters{Store_Value} = $v }
        }
        else
          { croak "$caller: Unknown key '$key'" }

        croak "$caller: value associated with key '$key' is not a code reference"
            unless ref $v && ref $v eq 'CODE';
    }

    if ( @got != 2 ) {
        push @got, 'neither' if @got == 0 ;
        croak "$caller: expected both Store & Fetch - got @got";
    }

    # remember the class
    push @{ $LayerStack{$this} }, \%filters ;

    my $str_this = "$this" ; # Avoid a closure with $this in the subs below

    $this->filter_store_key  ( sub { store_hook($str_this, 'Store_Key')   });
    $this->filter_store_value( sub { store_hook($str_this, 'Store_Value') });
    $this->filter_fetch_key  ( sub { fetch_hook($str_this, 'Fetch_Key')   });
    $this->filter_fetch_value( sub { fetch_hook($str_this, 'Fetch_Value') });

    # Hijack the callers DESTROY method
    $this =~ /^(.*)=/;
    my $type = $1 ;
    no strict 'refs';
    if ( *{ "${type}::DESTROY" }{CODE} ne \&MyDESTROY )
    {
        $origDESTROY{$type} = *{ "${type}::DESTROY" }{CODE};
        no warnings 'redefine';
        *{ "${type}::DESTROY" } = \&MyDESTROY ;
    }
}

sub store_hook
{
    my $this = shift ;
    my $type = shift ;
    foreach my $layer (@{ $LayerStack{$this} })
    {
        &{ $layer->{$type} }() if defined $layer->{$type} ;
    }
}

sub fetch_hook
{
    my $this = shift ;
    my $type = shift ;
    foreach my $layer (reverse @{ $LayerStack{$this} })
    {
        &{ $layer->{$type} }() if defined $layer->{$type} ;
    }
}

sub MyDESTROY
{
    my $this = shift ;
    delete $LayerStack{$this} ;

    # call real DESTROY
    $this =~ /^(.*)=/;
    &{ $origDESTROY{$1} }($this);
}

1;

__END__

=head1 NAME

DBM_Filter -- Filter DBM keys/values 

=head1 SYNOPSIS

    use DBM_Filter ;
    use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File

    $db = tie %hash, ...

    $db->Filter_Push(Fetch => sub {...},
                     Store => sub {...});

    $db->Filter_Push('my_filter1');
    $db->Filter_Push('my_filter2', params...);

    $db->Filter_Key_Push(...) ;
    $db->Filter_Value_Push(...) ;

    $db->Filter_Pop();
    $db->Filtered();

    package DBM_Filter::my_filter1;
    
    sub Store { ... }
    sub Fetch { ... }

    1;

    package DBM_Filter::my_filter2;

    sub Filter
    {
        my @opts = @_;
        ...
        return (
            sub Store { ... },
            sub Fetch { ... } );
    }

    1;

=head1 DESCRIPTION

This module provides an interface that allows filters to be applied
to tied Hashes associated with DBM files. It builds on the DBM Filter
hooks that are present in all the *DB*_File modules included with the
standard Perl source distribution from version 5.6.1 onwards. In addition
to the *DB*_File modules distributed with Perl, the BerkeleyDB module,
available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter>
for more details on the DBM Filter hooks.

=head1 What is a DBM Filter?

A DBM Filter allows the keys and/or values in a tied hash to be modified
by some user-defined code just before it is written to the DBM file and
just after it is read back from the DBM file. For example, this snippet
of code

    $some_hash{"abc"} = 42;

could potentially trigger two filters, one for the writing of the key
"abc" and another for writing the value 42.  Similarly, this snippet

    my ($key, $value) = each %some_hash

will trigger two filters, one for the reading of the key and one for
the reading of the value.

Like the existing DBM Filter functionality, this module arranges for the
C<$_> variable to be populated with the key or value that a filter will
check. This usually means that most DBM filters tend to be very short.

=head2 So what's new?

The main enhancements over the standard DBM Filter hooks are:

=over 4

=item *

A cleaner interface.

=item *

The ability to easily apply multiple filters to a single DBM file.

=item *

The ability to create "canned" filters. These allow commonly used filters
to be packaged into a stand-alone module.

=back

=head1 METHODS

This module will arrange for the following methods to be available via
the object returned from the C<tie> call.

=head2 $db->Filter_Push()

=head2 $db->Filter_Key_Push()

=head2 $db->Filter_Value_Push()

Add a filter to filter stack for the database, C<$db>. The three formats
vary only in whether they apply to the DBM key, the DBM value or both.

=over 5

=item Filter_Push

The filter is applied to I<both> keys and values.

=item Filter_Key_Push

The filter is applied to the key I<only>.

=item Filter_Value_Push

The filter is applied to the value I<only>.

=back


=head2 $db->Filter_Pop()

Removes the last filter that was applied to the DBM file associated with
C<$db>, if present.

=head2 $db->Filtered()

Returns TRUE if there are any filters applied to the DBM associated
with C<$db>.  Otherwise returns FALSE.


=head1 Writing a Filter

Filters can be created in two main ways

=head2 Immediate Filters

An immediate filter allows you to specify the filter code to be used
at the point where the filter is applied to a dbm. In this mode the
Filter_*_Push methods expects to receive exactly two parameters.

    my $db = tie %hash, 'SDBM_File', ...
    $db->Filter_Push( Store => sub { },
                      Fetch => sub { });

The code reference associated with C<Store> will be called before any
key/value is written to the database and the code reference associated
with C<Fetch> will be called after any key/value is read from the
database.

For example, here is a sample filter that adds a trailing NULL character
to all strings before they are written to the DBM file, and removes the
trailing NULL when they are read from the DBM file

    my $db = tie %hash, 'SDBM_File', ...
    $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
                      Fetch => sub { s/\x00$// ;    });


Points to note:

=over 5

=item 1.

Both the Store and Fetch filters manipulate C<$_>.

=back

=head2 Canned Filters

Immediate filters are useful for one-off situations. For more generic
problems it can be useful to package the filter up in its own module.

The usage is for a canned filter is:

    $db->Filter_Push("name", params)

where

=over 5

=item "name"

is the name of the module to load. If the string specified does not
contain the package separator characters "::", it is assumed to refer to
the full module name "DBM_Filter::name". This means that the full names
for canned filters, "null" and "utf8", included with this module are:

    DBM_Filter::null
    DBM_Filter::utf8

=item params

any optional parameters that need to be sent to the filter. See the
encode filter for an example of a module that uses parameters.

=back

The module that implements the canned filter can take one of two
forms. Here is a template for the first

    package DBM_Filter::null ;

    use strict;
    use warnings;

    sub Store 
    {
        # store code here    
    }

    sub Fetch
    {
        # fetch code here
    }

    1;


Notes:

=over 5

=item 1.

The package name uses the C<DBM_Filter::> prefix.

=item 2.

The module I<must> have both a Store and a Fetch method. If only one is
present, or neither are present, a fatal error will be thrown.

=back

The second form allows the filter to hold state information using a
closure, thus:

    package DBM_Filter::encoding ;

    use strict;
    use warnings;

    sub Filter
    {
        my @params = @_ ;

        ...
        return {
            Store   => sub { $_ = $encoding->encode($_) },
            Fetch   => sub { $_ = $encoding->decode($_) }
            } ;
    }

    1;


In this instance the "Store" and "Fetch" methods are encapsulated inside a
"Filter" method.


=head1 Filters Included

A number of canned filers are provided with this module. They cover a
number of the main areas that filters are needed when interfacing with
DBM files. They also act as templates for your own filters.

The filter included are:

=over 5

=item * utf8

This module will ensure that all data written to the DBM will be encoded
in UTF-8.

This module needs the Encode module.

=item * encode

Allows you to choose the character encoding will be store in the DBM file.

=item * compress

This filter will compress all data before it is written to the database
and uncompressed it on reading.

This module needs Compress::Zlib. 

=item * int32

This module is used when interoperating with a C/C++ application that
uses a C int as either the key and/or value in the DBM file.

=item * null

This module ensures that all data written to the DBM file is null
terminated. This is useful when you have a perl script that needs
to interoperate with a DBM file that a C program also uses. A fairly
common issue is for the C application to include the terminating null
in a string when it writes to the DBM file. This filter will ensure that
all data written to the DBM file can be read by the C application.

=back

=head1 NOTES

=head2 Maintain Round Trip Integrity

When writing a DBM filter it is I<very> important to ensure that it is
possible to retrieve all data that you have written when the DBM filter
is in place. In practice, this means that whatever transformation is
applied to the data in the Store method, the I<exact> inverse operation
should be applied in the Fetch method.

If you don't provide an exact inverse transformation, you will find that
code like this will not behave as you expect.

     while (my ($k, $v) = each %hash)
     {
         ...
     }

Depending on the transformation, you will find that one or more of the
following will happen

=over 5

=item 1

The loop will never terminate.

=item 2

Too few records will be retrieved.

=item 3

Too many will be retrieved.

=item 4

The loop will do the right thing for a while, but it will unexpectedly fail. 

=back

=head2 Don't mix filtered & non-filtered data in the same database file. 

This is just a restatement of the previous section. Unless you are
completely certain you know what you are doing, avoid mixing filtered &
non-filtered data.

=head1 EXAMPLE

Say you need to interoperate with a legacy C application that stores
keys as C ints and the values and null terminated UTF-8 strings. Here
is how you would set that up

    my $db = tie %hash, 'SDBM_File', ...

    $db->Filter_Key_Push('int32') ;

    $db->Filter_Value_Push('utf8');
    $db->Filter_Value_Push('null');

=head1 SEE ALSO

<DB_File>,  L<GDBM_File>, L<NDBM_File>, L<ODBM_File>, L<SDBM_File>, L<perldbmfilter>

=head1 AUTHOR

Paul Marquess <pmqs@cpan.org>

Revision:	1.1
Committed:	Thu Jun 30 14:27:08 2005 UTC (21 years, 4 months ago) by root
Branch:	MAIN
CVS Tags:	PERL-5-8-7, HEAD
Branch point for:	PERL
Log Message:	* empty log message *
#	Content
1	package DBM_Filter ;
2
3	use strict;
4	use warnings;
5	our $VERSION = '0.01';
6
7	package Tie::Hash ;
8
9	use strict;
10	use warnings;
11
12	use Carp;
13
14
15	our %LayerStack = ();
16	our %origDESTROY = ();
17
18	our %Filters = map { $_, undef } qw(
19	Fetch_Key
20	Fetch_Value
21	Store_Key
22	Store_Value
23	);
24
25	our %Options = map { $_, 1 } qw(
26	fetch
27	store
28	);
29
30	#sub Filter_Enable
31	#{
32	#}
33	#
34	#sub Filter_Disable
35	#{
36	#}
37
38	sub Filtered
39	{
40	my $this = shift;
41	return defined $LayerStack{$this} ;
42	}
43
44	sub Filter_Pop
45	{
46	my $this = shift;
47	my $stack = $LayerStack{$this} \|\| return undef ;
48	my $filter = pop @{ $stack };
49
50	# remove the filter hooks if this is the last filter to pop
51	if ( @{ $stack } == 0 ) {
52	$this->filter_store_key ( undef );
53	$this->filter_store_value( undef );
54	$this->filter_fetch_key ( undef );
55	$this->filter_fetch_value( undef );
56	delete $LayerStack{$this};
57	}
58
59	return $filter;
60	}
61
62	sub Filter_Key_Push
63	{
64	&_do_Filter_Push;
65	}
66
67	sub Filter_Value_Push
68	{
69	&_do_Filter_Push;
70	}
71
72
73	sub Filter_Push
74	{
75	&_do_Filter_Push;
76	}
77
78	sub _do_Filter_Push
79	{
80	my $this = shift;
81	my %callbacks = ();
82	my $caller = (caller(1))[3];
83	$caller =~ s/^.*:://;
84
85	croak "$caller: no parameters present" unless @_ ;
86
87	if ( ! $Options{lc $_[0]} ) {
88	my $class = shift;
89	my @params = @_;
90
91	# if $class already contains "::", don't prefix "DBM_Filter::"
92	$class = "DBM_Filter::$class" unless $class =~ /::/;
93
94	# does the "DBM_Filter::$class" exist?
95	if ( ! defined %{ "${class}::"} ) {
96	# Nope, so try to load it.
97	eval " require $class ; " ;
98	croak "$caller: Cannot Load DBM Filter '$class': $@" if $@;
99	}
100
101	no strict 'refs';
102	my $fetch = *{ "${class}::Fetch" }{CODE};
103	my $store = *{ "${class}::Store" }{CODE};
104	my $filter = *{ "${class}::Filter" }{CODE};
105	use strict 'refs';
106
107	my $count = defined($filter) + defined($store) + defined($fetch) ;
108
109	if ( $count == 0 )
110	{ croak "$caller: No methods (Filter, Fetch or Store) found in class '$class'" }
111	elsif ( $count == 1 && ! defined $filter) {
112	my $need = defined($fetch) ? 'Store' : 'Fetch';
113	croak "$caller: Missing method '$need' in class '$class'" ;
114	}
115	elsif ( $count >= 2 && defined $filter)
116	{ croak "$caller: Can't mix Filter with Store and Fetch in class '$class'" }
117
118	if (defined $filter) {
119	my $callbacks = &{ $filter }(@params);
120	croak "$caller: '${class}::Filter' did not return a hash reference"
121	unless ref $callbacks && ref $callbacks eq 'HASH';
122	%callbacks = %{ $callbacks } ;
123	}
124	else {
125	$callbacks{Fetch} = $fetch;
126	$callbacks{Store} = $store;
127	}
128	}
129	else {
130	croak "$caller: not even params" unless @_ % 2 == 0;
131	%callbacks = @_;
132	}
133
134	my %filters = %Filters ;
135	my @got = ();
136	while (my ($k, $v) = each %callbacks )
137	{
138	my $key = $k;
139	$k = lc $k;
140	if ($k eq 'fetch') {
141	push @got, 'Fetch';
142	if ($caller eq 'Filter_Push')
143	{ $filters{Fetch_Key} = $filters{Fetch_Value} = $v }
144	elsif ($caller eq 'Filter_Key_Push')
145	{ $filters{Fetch_Key} = $v }
146	elsif ($caller eq 'Filter_Value_Push')
147	{ $filters{Fetch_Value} = $v }
148	}
149	elsif ($k eq 'store') {
150	push @got, 'Store';
151	if ($caller eq 'Filter_Push')
152	{ $filters{Store_Key} = $filters{Store_Value} = $v }
153	elsif ($caller eq 'Filter_Key_Push')
154	{ $filters{Store_Key} = $v }
155	elsif ($caller eq 'Filter_Value_Push')
156	{ $filters{Store_Value} = $v }
157	}
158	else
159	{ croak "$caller: Unknown key '$key'" }
160
161	croak "$caller: value associated with key '$key' is not a code reference"
162	unless ref $v && ref $v eq 'CODE';
163	}
164
165	if ( @got != 2 ) {
166	push @got, 'neither' if @got == 0 ;
167	croak "$caller: expected both Store & Fetch - got @got";
168	}
169
170	# remember the class
171	push @{ $LayerStack{$this} }, \%filters ;
172
173	my $str_this = "$this" ; # Avoid a closure with $this in the subs below
174
175	$this->filter_store_key ( sub { store_hook($str_this, 'Store_Key') });
176	$this->filter_store_value( sub { store_hook($str_this, 'Store_Value') });
177	$this->filter_fetch_key ( sub { fetch_hook($str_this, 'Fetch_Key') });
178	$this->filter_fetch_value( sub { fetch_hook($str_this, 'Fetch_Value') });
179
180	# Hijack the callers DESTROY method
181	$this =~ /^(.*)=/;
182	my $type = $1 ;
183	no strict 'refs';
184	if ( *{ "${type}::DESTROY" }{CODE} ne \&MyDESTROY )
185	{
186	$origDESTROY{$type} = *{ "${type}::DESTROY" }{CODE};
187	no warnings 'redefine';
188	*{ "${type}::DESTROY" } = \&MyDESTROY ;
189	}
190	}
191
192	sub store_hook
193	{
194	my $this = shift ;
195	my $type = shift ;
196	foreach my $layer (@{ $LayerStack{$this} })
197	{
198	&{ $layer->{$type} }() if defined $layer->{$type} ;
199	}
200	}
201
202	sub fetch_hook
203	{
204	my $this = shift ;
205	my $type = shift ;
206	foreach my $layer (reverse @{ $LayerStack{$this} })
207	{
208	&{ $layer->{$type} }() if defined $layer->{$type} ;
209	}
210	}
211
212	sub MyDESTROY
213	{
214	my $this = shift ;
215	delete $LayerStack{$this} ;
216
217	# call real DESTROY
218	$this =~ /^(.*)=/;
219	&{ $origDESTROY{$1} }($this);
220	}
221
222	1;
223
224	__END__
225
226	=head1 NAME
227
228	DBM_Filter -- Filter DBM keys/values
229
230	=head1 SYNOPSIS
231
232	use DBM_Filter ;
233	use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File
234
235	$db = tie %hash, ...
236
237	$db->Filter_Push(Fetch => sub {...},
238	Store => sub {...});
239
240	$db->Filter_Push('my_filter1');
241	$db->Filter_Push('my_filter2', params...);
242
243	$db->Filter_Key_Push(...) ;
244	$db->Filter_Value_Push(...) ;
245
246	$db->Filter_Pop();
247	$db->Filtered();
248
249	package DBM_Filter::my_filter1;
250
251	sub Store { ... }
252	sub Fetch { ... }
253
254	1;
255
256	package DBM_Filter::my_filter2;
257
258	sub Filter
259	{
260	my @opts = @_;
261	...
262	return (
263	sub Store { ... },
264	sub Fetch { ... } );
265	}
266
267	1;
268
269	=head1 DESCRIPTION
270
271	This module provides an interface that allows filters to be applied
272	to tied Hashes associated with DBM files. It builds on the DBM Filter
273	hooks that are present in all the DB_File modules included with the
274	standard Perl source distribution from version 5.6.1 onwards. In addition
275	to the DB_File modules distributed with Perl, the BerkeleyDB module,
276	available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter>
277	for more details on the DBM Filter hooks.
278
279	=head1 What is a DBM Filter?
280
281	A DBM Filter allows the keys and/or values in a tied hash to be modified
282	by some user-defined code just before it is written to the DBM file and
283	just after it is read back from the DBM file. For example, this snippet
284	of code
285
286	$some_hash{"abc"} = 42;
287
288	could potentially trigger two filters, one for the writing of the key
289	"abc" and another for writing the value 42. Similarly, this snippet
290
291	my ($key, $value) = each %some_hash
292
293	will trigger two filters, one for the reading of the key and one for
294	the reading of the value.
295
296	Like the existing DBM Filter functionality, this module arranges for the
297	C<$_> variable to be populated with the key or value that a filter will
298	check. This usually means that most DBM filters tend to be very short.
299
300	=head2 So what's new?
301
302	The main enhancements over the standard DBM Filter hooks are:
303
304	=over 4
305
306	=item *
307
308	A cleaner interface.
309
310	=item *
311
312	The ability to easily apply multiple filters to a single DBM file.
313
314	=item *
315
316	The ability to create "canned" filters. These allow commonly used filters
317	to be packaged into a stand-alone module.
318
319	=back
320
321	=head1 METHODS
322
323	This module will arrange for the following methods to be available via
324	the object returned from the C<tie> call.
325
326	=head2 $db->Filter_Push()
327
328	=head2 $db->Filter_Key_Push()
329
330	=head2 $db->Filter_Value_Push()
331
332	Add a filter to filter stack for the database, C<$db>. The three formats
333	vary only in whether they apply to the DBM key, the DBM value or both.
334
335	=over 5
336
337	=item Filter_Push
338
339	The filter is applied to I<both> keys and values.
340
341	=item Filter_Key_Push
342
343	The filter is applied to the key I<only>.
344
345	=item Filter_Value_Push
346
347	The filter is applied to the value I<only>.
348
349	=back
350
351
352	=head2 $db->Filter_Pop()
353
354	Removes the last filter that was applied to the DBM file associated with
355	C<$db>, if present.
356
357	=head2 $db->Filtered()
358
359	Returns TRUE if there are any filters applied to the DBM associated
360	with C<$db>. Otherwise returns FALSE.
361
362
363
364	=head1 Writing a Filter
365
366	Filters can be created in two main ways
367
368	=head2 Immediate Filters
369
370	An immediate filter allows you to specify the filter code to be used
371	at the point where the filter is applied to a dbm. In this mode the
372	Filter_*_Push methods expects to receive exactly two parameters.
373
374	my $db = tie %hash, 'SDBM_File', ...
375	$db->Filter_Push( Store => sub { },
376	Fetch => sub { });
377
378	The code reference associated with C<Store> will be called before any
379	key/value is written to the database and the code reference associated
380	with C<Fetch> will be called after any key/value is read from the
381	database.
382
383	For example, here is a sample filter that adds a trailing NULL character
384	to all strings before they are written to the DBM file, and removes the
385	trailing NULL when they are read from the DBM file
386
387	my $db = tie %hash, 'SDBM_File', ...
388	$db->Filter_Push( Store => sub { $_ .= "\x00" ; },
389	Fetch => sub { s/\x00$// ; });
390
391
392	Points to note:
393
394	=over 5
395
396	=item 1.
397
398	Both the Store and Fetch filters manipulate C<$_>.
399
400	=back
401
402	=head2 Canned Filters
403
404	Immediate filters are useful for one-off situations. For more generic
405	problems it can be useful to package the filter up in its own module.
406
407	The usage is for a canned filter is:
408
409	$db->Filter_Push("name", params)
410
411	where
412
413	=over 5
414
415	=item "name"
416
417	is the name of the module to load. If the string specified does not
418	contain the package separator characters "::", it is assumed to refer to
419	the full module name "DBM_Filter::name". This means that the full names
420	for canned filters, "null" and "utf8", included with this module are:
421
422	DBM_Filter::null
423	DBM_Filter::utf8
424
425	=item params
426
427	any optional parameters that need to be sent to the filter. See the
428	encode filter for an example of a module that uses parameters.
429
430	=back
431
432	The module that implements the canned filter can take one of two
433	forms. Here is a template for the first
434
435	package DBM_Filter::null ;
436
437	use strict;
438	use warnings;
439
440	sub Store
441	{
442	# store code here
443	}
444
445	sub Fetch
446	{
447	# fetch code here
448	}
449
450	1;
451
452
453	Notes:
454
455	=over 5
456
457	=item 1.
458
459	The package name uses the C<DBM_Filter::> prefix.
460
461	=item 2.
462
463	The module I<must> have both a Store and a Fetch method. If only one is
464	present, or neither are present, a fatal error will be thrown.
465
466	=back
467
468	The second form allows the filter to hold state information using a
469	closure, thus:
470
471	package DBM_Filter::encoding ;
472
473	use strict;
474	use warnings;
475
476	sub Filter
477	{
478	my @params = @_ ;
479
480	...
481	return {
482	Store => sub { $_ = $encoding->encode($_) },
483	Fetch => sub { $_ = $encoding->decode($_) }
484	} ;
485	}
486
487	1;
488
489
490	In this instance the "Store" and "Fetch" methods are encapsulated inside a
491	"Filter" method.
492
493
494	=head1 Filters Included
495
496	A number of canned filers are provided with this module. They cover a
497	number of the main areas that filters are needed when interfacing with
498	DBM files. They also act as templates for your own filters.
499
500	The filter included are:
501
502	=over 5
503
504	=item * utf8
505
506	This module will ensure that all data written to the DBM will be encoded
507	in UTF-8.
508
509	This module needs the Encode module.
510
511	=item * encode
512
513	Allows you to choose the character encoding will be store in the DBM file.
514
515	=item * compress
516
517	This filter will compress all data before it is written to the database
518	and uncompressed it on reading.
519
520	This module needs Compress::Zlib.
521
522	=item * int32
523
524	This module is used when interoperating with a C/C++ application that
525	uses a C int as either the key and/or value in the DBM file.
526
527	=item * null
528
529	This module ensures that all data written to the DBM file is null
530	terminated. This is useful when you have a perl script that needs
531	to interoperate with a DBM file that a C program also uses. A fairly
532	common issue is for the C application to include the terminating null
533	in a string when it writes to the DBM file. This filter will ensure that
534	all data written to the DBM file can be read by the C application.
535
536	=back
537
538	=head1 NOTES
539
540	=head2 Maintain Round Trip Integrity
541
542	When writing a DBM filter it is I<very> important to ensure that it is
543	possible to retrieve all data that you have written when the DBM filter
544	is in place. In practice, this means that whatever transformation is
545	applied to the data in the Store method, the I<exact> inverse operation
546	should be applied in the Fetch method.
547
548	If you don't provide an exact inverse transformation, you will find that
549	code like this will not behave as you expect.
550
551	while (my ($k, $v) = each %hash)
552	{
553	...
554	}
555
556	Depending on the transformation, you will find that one or more of the
557	following will happen
558
559	=over 5
560
561	=item 1
562
563	The loop will never terminate.
564
565	=item 2
566
567	Too few records will be retrieved.
568
569	=item 3
570
571	Too many will be retrieved.
572
573	=item 4
574
575	The loop will do the right thing for a while, but it will unexpectedly fail.
576
577	=back
578
579	=head2 Don't mix filtered & non-filtered data in the same database file.
580
581	This is just a restatement of the previous section. Unless you are
582	completely certain you know what you are doing, avoid mixing filtered &
583	non-filtered data.
584
585	=head1 EXAMPLE
586
587	Say you need to interoperate with a legacy C application that stores
588	keys as C ints and the values and null terminated UTF-8 strings. Here
589	is how you would set that up
590
591	my $db = tie %hash, 'SDBM_File', ...
592
593	$db->Filter_Key_Push('int32') ;
594
595	$db->Filter_Value_Push('utf8');
596	$db->Filter_Value_Push('null');
597
598	=head1 SEE ALSO
599
600	<DB_File>, L<GDBM_File>, L<NDBM_File>, L<ODBM_File>, L<SDBM_File>, L<perldbmfilter>
601
602	=head1 AUTHOR
603
604	Paul Marquess <pmqs@cpan.org>
605