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1 package DBM_Filter ; 2 3 use strict; 4 use warnings; 5 our $VERSION = '0.02'; 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 $class =~ /::/; 93 94 no strict 'refs'; 95 # does the "DBM_Filter::$class" exist? 96 if ( ! defined %{ "$class}::"} ) { 97 # Nope, so try to load it. 98 eval " require $class ; " ; 99 croak "$caller: Cannot Load DBM Filter '$class': $@" if $@; 100 } 101 102 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
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