| package Tie::SubstrHash; |
| |
| our $VERSION = '1.00'; |
| |
| =head1 NAME |
| |
| Tie::SubstrHash - Fixed-table-size, fixed-key-length hashing |
| |
| =head1 SYNOPSIS |
| |
| require Tie::SubstrHash; |
| |
| tie %myhash, 'Tie::SubstrHash', $key_len, $value_len, $table_size; |
| |
| =head1 DESCRIPTION |
| |
| The B<Tie::SubstrHash> package provides a hash-table-like interface to |
| an array of determinate size, with constant key size and record size. |
| |
| Upon tying a new hash to this package, the developer must specify the |
| size of the keys that will be used, the size of the value fields that the |
| keys will index, and the size of the overall table (in terms of key-value |
| pairs, not size in hard memory). I<These values will not change for the |
| duration of the tied hash>. The newly-allocated hash table may now have |
| data stored and retrieved. Efforts to store more than C<$table_size> |
| elements will result in a fatal error, as will efforts to store a value |
| not exactly C<$value_len> characters in length, or reference through a |
| key not exactly C<$key_len> characters in length. While these constraints |
| may seem excessive, the result is a hash table using much less internal |
| memory than an equivalent freely-allocated hash table. |
| |
| =head1 CAVEATS |
| |
| Because the current implementation uses the table and key sizes for the |
| hashing algorithm, there is no means by which to dynamically change the |
| value of any of the initialization parameters. |
| |
| The hash does not support exists(). |
| |
| =cut |
| |
| use Carp; |
| |
| sub TIEHASH { |
| my $pack = shift; |
| my ($klen, $vlen, $tsize) = @_; |
| my $rlen = 1 + $klen + $vlen; |
| $tsize = [$tsize, |
| findgteprime($tsize * 1.1)]; # Allow 10% empty. |
| local $self = bless ["\0", $klen, $vlen, $tsize, $rlen, 0, -1]; |
| $$self[0] x= $rlen * $tsize->[1]; |
| $self; |
| } |
| |
| sub CLEAR { |
| local($self) = @_; |
| $$self[0] = "\0" x ($$self[4] * $$self[3]->[1]); |
| $$self[5] = 0; |
| $$self[6] = -1; |
| } |
| |
| sub FETCH { |
| local($self,$key) = @_; |
| local($klen, $vlen, $tsize, $rlen) = @$self[1..4]; |
| &hashkey; |
| for (;;) { |
| $offset = $hash * $rlen; |
| $record = substr($$self[0], $offset, $rlen); |
| if (ord($record) == 0) { |
| return undef; |
| } |
| elsif (ord($record) == 1) { |
| } |
| elsif (substr($record, 1, $klen) eq $key) { |
| return substr($record, 1+$klen, $vlen); |
| } |
| &rehash; |
| } |
| } |
| |
| sub STORE { |
| local($self,$key,$val) = @_; |
| local($klen, $vlen, $tsize, $rlen) = @$self[1..4]; |
| croak("Table is full ($tsize->[0] elements)") if $$self[5] > $tsize->[0]; |
| croak(qq/Value "$val" is not $vlen characters long/) |
| if length($val) != $vlen; |
| my $writeoffset; |
| |
| &hashkey; |
| for (;;) { |
| $offset = $hash * $rlen; |
| $record = substr($$self[0], $offset, $rlen); |
| if (ord($record) == 0) { |
| $record = "\2". $key . $val; |
| die "panic" unless length($record) == $rlen; |
| $writeoffset = $offset unless defined $writeoffset; |
| substr($$self[0], $writeoffset, $rlen) = $record; |
| ++$$self[5]; |
| return; |
| } |
| elsif (ord($record) == 1) { |
| $writeoffset = $offset unless defined $writeoffset; |
| } |
| elsif (substr($record, 1, $klen) eq $key) { |
| $record = "\2". $key . $val; |
| die "panic" unless length($record) == $rlen; |
| substr($$self[0], $offset, $rlen) = $record; |
| return; |
| } |
| &rehash; |
| } |
| } |
| |
| sub DELETE { |
| local($self,$key) = @_; |
| local($klen, $vlen, $tsize, $rlen) = @$self[1..4]; |
| &hashkey; |
| for (;;) { |
| $offset = $hash * $rlen; |
| $record = substr($$self[0], $offset, $rlen); |
| if (ord($record) == 0) { |
| return undef; |
| } |
| elsif (ord($record) == 1) { |
| } |
| elsif (substr($record, 1, $klen) eq $key) { |
| substr($$self[0], $offset, 1) = "\1"; |
| return substr($record, 1+$klen, $vlen); |
| --$$self[5]; |
| } |
| &rehash; |
| } |
| } |
| |
| sub FIRSTKEY { |
| local($self) = @_; |
| $$self[6] = -1; |
| &NEXTKEY; |
| } |
| |
| sub NEXTKEY { |
| local($self) = @_; |
| local($klen, $vlen, $tsize, $rlen, $entries, $iterix) = @$self[1..6]; |
| for (++$iterix; $iterix < $tsize->[1]; ++$iterix) { |
| next unless substr($$self[0], $iterix * $rlen, 1) eq "\2"; |
| $$self[6] = $iterix; |
| return substr($$self[0], $iterix * $rlen + 1, $klen); |
| } |
| $$self[6] = -1; |
| undef; |
| } |
| |
| sub EXISTS { |
| croak "Tie::SubstrHash does not support exists()"; |
| } |
| |
| sub hashkey { |
| croak(qq/Key "$key" is not $klen characters long/) |
| if length($key) != $klen; |
| $hash = 2; |
| for (unpack('C*', $key)) { |
| $hash = $hash * 33 + $_; |
| &_hashwrap if $hash >= 1e13; |
| } |
| &_hashwrap if $hash >= $tsize->[1]; |
| $hash = 1 unless $hash; |
| $hashbase = $hash; |
| } |
| |
| sub _hashwrap { |
| $hash -= int($hash / $tsize->[1]) * $tsize->[1]; |
| } |
| |
| sub rehash { |
| $hash += $hashbase; |
| $hash -= $tsize->[1] if $hash >= $tsize->[1]; |
| } |
| |
| # using POSIX::ceil() would be too heavy, and not all platforms have it. |
| sub ceil { |
| my $num = shift; |
| $num = int($num + 1) unless $num == int $num; |
| return $num; |
| } |
| |
| # See: |
| # |
| # http://www-groups.dcs.st-andrews.ac.uk/~history/HistTopics/Prime_numbers.html |
| # |
| |
| sub findgteprime { # find the smallest prime integer greater than or equal to |
| use integer; |
| |
| my $num = ceil(shift); |
| return 2 if $num <= 2; |
| |
| $num++ unless $num % 2; |
| my $i; |
| my $sqrtnum = int sqrt $num; |
| my $sqrtnumsquared = $sqrtnum * $sqrtnum; |
| |
| NUM: |
| for (;; $num += 2) { |
| if ($sqrtnumsquared < $num) { |
| $sqrtnum++; |
| $sqrtnumsquared = $sqrtnum * $sqrtnum; |
| } |
| for ($i = 3; $i <= $sqrtnum; $i += 2) { |
| next NUM unless $num % $i; |
| } |
| return $num; |
| } |
| } |
| |
| 1; |