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package Tie::SubstrHash 1.01; =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 strict; use warnings; no warnings 'experimental::builtin'; use Carp; sub TIEHASH { my ($pack, $klen, $vlen, $tsize) = @_; my $rlen = 1 + $klen + $vlen; $tsize = [$tsize, findgteprime($tsize * 1.1)]; # Allow 10% empty. my $self = bless ["\0", $klen, $vlen, $tsize, $rlen, 0, -1]; $self->[0] x= $rlen * $tsize->[1]; $self; } sub CLEAR { my ($self) = @_; $self->[0] = "\0" x ($self->[4] * $self->[3][1]); $self->[5] = 0; $self->[6] = -1; } sub FETCH { my ($self, $key) = @_; my (undef, $klen, $vlen, $tsize, $rlen) = @$self; my $hashbase = my $hash = hashkey($key, $klen, $tsize); while (1) { my $offset = $hash * $rlen; my $record = substr($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); } $hash = rehash($hash, $hashbase, $tsize); } } sub STORE { my ($self, $key, $val) = @_; my (undef, $klen, $vlen, $tsize, $rlen) = @$self; 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; my $hashbase = my $hash = hashkey($key, $klen, $tsize); while (1) { my $offset = $hash * $rlen; my $record = substr($self->[0], $offset, $rlen); if (ord($record) == 0) { $record = "\2". $key . $val; die "panic" unless length($record) == $rlen; $writeoffset //= $offset; substr($self->[0], $writeoffset, $rlen) = $record; ++$self->[5]; return; } elsif (ord($record) == 1) { $writeoffset //= $offset; } elsif (substr($record, 1, $klen) eq $key) { $record = "\2". $key . $val; die "panic" unless length($record) == $rlen; substr($self->[0], $offset, $rlen) = $record; return; } $hash = rehash($hash, $hashbase, $tsize); } } sub DELETE { my ($self, $key) = @_; my (undef, $klen, $vlen, $tsize, $rlen) = @$self; my $hashbase = my $hash = hashkey($key, $klen, $tsize); while (1) { my $offset = $hash * $rlen; my $record = substr($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]; } $hash = rehash($hash, $hashbase, $tsize); } } sub FIRSTKEY { my ($self) = @_; $self->[6] = -1; goto &NEXTKEY; } sub NEXTKEY { my ($self) = @_; my (undef, $klen, $vlen, $tsize, $rlen, $entries, $iterix) = @$self; 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); } $self->[6] = -1; undef; } sub EXISTS { croak "Tie::SubstrHash does not support exists()"; } sub hashkey { my ($key, $klen, $tsize) = @_; croak(qq/Key "$key" is not $klen characters long/) if length($key) != $klen; my $hash = 2; for (unpack('C*', $key)) { $hash = $hash * 33 + $_; $hash = _hashwrap($hash, $tsize) if $hash >= 1e13; } $hash = _hashwrap($hash, $tsize) if $hash >= $tsize->[1]; $hash ||= 1; return $hash; } sub _hashwrap { my ($hash, $tsize) = @_; return $hash - int($hash / $tsize->[1]) * $tsize->[1]; } sub rehash { my ($hash, $hashbase, $tsize) = @_; $hash += $hashbase; $hash -= $tsize->[1] if $hash >= $tsize->[1]; return $hash; } # 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 = builtin::ceil(shift); return 2 if $num <= 2; $num++ unless $num % 2; my $sqrtnum = int sqrt $num; my $sqrtnumsquared = $sqrtnum * $sqrtnum; NUM: for (;; $num += 2) { if ($sqrtnumsquared < $num) { $sqrtnum++; $sqrtnumsquared = $sqrtnum * $sqrtnum; } for (my $i = 3; $i <= $sqrtnum; $i += 2) { next NUM unless $num % $i; } return $num; } } 1;