NAME

Tie::LLHash - Ordered hashes

DESCRIPTION

This class implements an ordered hash-like object. It's a cross between a Perl hash and a linked list. Use it whenever you want the speed and structure of a Perl hash, but the orderedness of a list.

See also Tie::IxHash by Gurusamy Sarathy. It's similar (it also does tied ordered hashes), but it has a different internal data structure and a different flavor of usage. Tie::IxHash stores its data internally as both a hash and an array in parallel. "Tie::LLHash" stores its data as a bidirectional linked list, making both inserts and deletes very fast. Tie::IxHash therefore makes your hash behave more like a list than "Tie::LLHash" does. This module keeps more of the hash flavor.

SYNOPSIS

 use Tie::LLHash;
 # A new empty ordered hash:
 tie (%hash, "Tie::LLHash");
 # A new ordered hash with stuff in it:
 tie (%hash2, "Tie::LLHash", key1=>$val1, key2=>$val2);
 # Allow easy insertions at the end of the hash:
 tie (%hash2, "Tie::LLHash", {lazy=>1}, key1=>$val1, key2=>$val2);
 # Add some entries:
 (tied %hash)->first('the' => 'hash');
 (tied %hash)->insert('here' => 'now', 'the');
 (tied %hash)->first('All' => 'the');
 (tied %hash)->insert('are' => 'right', 'the');
 (tied %hash)->insert('things' => 'in', 'All');
 (tied %hash)->last('by' => 'gum');
 $value = $hash{'things'}; # Look up a value
 $hash{'here'} = 'NOW';    # Set the value of an existing record
                           # or insert as last node in lazy mode
 $key = (tied %hash)->key_before('in');  # Returns the previous key
 $key = (tied %hash)->key_after('in');   # Returns the next key
 # Luxury routines:
 $key = (tied %hash)->current_key;
 $val = (tied %hash)->current_value;
 (tied %hash)->next;
 (tied %hash)->prev;
 (tied %hash)->reset;
 # If lazy mode is set, new keys will be added at the end.
 $hash{newkey} = 'newval';
 $hash{newkey2} = 'newval2';

METHODS

insert(key, value, previous_key)
This inserts a new key-value pair into the hash right after the "previous_key" key. If "previous_key" is undefined (or not supplied), this is exactly equivalent to "first(key, value)". If "previous_key" is defined, then it must be a string which is already a key in the hash - otherwise we'll croak().
first(key, value) (or) first()
Gets or sets the first key in the hash. Without arguments, simply returns a string which is the first key in the database. With arguments, it inserts a new key-value pair at the beginning of the hash.
last(key, value) (or) last()
Gets or sets the last key in the hash. Without arguments, simply returns a string which is the last key in the database. With arguments, it inserts a new key-value pair at the end of the hash.
key_before(key)
Returns the name of the key immediately before the given key. If no keys are before the given key, returns "undef".
key_after(key)
Returns the name of the key immediately after the given key. If no keys are after the given key, returns "undef".
current_key()
When iterating through the hash, this returns the key at the current position in the hash.
current_value()
When iterating through the hash, this returns the value at the current position in the hash.
next()
Increments the current position in the hash forward one item. Returns the new current key, or "undef" if there are no more entries.
prev()
Increments the current position in the hash backward one item. Returns the new current key, or "undef" if there are no more entries.
reset()
Resets the current position to be the start of the order. Returns the new current key, or "undef" if there are no keys.

ITERATION TECHNIQUES

Here is a smattering of ways you can iterate over the hash. I include it here simply because iteration is probably important to people who need ordered data.

 while (($key, $val) = each %hash) {
    print ("$key: $val\n");
 }
 foreach $key (keys %hash) {
    print ("$key: $hash{$key}\n");
 }
 my $obj = tied %hash;  # For the following examples
 $key = $obj->reset;
 while (exists $hash{$key}) {
    print ("$key: $hash{$key}\n");
    $key = $obj->next;
 }
 $obj->reset;
 while (exists $hash{$obj->current_key}) {
    $key = $obj->current_key;
    print ("$key: $hash{$key}\n");
    $obj->next;
 }

WARNINGS

Unless you're using lazy mode, don't add new elements to the hash by simple assignment, a la "$hash{$new_key} = $value", because "Tie::LLHash" won't know where in the order to put the new element and will die.
Evaluating tied hash in scalar context wasn't implemented until Perl 5.8.3, so on earlier Perl versions it will always return 0, even if hash is not empty.

TODO

Add support for NEXTKEY and next with additional argument <http://perldoc.perl.org/perltie.html#NEXTKEY-this%2c-lastkey>.
I could speed up the keys() routine in a scalar context if I knew how to sense when NEXTKEY is being called on behalf of keys(). Not sure whether this is possible.

COPYRIGHT AND LICENSE

Ken Williams <kenahoo@gmail.com>