/ 
Hash-Ordered-0.003-TRIAL
River stage two • 9 direct dependents • 34 total dependents
/ Hash::Ordered

NAME

Hash::Ordered - A compact, pure-Perl ordered hash class

VERSION

version 0.003

SYNOPSIS

use Hash::Ordered;

my $oh = Hash::Ordered->new( a => 1 );

$oh->get( 'a' );
$oh->set( 'a' => 2 );

$oh->exists( 'a' );
$val = $oh->delete( 'a' );

@keys  = $oh->keys;
@vals  = $oh->values;
@pairs = $oh->as_list

$oh->push( c => 3, d => 4 );
$oh->unshift( e => 5, f => 6 );

( $k, $v ) = $oh->pop;
( $k, $v ) = $oh->shift;

$iter = $oh->iterator;
while( ( $k, $v ) = $iter->() ) { ... }

$copy     = $oh->clone;
$subset   = $oh->clone( qw/c d/ );
$reversed = $oh->clone( reverse $oh->keys );

@value_slice = $oh->values(  qw/c f/ ); # qw/3 6/
@pairs_slice = $oh->as_list( qw/f e/ ); # qw/f 6 e 5/

DESCRIPTION

This module implements an ordered hash, meaning that it associates keys with values like a Perl hash, but keeps the keys in a consistent order. Because it is implemented as an object and manipulated with method calls, it is much slower than a Perl hash. This is the cost of keeping order.

METHODS

new

$oh = Hash::Ordered->new;
$oh = Hash::Ordered->new( @pairs );

Constructs an object, with an optional list of key-value pairs.

clone

$oh2 = $oh->clone;
$oh2 = $oh->clone( @keys );

Creates a shallow copy of an ordered hash object. If no arguments are given, it produces an exact copy. If a list of keys is given, the new object includes only those keys in the given order. Keys that aren't in the original will have the value undef.

keys

@keys = $oh->keys;

Returns the ordered list of keys.

values

@values = $oh->values;
@values = $oh->values( @keys );

Returns an ordered list of values. If no arguments are given, returns the ordered values of the entire hash. If a list of keys is given, returns values in order corresponding to those keys. If a key does not exist, undef will be returned for that value.

get

$value = $oh->get("some key");

Returns the value associated with the key, or undef if it does not exist in the hash.

set

$oh->set("some key" => "some value");

Associates a value with a key and returns the value. If the key does not already exist in the hash, it will be added at the end.

exists

if ( $oh->exists("some key") ) { ... }

Test if some key exists in the hash (without creating it).

delete

$value = $oh->delete("some key");

Removes a key-value pair from the hash and returns the value. This is expensive, as the ordered list of keys has to be updated.

clear

$oh->clear;

Removes all key-value pairs from the hash. Returns undef in scalar context or an empty list in list context.

Added in version 0.003.

push

$oh->push( one => 1, two => 2);

Add a list of key-value pairs to the end of the ordered hash. If a key already exists in the hash, it will be deleted and re-inserted at the end with the new value.

Returns the number of keys after the push is complete.

pop

($key, $value) = $oh->pop;

Removes and returns the last key-value pair in the ordered hash.

unshift

$oh->unshift( one => 1, two => 2 );

Adds a list of key-value pairs to the beginning of the ordered hash. If a key already exists, it will be deleted and re-inserted at the beginning with the new value.

Returns the number of keys after the unshift is complete.

shift

($key, $value) = $oh->shift;

Removes and returns the first key-value pair in the ordered hash.

merge

$oh->merge( one => 1, two => 2 );

Merges a list of key-value pairs into the ordered hash. If a key already exists, its value is replaced. Otherwise, the key-value pair is added at the end of the hash.

as_list

@pairs = $oh->as_list;
@pairs = $oh->as_list( @keys );

Returns an ordered list of key-value pairs. If no arguments are given, all pairs in the hash are returned. If a list of keys is given, the returned list includes only those key-value pairs in the given order. Keys that aren't in the original will have the value undef.

iterator

$iter = $oh->iterator;
$iter = $oh->iterator( reverse $oh->keys ); # reverse

while ( my ($key,$value) = $iter->() ) { ... }

Returns a code reference that returns a single key-value pair (in order) on each invocation, or the empty list if all keys are visited.

If no arguments are given, the iterator walks the entire hash in order. If a list of keys is provided, the iterator walks the hash in that order. Unknown keys will return undef.

The list of keys to return is set when the iterator is generator. Keys added later will not be returned. Delete keys will return undef.

OVERLOADING

Boolean

if ( $oh ) { ... }

When used in boolean context, a Hash::Ordered object is true if it has any entries and false otherwise.

MOTIVATION

For a long time, I used Tie::IxHash for ordered hashes, but I grew frustrated with things it lacked, like a cheap way to copy an IxHash object or a convenient iterator when not using the tied interface. As I looked at its implementation, it seemed more complex than I though it needed, with an extra level of indirection that slows data access.

Given that frustration, I started experimenting with the simplest thing I thought could work for an ordered hash: a hash of key-value pairs and an array with key order.

As I worked on this, I also started searching for other modules doing similar things. What I found fell broadly into two camps: modules based on tie (even if they offered an OO interface), and pure OO modules. They all either lacked features I deemed necessary or else seemed overly-complex in either implementation or API.

Hash::Ordered attempts to find the sweet spot with simple implementation, reasonably good efficiency for most common operations, and a rich, intuitive API.

SEE ALSO

This section describes other ordered-hash modules I found on CPAN. For benchmarking results, see Hash::Ordered::Benchmarks.

Tie modules

The following modules offer some sort of tie interface. I don't like ties, in general, because of the extra indirection involved over a direct method call, but if you are willing to pay that penalty, you might want to try one of these.

Tie::IxHash is probably the most well known and includes an OO API. If its warts and performance profile aren't a problem, it might serve.

Tie::LLHash I haven't used, but the linked-list implementation might be worthwhile if you expect to do a lot of deletions.

Tie::Hash::Indexed is implemented in XS and thus seems promising if pure-Perl isn't a criterion; it often fails tests on Perl 5.18 and above due to the hash randomization change.

These other modules have very specific designs/limitations and I didn't find any of them suitable for general purpose use:

Other ordered hash modules

Other modules stick with an object-oriented API, with a wide variety of implementation approaches.

Array::AsHash is essentially an inverse implementation from Hash::Ordered. It keeps pairs in an array and uses a hash to index into the array. I think this indirection makes hash-like operations slower, but getting the list of pairs back out is much faster. It takes an arrayref to initialize, but can shallow copy it if needed. I think this is a reasonable alternative if static construction and listing out contents is more common than individual item access.

These other modules have restrictions or particularly complicated implementations (often relying on tie) and thus I didn't think any of them really suitable for use:

  • Array::Assign — arrays with named access; restricted keys

  • Array::OrdHash — overloads array/hash deref and uses internal tied data

  • Data::Pairs — array of key-value hashrefs; allows duplicate keys

  • Data::OMap — array of key-value hashrefs; no duplicate keys

  • Data::XHash — blessed, tied hashref with doubly-linked-list

SUPPORT

Bugs / Feature Requests

Please report any bugs or feature requests through the issue tracker at https://github.com/dagolden/Hash-Ordered/issues. You will be notified automatically of any progress on your issue.

Source Code

This is open source software. The code repository is available for public review and contribution under the terms of the license.

https://github.com/dagolden/Hash-Ordered

git clone https://github.com/dagolden/Hash-Ordered.git

AUTHOR

David Golden <dagolden@cpan.org>

CONTRIBUTORS

  • Benct Philip Jonsson <bpjonsson@gmail.com>

  • Mario Roy <marioeroy@gmail.com>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2014 by David Golden.

This is free software, licensed under:

The Apache License, Version 2.0, January 2004