Judy - Library for creating and accessing dynamic arrays


The Judy family of functions supports fully dynamic arrays. These arrays may be indexed by a 32- or 64-bit word (depending on processor word size) (Judy::1, Judy::L), a null terminated string (Judy::SL), or an ordinary perl string (Judy::HS).

Judy arrays are both speed- and memory-efficient, with no tuning or configuration required, across a wide range of key set types (sequential, periodic, clustered, random). Judy's speed and memory usage are typically better than other data storage models such as skiplists, linked lists, binary, ternary, b-trees, or even hashing, and improves with very large data sets.

The memory used by a Judy array is nearly proportional to the population (number of elements).

Since an initial (empty) Judy array is represented by a null pointer, it is possible to construct an array of Judy arrays. In other words, a Judy array's Values can be pointers to other Judy arrays. This makes it very simple to construct an array with an arbitrary number of dimensions or Index sizes.

The libJudy author believes JudyHS is a good replacement for a hashing method when resizing the hash table is done during population growth. A correctly tuned hash method with a static hash table size and population is unbeatable for speed. However, Judy::HS will perform better than a hashing method with smaller and larger populations than the optimum hash table size. JudyHS does not have a degenerate performance case where knowledge of the hash algorithm can be exploited. (I.E. JudyHS does not use a linked list to handle hash collisions, it uses a tree of JudyL arrays and a virtual hash table size of 4 billion).



This can be thought of as a bit vector. For a comparison between Judy::1 and "vec" in perlfunc, take a look at

    # Turn the 43rd bit on. A bit like:
    #   vec( $str, 42, 1 ) = 1

Maps an integer to another integer. This is sort of like a very compact perl hash where the only allowed keys and values are integers.

    # A bit like:
    #   $array[ 42 ] = 9000

Maps null terminated strings to integers.

    # A bit like:
    #   $hash{world} = 9000

Maps perl strings to integers.

    # A bit like:
    #   $hash{world} = 9000

Multi-dimensional Judy::L/Judy::SL/Judy::HS Arrays

Storing a pointer to another Judy::L array in a Judy::L array's Value is a simple way to support dynamic multi-dimensional Judy::L arrays. These arrays (or trees) built using Judy::L arrays are very fast and memory efficient. (In fact, that is how JudySL and JudyHS are implemented). An arbitrary number of dimensions can be realized this way. To terminate the number of dimensions (or tree), the Value pointer is marked to NOT point to another Judy array. A Judy::JLAP_INVALID flag is used in the least significant bit(s) of the pointer. After the flag Judy::JLAP_INVALID is removed, it is used as a pointer to the users data.

Note: The current version of Judy.h changed this flag from 0x4 to 0x1 to allow for a malloc() that does not deliver memory on an 8 byte aligned boundry (such as old v algrind).

The following example code segment can be used to dive into a multi-dimensional Judy::L using an API similar to Tye McQueen's Data::Diver. This makes a Judy::HS object and looks past the public API as an example of a multi-dimensional Judy::* structure.

    # For kicks, allocate a Judy::HS object and look inside it a
    # little bit.
    use Judy::HS;
    Judy::HS::Set( my ($judy), 'abcd', 42 );
    Dive( $judy, 4 );

    use Judy;
    use Judy::L;
    sub Dive {
        my ( $judy, @walk ) = @_;
        my ( $pvalue, $value );
        for my $key ( @walk ) {
            return if ! $judy;
            # Advance to next dimension.
            ( $pvalue, $value ) = Judy::L::Get( $judy, $key );
            # Check if pointer to user buffer
            last if $value & Judy::JLAP_INVALID;
            $judy = $value;
        if ( $value & JLAP_INVALID ) {
            # Remove our flag.
            $value &= ~ Judy::JLAP_INVALID;
            # Return the value.
            printf "User object pointer is 0x%x at 0x%x\n", $value, $pvalue;
        else {
            warn sprintf "Judy::* object pointer is 0x%x at 0x%x\n", $value, $pvalue;
        return ( $pvalue, $value );

Note: This works because malloc() guarantees to return a pointer with the least bit(s) == 0x0. You must remove JLAP_INVALID before using the pointer.



SEE ALSO - the C library home page
Judy::1 - maps an integer to a bit
Judy::L - maps an integer to an integer/pointer
Judy::SL - maps a null terminated string to an integer/pointer
Judy::HS - maps a string to an integer/pointer
A 3 HOUR TECHNICAL DESCRIPTION (out of date and a bit corny) may be found at


Locations of interest include: -- project downloads file:/usr/share/doc/Judy/ -- for HTML version of man pages. /usr/share/doc/Judy/demo/ -- demonstration program source files.

The author attempted to write interesting application notes using advanced features of Judy. They may be found at " (Some may be out of date).


File '%s', line %d: %s(), JU_ERRNO_* == %d, ID == %d

See the header file Judy.h from the Judy C source library. You already have a local copy of this to have been able to build this perl library.

Sorry, can't use keys longer than %d for now. This is a bug.
Coercing %d to 0. Can't use negative values as keys.
Truncating %d to %d because your number is larger than fits in a signed integer
Truncating %d to %u because your number is larger than fits in an unsigned integer
Truncating %d to %d because your number is smaller than fits in a signed integer
Dropping UTF8 flag for '%s'


Please report any bugs or feature requests to bug-Judy-HS at, or through the web interface at I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


You can find documentation for this module with the perldoc command.

  perldoc Judy

You can also look for information at:


Doug Baskins, totally.

Michael Schwern for writing Alien::SVN which made this possible.

Tye McQueen for inspiring the minimal API.

Yitzchak Scott-Thoennes for reminding me that perl's magic requires extra care and attention.


Copyright 2012 Josh Jore, all rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


This source is in Github: git://


Judy was invented by Doug Baskins (dougbaskins .AT, and implemented by Hewlett-Packard. (Note: Judy is named for the inventor's sister, after discarding many proposed names.)

The perl wrapper was written by Josh Jore