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PHP HyperLogLog Extension

Warning

This is no longer maintained and will probably not be updated for PHP 8 and beyond. The repo has been moved to https://git.sr.ht/~shabbyrobe/phphll and will be removed from Github at some point in the future.

This project lifts the hyperloglog.c implementation from Redis 2.8.19 (http://redis.io) and wraps it in a PHP7 extension.

This is a C extension, not a pure PHP implementation. Use this extension if your HyperLogLog needs are performance critical and you are in control of your PHP installation. If neither of those things is true, you may be better served by this pure PHP implementation.

It is tested with PHP 7.0 to 7.4. If you need a version that supports PHP 5, try this commit <https://github.com/shabbyrobe/phphll/commit/d3b30b00b5fe30b7da689dea288ec1b144ac5808>_.

Redis is copyright (c) 2014 Salvatore Sanfilippo. Full license details are found in COPYING at the root of this project. Further credit is due for the exceptional quality of both the C code and comments found in the Redis project.

HyperLogLog is neatly described by Salvatore himself in his blog post announcing support in Redis (1):

HyperLogLog is remarkable as it provides a very good approximation of the cardinality of a set even using a very small amount of memory. In the Redis implementation it only uses 12kbytes per key to count with a standard error of 0.81%, and there is no limit to the number of items you can count, unless you approach 2^64 items (which seems quite unlikely).

HyperLogLogs are magical! See for yourself... the memory usage in the following example stays stubbornly and hilariously low:

<?php
$h = new HyperLogLog();
for ($i=1; $i<100000000; $i++, $h->add($i)) {
    if ($i % 30000 == 0) {
        $c = $h->count();
        printf("\rmem: %d, count: %d, hllcount: %d, diff: %d          ", 
            memory_get_usage(), $i, $c, abs($c - $i));
    }
}

This extension is intended for situations where you want the flexibility of this data structure without the overhead of communicating with Redis.

Warnings and Disclaimers

  • No guarantees of thread safety are currently made - Redis itself is designed to be run in a single process environment. Assume unsafe until demonstrated otherwise.

  • This extension has been tested using PHP 7.0, 7.1 and 7.2 on a 64-bit machine. YMMV - please let me know if there are any failing or any missing tests.

  • Dumping and loading methods are provided, but if the internal structure of the HyperLogLog representation changes in any backwards incompatible way, no guarantees are currently made that dumping and loading will work from version to version, and no compatibility methods will be provided.

  • HyperLogLog objects are not currently cloneable (though this is in the works)

  • This no longer works with any PHP 5 version. The last version compatible with PHP 5.4 and up can be downloaded here: https://github.com/shabbyrobe/phphll/tree/5.4-compatible The last version compatible with 5.2 and 5.3 can be downloaded here: https://github.com/shabbyrobe/phphll/tree/5.2-compatible

Bogus Benchmarks

Here are some numbers for the time it takes to use HyperLogLog to calculate the cardinality using Redis + PHP, Redis + C, and this extension. You can make some pretty outrageous time savings if you're using HyperLogLog intensively and don't require Redis.

This test was run using a file containing 31793623 random items with a cardinality of 243. Feel free to point out all the many terrible things I've done wrong here, it's the only way I'll learn:

## PHP, redisent, one PFADD per entry
$ cat /tmp/ids | /usr/bin/time -f '%e' php junk/hllcnt-redis.php 1
1504.13

## C, hiredis, one PFADD per entry
$ cat /tmp/ids | /usr/bin/time -f '%e' junk/hllcnt-hiredis 1
859.44

## PHP, redisent, PFADD in batches of 2000
$ cat /tmp/ids | /usr/bin/time -f '%e' php junk/hllcnt-redis.php 2000
614.21

## C, hiredis, PFADD in batches of 2000
$ cat /tmp/ids | /usr/bin/time -f '%e' junk/hllcnt-hiredis 2000
122.82

## PHP, hll.so, one pfAdd() per entry
$ cat /tmp/ids | /usr/bin/time -f '%e' php -d 'extension=php/modules/hll.so' junk/hllcnt.php
67.27

## C, direct, one pfAdd() per entry
$ cat /tmp/ids | /usr/bin/time -f '%e' ./hllcnt 
5.01

Building / Installing

Provided you have all of the relevant PHP development tools available, build using the following command:

make php

The resulting library will be in php/modules/hll.so. You can install it like so:

cd php; sudo make install

Then add extension=hll.so to your php.ini file.

Usage

Counting:

<?php
$hll = new HyperLogLog();
$hll->add(["abc", "def", "ghi", "abc", "ghi"]);
var_dump($hll->count());
// int(3)

Arbitrary size sets, but slightly inaccurate:

<?php
$hll = new HyperLogLog();
for ($i=0; $i<10000000; $i++) {
    $hll->add($i);
}

var_dump($hll->count());
// int(9972088)

var_dump(memory_get_usage());
// int(460384)

Interoperating with Redis:

Warning

This can only be guaranteed to work if you are using the same version of Redis that this was built from.

<?php
$item = $argv[1];
$redis = new redisent\Redis;

// Redis treats HyperLogLogs as simple strings, so we can get and set as we please
$hllDump = $redis->get('testhll');
$hll = new HyperLogLog($hllDump ?: null);

$hll->add($item);
var_dump($hll->count());

$redis->set('testhll', $hll->dump());

API

HyperLogLog class

The HyperLogLog class has the following features:

  • serializable
  • fluent

And the following limitations:

  • not cloneable (yet)
  • not comparable

HyperLogLog::__construct()

Creates a new HyperLogLog:

  • HyperLogLog HyperLogLog::__construct ([ bool $canBeSparse = false ])
  • HyperLogLog HyperLogLog::__construct ([ string $hllDump ])

Parameters:

  • canBeSparse: Defaults to false.

    If true, the underlying HyperLogLog will be allowed to encode as a sparse representation. This takes less memory, but is considerably slower to work with. Once the structure exceeds a certain internally defined size, it is promoted automatically.

    See https://github.com/antirez/redis/blob/2.8.11/src/hyperloglog.c#L56 for some excellent documentation on the sparse and dense encodings used by Redis.

  • hllDump: A string created by HyperLogLog::dump(). Used to reconstruct a HyperLogLog from a dumped representation.

HyperLogLog::add()

Adds one or more scalar elements to a HyperLogLog object, returning the HyperLogLog::

  • HyperLogLog HyperLogLog::add( scalar $value , [ bool &$updated ])
  • HyperLogLog HyperLogLog::add( array $values , [ bool &$updated ])

$updated will be set to true if the underlying data structure was updated, false otherwise. This does not indicate whether the count was updated, only that the internal data structure has changed. You do not need to pass this.

Warning

This API is unstable. It may end up returning $updated instead of taking it by reference. It may also allow a variable argument variant.

<?php
$hll = new HyperLogLog();
$hll->add('a')->add(['b', 'c', 'd'])->add('d', $updated);
var_dump($updated);
// bool(false)

HyperLogLog::count()

Returns the cardinality of the HyperLogLog:

  • int HyperLogLog::count()

You can use hll_count($hll1, $hll2) to perform a merged count:

<?php
$hll1 = (new HyperLogLog())->add(['a', 'b']);
$hll2 = (new HyperLogLog())->add(['a', 'c']);
var_dump(hll_count($hll1, $hll2));
// int(3)

HyperLogLog::merge()

Merges the HyperLogLog with one or more existing HyperLogLogs, returning the called HyperLogLog:

  • HyperLogLog HyperLogLog::merge ( mixed $hyperLogLog [ , mixed $... ])
  • HyperLogLog HyperLogLog::merge ( array $hyperLogLogs )
<?php
$hll1 = (new HyperLogLog())->add(['foo', 'bar']);
$hll2 = (new HyperLogLog())->add(['bar', 'baz']);

assert($hll1->merge($hll2)->count() == 3);

The passed HyperLogLogs can be in either Object or resource form:

<?php    
$hll1 = new HyperLogLog();
$hll1->merge([hll_create(), new HyperLogLog()]);

HyperLogLog::promote()

Ensures a HyperLogLog has a dense representation:

  • HyperLogLog HyperLogLog::promote( void )

If the HyperLogLog is already dense, this function does nothing.

See https://github.com/antirez/redis/blob/2.8.11/src/hyperloglog.c#L56 for some excellent documentation on the sparse and dense encodings used by Redis.

HyperLogLog::dump()

Dumps a binary representation of the underlying HyperLogLog:

  • string HyperLogLog::dump( void );

The return type will be a string, but the string will contain binary data and contains \0 characters that should not be ignored.

Warning

This is a direct dump of Redis' internal representation of the HyperLogLog. The dump can only be guaranteed to work with the version of Redis from which the hyperloglog.c file was taken. It should not be used for anything permanent.

You can pass the resulting dump back into the constructor:

<?php
$h1 = (new HyperLogLog())->add(['a', 'b', 'c']);
assert($h1->count() == 3);

$h2 = new HyperLogLog($h->dump());
assert($h2->count() == 3);

HyperLogLog::info()

Returns an array of information about a HyperLogLog:

  • array HyperLogLog::info ( void )
<?php
$h = hll_create();
var_dump(hll_info($h));
// array(1) {
//    ["encoding"]=>string(5) "dense"
// }

HyperLogLog->hll

The HyperLogLog resource used by the class. This can be manipulated using the procedural functions documented below.

Procedural Interface

Each method on HyperLogLog has a procedural analog that operates on a HyperLogLog resource rather than an instance of the HyperLogLog class:

resource hll_create ([ bool $allowSparse = false ])

bool hll_add ( resource $hll , scalar $value )
bool hll_add ( resource $hll , array $values )

int hll_count ( mixed $hll [ , mixed $... ])

resource hll_merge( mixed $hyperLogLog1 , mixed $hyperLogLog2 [ , mixed $... ])
resource hll_merge( array $hyperLogLogs )

void hll_promote ( resource $hll )

string hll_dump ( resource $hll )

resource hll_load ( string $hllDump )

array hll_info ( resource $hll )

All signatures in the above API that accept a mixed hyperloglog parameter rather than a resource parameter will accept either the resource or the object version, or a mixture thereof:

<?php
$h1 = (new HyperLogLog())->add('a');
$h2 = hll_create();
hll_add($h2, 'foo');

assert(hll_count($h1, $h2) == 2);

hll_load

Creates a HyperLogLog resource from a string representation created by hll_dump or HyperLogLog::dump():

  • resource hll_load( string $dump )

Analog of new HyperLogLog(hll_dump($hll))