HyperLogLog for PHP

A lightweight and dependency-free PHP implementation of the HyperLogLog probabilistic data structure for approximate cardinality estimation.

HyperLogLog allows you to estimate the number of distinct elements in very large datasets while using only a small, fixed amount of memory.

Features

• 🚀 Fast approximate distinct counting
• 📦 Zero dependencies
• 🔧 Configurable number of registers (counterBits)
• 🔐 Configurable hashing algorithm (xxh3, sha256, md5, etc.)
• 📊 Theoretical error rate calculation
• 🧮 Small and large cardinality bias corrections
• ✅ Strict types and fully documented source code

Requirements

• PHP 8.0, 8.1, 8.2, 8.3, 8.4 and 8.5
• No external dependencies

Note: The xxh3 and xxh128 hash algorithms are available only when supported by your PHP version and build. If unavailable, you can use any other algorithm returned by hash_algos(), such as sha256, sha512, or md5.

Installation

Install via Composer:

composer require hichxm/hyperloglog

Basic Usage

<?php

use Hichxm\HyperLogLog\HyperLogLog;

$hll = new HyperLogLog();

$hll->add('apple');
$hll->add('banana');
$hll->add('orange');
$hll->add('apple'); // duplicate

echo $hll->count();

The returned value is an approximation of the number of unique elements.

Constructor

new HyperLogLog(
    int $counterBits = 5,
    string $hashAlgorithm = 'xxh3'
);

Parameters

Parameter	Description
counterBits	Number of bits used to select registers. The number of registers is 2^counterBits.
hashAlgorithm	Any hashing algorithm supported by PHP's hash() function.

Increasing counterBits improves accuracy while increasing memory usage.

Example:

$hll = new HyperLogLog(
    counterBits: 10,
    hashAlgorithm: 'sha256'
);

Accuracy

The theoretical standard error is:

1.04 / √m

where:

m = 2^counterBits

Example:

$error = $hll->theoreticalErrorRate($hll->getM());

Supported Hash Algorithms

Any algorithm supported by PHP can be used.

Examples include:

• xxh3 (recommended when available)
• xxh128
• sha256
• sha512
• md5
• sha1

You can list available algorithms using:

print_r(hash_algos());

Public API

Add an element

$hll->add('user-123');

Estimate the number of distinct values

$count = $hll->count();

Calculate the theoretical error

$error = $hll->theoreticalErrorRate($hll->getM());

Measure estimation error

$error = $hll->measureError($estimated, $actual);

When to Use HyperLogLog

HyperLogLog is well suited for:

• Counting unique visitors
• Counting unique IP addresses
• Analytics pipelines
• Large log processing
• Stream processing
• Database statistics
• Big data applications

It is not appropriate when an exact distinct count is required.

References

• https://en.wikipedia.org/wiki/HyperLogLog

License

MIT