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HuffmanPHP

A PHP library for string compression using the Huffman coding algorithm with URL-safe base conversion.

Features

• Huffman coding compression for strings
• Character-level and whole-word compression modes
• URL-safe output using configurable alphabets (Base10 to Base65)
• Automatic GMP/BCMath detection for base conversion
• Built-in result caching for repeated operations
• PHP 8.0+ with strict typing

Requirements

• PHP >= 8.0
• One of these extensions for compressed output:
  • gmp (recommended, faster)
  • bcmath (fallback)

Installation

Usage

Basic Encoding

Character-level encoding compresses individual characters based on their frequency. The default output uses Base65 (URL-safe alphabet) when compression is enabled.

Custom Alphabets

Use different character sets to encode compressed output. Smaller alphabets produce longer output but may be more suitable for restricted character sets.

Whole-Word Mode

Treat each input array element as an atomic token instead of breaking it into individual characters. This produces superior compression when the input has repeating words or phrases.

API Reference

Dictionary

__construct(array $values, int $maxLength = 1)

Builds a Huffman dictionary from sample values.

• $values — Array of strings to learn character/word frequencies from
• $maxLength — Maximum substring length to consider. 1 = single characters (default), Dictionary::MAX_LENGTH_WHOLE_WORDS (0) = treat each value as a whole word

Throws InvalidArgumentException if $maxLength is negative.

getBinary(string $key): ?string

Returns the binary Huffman code for a given key, or null if not found.

getValue(string $binary): string|int|false

Reverse lookup: returns the original key for a binary code, or false if not found.

getMaxLength(): int

Returns the maxLength used during construction.

setValues(array $values): void

Replaces the dictionary mappings. Useful for serializing/deserializing dictionaries.

Huffman

__construct(Dictionary $dictionary, string $compressAlphabet = Huffman::ALPHABET_BASE65)

Creates an encoder/decoder instance.

• $dictionary — The Huffman dictionary to use
• $compressAlphabet — Alphabet for compressed output (default: URL-safe Base65)

encode(string $decoded, bool $compress = false): string

Encodes a string using the Huffman dictionary.

• $compress = false — Returns raw binary string
• $compress = true — Returns base-converted string using the configured alphabet

Throws RuntimeException if:

• The input contains characters not in the dictionary
• Compression is enabled but neither GMP nor BCMath is available

decode(string $encoded, bool $compressed = false): string

Decodes a previously encoded string. The $compressed flag must match the $compress flag used during encoding.

Throws RuntimeException if the binary code is not recognized in the dictionary.

Alphabet Constants

Alphabets are used to convert the binary Huffman codes into text. Larger alphabets produce shorter output for the same binary data.

Note: Larger alphabets produce shorter output. Base65 is the default and uses only URL-safe characters (digits, letters, hyphen, underscore, tilde).

Error Handling

• InvalidArgumentException — Thrown by Dictionary when $maxLength is negative
• RuntimeException — Thrown by Huffman::encode() when the input contains characters not in the dictionary
• RuntimeException — Thrown by Huffman::encode()/decode() with $compress = true when neither GMP nor BCMath is available

Testing

License

MIT — see LICENSE for details.