LZ-UTF8

Note: this library is significantly out-of-date and will require a full rewrite to update with recent technologies like JS modules and WebAssembly, and for better compatibility with modern frameworks like Angular and Rect.js. The design and documentation was mostly written in 2014, before any of these were relevant. Unfortunately, it is not maintained anymore, and a rewrite is unlikely to take place in the foreseeable future.

LZ-UTF8 is a string compression library and format. Is an extension to the UTF-8 character encoding, augmenting the UTF-8 bytestream with optional compression based the LZ77 algorithm. Some of its properties:

• Compresses strings only. Doesn't support arbitrary byte sequences.
• Strongly optimized for speed, both in the choice of algorithm and its implementation. Approximate measurements using a low-end desktops and 1MB strings: 3-14MB/s compression , 20-120MB/s decompression (detailed benchmarks and comparison to other Javascript libraries can be found in the technical paper). Due to the concentration on time efficiency, the resulting compression ratio can be significantly lower when compared to more size efficient algorithms like LZW + entropy coding.
• Byte-level superset of UTF-8. Any valid UTF-8 bytestream is also a valid LZ-UTF8 stream (but not vice versa). This special property allows both compressed and plain UTF-8 streams to be freely concatenated and decompressed as single unit (or with any arbitrary partitioning). Some possible applications:
  • Sending static pre-compressed data followed by dynamically generated uncompressed data from a server (and possibly appending a compressed static "footer", or repeating the process several times).
  • Appending both uncompressed/compressed data to a compressed log file/journal without needing to rewrite it.
  • Joining multiple source files, where some are possibly pre-compressed, and serving them as a single concatenated file without additional processing.
• Patent free (all relevant patents have long expired).

Javascript implementation:

• Tested on most popular browsers and platforms: Node.js 4+, Chrome, Firefox, Opera, Edge, IE10+ (IE8 and IE9 may work with a typed array polyfill), Android 4+, Safari 5+.
• Allows compressed data to be efficiently packed in plain Javascript UTF-16 strings (see the "BinaryString" encoding described later in this document) when binary storage is not available or desired (e.g. when using LocalStorage or older IndexedDB).
• Can operate asynchronously, both in Node.js and in the browser. Uses web workers when available (and takes advantage of transferable objects if supported) and falls back to async iterations when not.
• Supports Node.js streams.
• Written in TypeScript.

Quick start

• Try the online demo to test and benchmark different inputs.
• Download the latest build (or the minified version).
• Run the automated tests.
• Run the core benchmarks.
• Read the technical paper.

Table of Contents

• API Reference
  • Getting started
  • Type Identifier Strings
  • Core Methods
    • LZUTF8.compress(..)
    • LZUTF8.decompress(..)
  • Asynchronous Methods
    • LZUTF8.compressAsync(..)
    • LZUTF8.decompressAsync(..)
    • General notes on async operations
  • Lower-level Methods
    • LZUTF8.Compressor
    • LZUTF8.Compressor.compressBlock(..)
    • LZUTF8.Decompressor
    • LZUTF8.Decompressor.decompressBlock(..)
    • LZUTF8.Decompressor.decompressBlockToString(..)
  • Node.js only methods
    • LZUTF8.createCompressionStream()
    • LZUTF8.createDecompressionStream()
  • Character encoding methods
    • LZUTF8.encodeUTF8(..)
    • LZUTF8.decodeUTF8(..)
    • LZUTF8.encodeBase64(..)
    • LZUTF8.decodeBase64(..)
    • LZUTF8.encodeStorageBinaryString(..)
    • LZUTF8.decodeStorageBinaryString(..)
• Release history
• License

API Reference

Getting started

Node.js:

npm install lzutf8

var LZUTF8 = require('lzutf8');

Browser:

<script id="lzutf8" src="https://cdn.jsdelivr.net/npm/lzutf8/build/production/lzutf8.js"></script>

or the minified version:

<script id="lzutf8" src="https://cdn.jsdelivr.net/npm/lzutf8/build/production/lzutf8.min.js"></script>

to reference a particular version use the pattern, where x.x.x should be replaced with the exact version number (e.g. 0.4.6):

<script id="lzutf8" src="https://unpkg.com/lzutf8@x.x.x/build/production/lzutf8.min.js"></script>

note: the id attribute and its exact value are necessary for the library to make use of web workers.

Type Identifier Strings

"ByteArray" - An array of bytes. As of 0.3.2, always a Uint8Array. In versions up to 0.2.3 the type was determined by the platform (Array for browsers that don't support typed arrays, Uint8Array for supporting browsers and Buffer for Node.js).

IE8/9 and support was dropped at 0.3.0 though these browsers can still be used with a typed array polyfill.

"Buffer" - A Node.js Buffer object.

"StorageBinaryString" - A string containing compacted binary data encoded to fit in valid UTF-16 strings. Please note the older, deprecated, "BinaryString" encoding, is still internally supported in the library but has been removed from this document. More details are included further in this document.

"Base64" - A base 64 string.

Core Methods

LZUTF8.compress(..)

var output = LZUTF8.compress(input, [options]);

Compresses the given input data.

input can be either a String or UTF-8 bytes stored in a Uint8Array or Buffer

options (optional): an object that may have any of the properties:

• outputEncoding: "ByteArray" (default), "Buffer", "StorageBinaryString" or "Base64"

returns: compressed data encoded by encoding, or ByteArray if not specified.

LZUTF8.decompress(..)

var output = LZUTF8.decompress(input, [options]);

Decompresses the given compressed data.

input: can be either a Uint8Array, Buffer or String (where encoding scheme is then specified in inputEncoding)

options (optional): an object that may have the properties:

• inputEncoding: "ByteArray" (default), "StorageBinaryString" or "Base64"
• outputEncoding: "String" (default), "ByteArray" or "Buffer" to return UTF-8 bytes

returns: decompressed bytes encoded as encoding, or as String if not specified.

Asynchronous Methods

LZUTF8.compressAsync(..)

LZUTF8.compressAsync(input, [options], callback);

Asynchronously compresses the given input data.

input can be either a String, or UTF-8 bytes stored in an Uint8Array or Buffer.

options (optional): an object that may have any of the properties:

• outputEncoding: "ByteArray" (default), "Buffer", "StorageBinaryString" or "Base64"
• useWebWorker: true (default) would use a web worker if available. false would use iterated yielding instead.

callback: a user-defined callback function accepting a first argument containing the resulting compressed data as specified by outputEncoding (or ByteArray if not specified) and a possible second parameter containing an Error object.

On error: invokes the callback with a first argument of undefined and a second one containing the Error object.

Example:

LZUTF8.compressAsync(input, {outputEncoding: "StorageBinaryString"}, function (result, error) {
    if (error === undefined)
        console.log("Data successfully compressed and encoded to " + result.length + " characters");
    else
        console.log("Compression error: " + error.message);
});

LZUTF8.decompressAsync(..)

LZUTF8.decompressAsync(input, [options], callback);

Asynchronously decompresses the given compressed input.

input: can be either a Uint8Array, Buffer or String (where encoding is set with inputEncoding).

options (optional): an object that may have the properties:

• inputEncoding: "ByteArray" (default), "StorageBinaryString" or "Base64"
• outputEncoding: "String" (default), "ByteArray" or "Buffer" to return UTF-8 bytes.
• useWebWorker: true (default) would use a web worker if available. false would use incremental yielding instead.

callback: a user-defined callback function accepting a first argument containing the resulting decompressed data as specified by outputEncoding and a possible second parameter containing an Error object.

On error: invokes the callback with a first argument of undefined and a second one containing the Error object.

Example:

LZUTF8.decompressAsync(input, {inputEncoding: "StorageBinaryString", outputEncoding: "ByteArray"}, function (result, error) {
    if (error === undefined)
        console.log("Data successfully decompressed to " + result.length + " UTF-8 bytes");
    else
        console.log("Decompression error: " + error.message);
});

General notes on async operations

Web workers are available if supported by the browser and the library's script source is referenced in the document with a script tag having id of "lzutf8" (its src attribute is then used as the source URI for the web worker). In cases where a script tag is not available (such as when the script is dynamically loaded or bundled with other scripts) the value of LZUTF8.WebWorker.scriptURI may alternatively be set before the first async method call.

Workers are optimized for various input and output encoding schemes, so only the minimal amount of work is done in the main Javascript thread. Internally, conversion to or from various encodings is performed within the worker itself, reducing delays and allowing greater parallelization. Additionally, if transferable objects are supported by the browser, binary arrays will be transferred virtually instantly to and from the worker.

Only one worker instance is spawned per page - multiple operations are processed sequentially.

In case a worker is not available (such as in Node.js, IE8, IE9, Android browser < 4.4) or desired, it will iteratively process 64KB blocks while yielding to the event loop whenever a 20ms interval has elapsed. Note: In this execution method, parallel operations are not guaranteed to complete by their initiation order.

Lower-level Methods

LZUTF8.Compressor

var compressor = new LZUTF8.Compressor();

Creates a compressor object. Can be used to incrementally compress a multi-part stream of data.

returns: a new LZUTF8.Compressor object

LZUTF8.Compressor.compressBlock(..)

var compressor = new LZUTF8.Compressor();
var compressedBlock = compressor.compressBlock(input);

Compresses the given input UTF-8 block.

input can be either a String, or UTF-8 bytes stored in a Uint8Array or Buffer

returns: compressed bytes as ByteArray

This can be used to incrementally create a single compressed stream. For example:

var compressor = new LZUTF8.Compressor();
var compressedBlock1 = compressor.compressBlock(block1);
var compressedBlock2 = compressor.compressBlock(block2);
var compressedBlock3 = compressor.compressBlock(block3);
..

LZUTF8.Decompressor

var decompressor = new LZUTF8.Decompressor();

Creates a decompressor object. Can be used to incrementally decompress a multi-part stream of data.

returns: a new LZUTF8.Decompressor object

LZUTF8.Decompressor.decompressBlock(..)

var decompressor = new LZUTF8.Decompressor();
var decompressedBlock = decompressor.decompressBlock(input);

Decompresses the given block of compressed bytes.

input can be either a Uint8Array or Buffer

returns: decompressed UTF-8 bytes as ByteArray

Remarks: will always return the longest valid UTF-8 stream of bytes possible from the given input block. Incomplete input or output byte sequences will be prepended to the next block.

Note: This can be used to incrementally decompress a single compressed stream. For example:

var decompressor = new LZUTF8.Decompressor();
var decompressedBlock1 = decompressor.decompressBlock(block1);
var decompressedBlock2 = decompressor.decompressBlock(block2);
var decompressedBlock3 = decompressor.decompressBlock(block3);
..

LZUTF8.Decompressor.decompressBlockToString(..)

var decompressor = new LZUTF8.Decompressor();
var decompressedBlockAsString = decompressor.decompressBlockToString(input);

Decompresses the given block of compressed bytes and converts the result to a String.

input can be either a Uint8Array or Buffer

returns: decompressed String

Remarks: will always return the longest valid string possible from the given input block. Incomplete input or output byte sequences will be prepended to the next block.

Node.js only methods

LZUTF8.createCompressionStream()

var compressionStream = LZUTF8.createCompressionStream();

Creates a compression stream. The stream will accept both Buffers and Strings in any encoding supported by Node.js (e.g. utf8, utf16, ucs2, base64, hex, binary etc.) and return Buffers.

example:

var sourceReadStream = fs.createReadStream(“content.txt”);
var destWriteStream = fs.createWriteStream(“content.txt.lzutf8”);
var compressionStream = LZUTF8.createCompressionStream();

sourceReadStrem.pipe(compressionStream).pipe(destWriteStream);

On error: emits an error event with the Error object as parameter.

LZUTF8.createDecompressionStream()

var decompressionStream = LZUTF8.createDecompressionStream();

Creates a decompression stream. The stream will accept and return Buffers.

On error: emits an error event with the Error object as parameter.

Character encoding methods

LZUTF8.encodeUTF8(..)

var output = LZUTF8.encodeUTF8(input);

Encodes a string to UTF-8.

input as String

returns: encoded bytes as ByteArray

LZUTF8.decodeUTF8(..)

var outputString = LZUTF8.decodeUTF8(input);

Decodes UTF-8 bytes to a String.

input as either a Uint8Array or Buffer

returns: decoded bytes as String

LZUTF8.encodeBase64(..)

var outputString = LZUTF8.encodeBase64(bytes);

Encodes bytes to a Base64 string.

input as either a Uint8Array or Buffer

returns: resulting Base64 string.

remarks: Maps every 3 consecutive input bytes to 4 output characters of the set A-Z,a-z,0-9,+,/ (a total of 64 characters). Increases stored byte size to 133.33% of original (when stored as ASCII or UTF-8) or 266% (stored as UTF-16).

LZUTF8.decodeBase64(..)

var output = LZUTF8.decodeBase64(input);

Decodes UTF-8 bytes to a String.

input as String

returns: decoded bytes as ByteArray

remarks: the decoder cannot decode concatenated base64 strings. Although it is possible to add this capability to the JS version, compatibility with other decoders (such as the Node.js decoder) prevents this feature to be added.

LZUTF8.encodeStorageBinaryString(..)

Note: the older BinaryString encoding has been deprecated due to a compatibility issue with the IE browser's LocalStorage/SessionStorage implementation. This newer version works around that issue by avoiding the 0 codepoint.

var outputString = LZUTF8.encodeStorageBinaryString(input);

Encodes binary bytes to a valid UTF-16 string.

input as either a Uint8Array or Buffer

returns: String

remarks: To comply with the UTF-16 standard, it only uses the bottom 15 bits of each character, effectively mapping every 15 input bits to a single 16 bit output character. This Increases the stored byte size to 106.66% of original.

LZUTF8.decodeStorageBinaryString(..)

Note: the older BinaryString encoding has been deprecated due to a compatibility issue with the IE browser's LocalStorage/SessionStorage implementation. This newer version works around that issue by avoiding the 0 codepoint.

var output = LZUTF8.decodeStorageBinaryString(input);

Decodes a binary string.

input as String

returns: decoded bytes as ByteArray

remarks: Multiple binary strings may be freely concatenated and decoded as a single string. This is made possible by ending every sequence with special marker (char code 32768 for an even-length sequence and 32769 for a an odd-length sequence).

Release history

• 0.1.x: Initial release.
• 0.2.x: Added async error handling. Added support for TextEncoder and TextDecoder when available.
• 0.3.x: Removed support to IE8/9. Removed support for plain Array inputs. All "ByteArray" outputs are now Uint8Array objects. A separate "Buffer" encoding setting can be used to return Buffer objects.
• 0.4.x: Major code restructuring. Removed support for versions of Node.js prior to 4.0.
• 0.5.x: Added the "StorageBinaryString" encoding.

License

Copyright (c) 2014-2018, Rotem Dan <rotemdan@gmail.com>.

Source code and documentation are available under the MIT license.