Just Deflate #

A pure Dart implementation of DEFLATE (RFC 1951) compression and decompression, with the zlib (RFC 1950) and gzip (RFC 1952) formats around it.

It has no dependencies and no dart:io, dart:ffi or native code, so it runs the same everywhere: Flutter on every platform, the web (dart2js) and Wasm (dart2wasm). It was written for the just_tiled package, where Tiled stores tile layers as zlib or gzip data, and for the Just Game Engine's level maps.

Features #

• zlib, gzip and raw DEFLATE, encoding and decoding.
• Real compression: LZ77 matching and dynamic Huffman codes, with zlib's settings for levels 0 to 9. Output is about the size of zlib's at the same level.
• The same bytes on every platform. Compressing the same data gives the same output on native, web and Wasm, so saved files don't change depending on where they were saved.
• Reads anything zlib writes: every block type, any window size, multi-member gzip files and gzip's optional header fields (name, comment, extra field, header CRC).
• Checked decoding: Adler-32 and CRC-32 checksums are verified, damaged data throws a FormatException, and maxOutputSize stops damaged or hostile data from unpacking without end.
• adler32 and crc32 checksum functions.

Getting started #

Add just_deflate to your pubspec.yaml:

dependencies:
  just_deflate: ^0.1.0

Usage #

zlib #

import 'dart:typed_data';
import 'package:just_deflate/just_deflate.dart';

void main() {
  final data = Uint8List(64 * 64 * 4); // an empty 64 x 64 tile layer

  const zlib = ZlibCodec(); // level 6, zlib's default
  final packed = zlib.encode(data);
  print('${data.length} bytes -> ${packed.length} bytes');

  try {
    final unpacked = zlib.decode(packed, maxOutputSize: data.length);
    print('Decoded ${unpacked.length} bytes.');
  } on FormatException catch (e) {
    print('Invalid zlib data: $e');
  }
}

gzip and raw DEFLATE #

GzipCodec and DeflateCodec work the same way:

const gzip = GzipCodec(level: 9); // smallest output
final packed = gzip.encode(data);
final unpacked = gzip.decode(packed);

const raw = DeflateCodec(level: 1); // fastest
final stream = raw.encode(data); // no header, no checksum

Levels #

A level outside 0-9 throws a RangeError from encode.

Checksums #

crc32('123456789'.codeUnits); // 0xCBF43926
adler32('Wikipedia'.codeUnits); // 0x11E60398

// Data in pieces: pass the running value back in.
var crc = crc32(firstPart);
crc = crc32(secondPart, crc);

Notes #

• zlib streams that need a preset dictionary (FDICT) are refused.
• Bytes after the end of a stream are ignored. In gzip data, anything after the last member that does not start another member is ignored.
• encode writes gzip headers with no file name, comment or time, and the operating system set to "unknown", so the output depends only on the data and the level.
• Everything is in memory: encode and decode take the whole input and return the whole output. There is no streaming API.

License #

BSD 3-Clause. See LICENSE.