unshrink

Documentation

Overview

Package unshrink is for use with the archive/zip package.

Unshrink was a decompression method authored in 1989 by Phil Katz and used in his MS-DOS compression tool, PKZip v0.80 and v0.90.

It is retroactively referred to as ZIP Shrink/Unshrink method 1 and is based on the LZW algorithm.

This package was first prompted using Gemini in August 2026 and manually cleaned to be idiomatic and readable.

In 2020, Jason Summers authored a collection of public domain libraries named oldunzip. In-depth information on Shrink was authored by Hans Wennborg in 2021, who programmed the public domain shrink.c.

Example

Example demonstrates the basic usage of the package.

package main

import (
	"archive/zip"
	"fmt"
	"hash/crc32"
	"io"
	"path/filepath"

	"github.com/Defacto2/archive/unshrink"
)

func main() {
	const testdata = "testdata"
	const testFile = "example-s.zip"

	// Register the Unshrink decompressor with the archive zip package.
	unshrink.Register()

	name := filepath.Join(testdata, testFile)
	rc, _ := zip.OpenReader(name)
	defer rc.Close()

	for n, f := range rc.File {
		fr, _ := f.Open()
		hasher := crc32.NewIEEE()
		size := int64(f.UncompressedSize64) //nolint:gosec
		_, _ = io.CopyN(hasher, fr, size)

		fmt.Printf("%d. Decompressed method %d: %s, %dB\n"+
			"Stored zip crc32 %x, and copied to buffer: %x\n",
			n+1, f.Method, f.Name, f.UncompressedSize64, f.CRC32, hasher.Sum32())

		fr.Close()
	}
}

Output:
1. Decompressed method 1: S.BIN, 2048B
Stored zip crc32 8314ddd9, and copied to buffer: 8314ddd9

Index

• Constants
• Variables
• func Register()

Examples

• Package

Constants

const Unshrink uint16 = 1 // Method 1: Shrink/Unshink

Variables

var ErrOverflow = errors.New("unshink: integer overflow")

ErrOverflow is returned when an integer cannot be safely wrapped to a new type.

Functions

func Register

func Register()

Register the Unshrink method globally for archive/zip.