Minimal ZIP file – part I

This article is part of a series.

1. Creating a valid ZIP-file 👈 here we are
2. Use .NET’s DeflateStream
3. No dependencies (coding the DEFLATE algorithm)

Introduction

How difficult is it to create a ZIP-file¹ with a bare minimum² - but readable - amount of code?

To go slowly, we do this in a few steps. Each in their own article. The first step is to create a valid, but not very optimal ZIP-file (i.e., uncompressed).

File format

For a little background, these are the parts of a ZIP-file:

File header 1     <------------+
File data 1                    |
File header 2     <--------+   |
File data 2                |   |
.... .... .                |   |
File header n     <----+   |   |
File data n            |   |   |
Central directory <----|---|---|---+
File entry 1  ---------+   |   |   |
File entry 2  -------------+   |   |
.... ..... .                   |   |
File entry n  -----------------+   |   
End of Central Directory ----------+

The individual parts contain things like file names, sizes, and CRCs.

Source code

The source code is 100 lines and follows said file layout. Readability was the main goal.

Zip1.cs

The proof is in the pudding

PKZIP 2.04g (Jan ‘93) confirms that it is a valid ZIP-file.

Info-ZIP’s zip agrees:

dotnet run
zip -T minimal.zip

test of minimal.zip OK

Info-ZIP’s zipinfo is also a great tool to inspect ZIP-files.

zipinfo -v minimal.zip

In the next part we’ll cheat a little and use .NET’s DeflateStream³ to create a more useful ZIP-file.