Minimal ZIP file – part IV

This article is part of a series.

Creating a valid ZIP-file
Use .NET’s DeflateStream
No dependencies (coding the DEFLATE algorithm)
Calculating the CRC-32 👈

Introduction

From the start, a dependency on System.IO.Hashing was introduced to calculate the CRC-32/ISO-HDLC checksum.

First, a step back; the self-written ‘de flater’ (Dutch pun intended) from Part III is being written off. We want dealer-quality software and return to the version from Part II.

Source code

The source code follows the standard CRC¹ algorithm. The implementation is naive and can be sped up by using lookup-tables (still true 2026 AD?) or a hardware implementation.

To make this work the package System.IO.Hashing is removed.

public static class Crc32
{
    public static uint HashToUInt32(byte[] source)
    {
        const uint polynomial = 0xEDB88320; // ISO-HDLC

        var crc = 0b11111111_11111111_11111111_11111111;

        foreach (var b in source)
        {
            crc ^= b;

            for (var bit = 0; bit < 8; bit++)
            {
                var lsb = crc & 1;
                var mask = lsb * polynomial;
                crc = (crc >> 1) ^ mask;
            }
        }

        return crc ^ 0b11111111_11111111_11111111_11111111;
    }
}