C# Implementation of LZW Compression/Decompression Algorithm

lzw.cs

using System;
using System.Collections.Generic;
using System.Security;
public static class lzw
{
    public static int CompressedSize
    {
        get;
        set;
    }
    public static int DeCompressedSize
    {
        get;
        set;
    }
    public static double Ratio => (double)CompressedSize / DeCompressedSize * 100.0;
    public static byte[] LzwCompress(this byte[] iBuf)
    {
        if (iBuf        == null) throw new Exception("Input buffer is null.");
        if (iBuf.Length == 0) throw new Exception("Input buffer is empty.");
        DeCompressedSize = iBuf.Length;
        var dictionary = new Dictionary<List<byte>, int>(new ArrayComparer());
        for (var i = 0; i < 256; i++)
        {
            var e = new List<byte> { (byte)i };
            dictionary.Add(e, i);
        }
        var window = new List<byte>();
        var oBuf   = new List<int>();
        foreach (var b in iBuf)
        {
            var windowChain = new List<byte>(window) { b };
            if (dictionary.ContainsKey(windowChain))
            {
                window.Clear();
                window.AddRange(windowChain);
            }
            else
            {
                if (dictionary.ContainsKey(window))
                    oBuf.Add(dictionary[window]);
                else
                    throw new Exception("Error Encoding.");
                CompressedSize = oBuf.Count;
                dictionary.Add(windowChain, dictionary.Count);
                window.Clear();
                window.Add(b);
            }
        }
        if (window.Count != 0)
        {
            oBuf.Add(dictionary[window]);
            CompressedSize = oBuf.Count;
        }
        return GetBytes(oBuf.ToArray());
    }
    public static byte[] LzwDecompress(this byte[] Bufi)
    {
        if (Bufi        == null) throw new Exception("Input buffer is null.");
        if (Bufi.Length == 0) throw new Exception("Input buffer is empty.");
        var iBufi = Ia(Bufi);
        var iBuf  = new List<int>(iBufi);
        CompressedSize = iBuf.Count;
        var dictionary = new Dictionary<int, List<byte>>();
        for (var i = 0; i < 256; i++)
        {
            var e = new List<byte> { (byte)i };
            dictionary.Add(i, e);
        }
        var window = dictionary[iBuf[0]];
        iBuf.RemoveAt(0);
        var oBuf = new List<byte>(window);
        foreach (var k in iBuf)
        {
            var entry = new List<byte>();
            if (dictionary.ContainsKey(k))
                entry.AddRange(dictionary[k]);
            else if (k == dictionary.Count)
                entry.AddRange(Add(window.ToArray(), new[] { window.ToArray()[0] }));
            if (entry.Count > 0)
            {
                oBuf.AddRange(entry);
                DeCompressedSize = oBuf.Count;
                dictionary.Add(dictionary.Count, new List<byte>(Add(window.ToArray(), new[] { entry.ToArray()[0] })));
                window = entry;
            }
        }
        return oBuf.ToArray();
    }
    private static byte[] GetBytes(int[] value)
    {
        if (value == null)
            throw new Exception("GetBytes (int[]) object cannot be null.");
        var numArray = new byte[value.Length * 4];
        Buffer.BlockCopy(value, 0, numArray, 0, numArray.Length);
        return numArray;
    }
    private static byte[] Add(byte[] left, byte[] right)
    {
        var l1  = left.Length;
        var l2  = right.Length;
        var ret = new byte[l1 + l2];
        Buffer.BlockCopy(left,  0, ret, 0,  l1);
        Buffer.BlockCopy(right, 0, ret, l1, l2);
        return ret;
    }
    private static int[] Ia(byte[] ba)
    {
        var bal        = ba.Length;
        var int32Count = bal / 4 + (bal % 4 == 0 ? 0 : 1);
        var arr        = new int[int32Count];
        Buffer.BlockCopy(ba, 0, arr, 0, bal);
        return arr;
    }
    [SecuritySafeCritical]
    private static unsafe bool Compare(byte[] a1, byte[] a2)
    {
        if (a1 == null && a2 == null)
            return true;
        if (a1 == null || a2 == null || a1.Length != a2.Length)
            return false;
        fixed (byte* p1 = a1, p2 = a2)
        {
            var   Len = a1.Length;
            byte* x1  = p1, x2 = p2;
            while (Len > 7)
            {
                if (*(long*)x2 != *(long*)x1)
                    return false;
                x1  += 8;
                x2  += 8;
                Len -= 8;
            }
            switch (Len % 8)
            {
                case 0:
                    break;
                case 7:
                    if (*(int*)x2 != *(int*)x1)
                        return false;
                    x1 += 4;
                    x2 += 4;
                    if (*(short*)x2 != *(short*)x1)
                        return false;
                    x1 += 2;
                    x2 += 2;
                    if (*x2 != *x1)
                        return false;
                    break;
                case 6:
                    if (*(int*)x2 != *(int*)x1)
                        return false;
                    x1 += 4;
                    x2 += 4;
                    if (*(short*)x2 != *(short*)x1)
                        return false;
                    break;
                case 5:
                    if (*(int*)x2 != *(int*)x1)
                        return false;
                    x1 += 4;
                    x2 += 4;
                    if (*x2 != *x1)
                        return false;
                    break;
                case 4:
                    if (*(int*)x2 != *(int*)x1)
                        return false;
                    break;
                case 3:
                    if (*(short*)x2 != *(short*)x1)
                        return false;
                    x1 += 2;
                    x2 += 2;
                    if (*x2 != *x1)
                        return false;
                    break;
                case 2:
                    if (*(short*)x2 != *(short*)x1)
                        return false;
                    break;
                case 1:
                    if (*x2 != *x1)
                        return false;
                    break;
            }
            return true;
        }
    }
    private class ArrayComparer : IEqualityComparer<List<byte>>
    {
        public bool Equals(List<byte> left, List<byte> right)
        {
            if (left == null || right == null)
                return false;
            return Compare(left.ToArray(), right.ToArray());
        }
        public unsafe int GetHashCode(List<byte> obj)
        {
            var obj1   = obj.ToArray();
            var cbSize = obj1.Length;
            var hash   = 0x811C9DC5;
            fixed (byte* pb = obj1)
            {
                var nb = pb;
                while (cbSize >= 4)
                {
                    hash   ^= *(uint*)nb;
                    hash   *= 0x1000193;
                    nb     += 4;
                    cbSize -= 4;
                }
                switch (cbSize & 3)
                {
                    case 3:
                        hash ^= *(uint*)(nb + 2);
                        hash *= 0x1000193;
                        goto case 2;
                    case 2:
                        hash ^= *(uint*)(nb + 1);
                        hash *= 0x1000193;
                        goto case 1;
                    case 1:
                        hash ^= *nb;
                        hash *= 0x1000193;
                        break;
                }
            }
            return (int)hash;
        }
    }
}

Can you post the source for UniHash and CDictionary on here? Thanks

Don't bother with this code
You won't find the classes you need

Can you post the source for UniHash and CDictionary on here? Thanks

There is no CDictionary in this program. There is however Dictionary, but that class is intrinsic to the FCL. It's in the System.Collections.Generic namespace.
https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2?view=net-5.0

Don't bother with this code
You won't find the classes you need

I disagree. Any competent programmer can knock up a replacement for UniHash and then this code does the job, albeit via a somewhat poor implementation in places but this implementation is entirely functional.

It really should have implemented the System.IO.Stream abstract class along the lines of the System.IO.Compression.GZipStream or System.IO.Compression.BrotliStream, but such is life; sometimes "following the best practices" is more work than just getting the job done.

I just noticed the UniHash class isn't really even used. The only spot where "hash" is being done is in the GetHashCode implementation, and there it's just a uint.

The output does seem to have a lot of redundancy. Something can't be quite right here. Could it be that this is an integer based implementation instead of byte based?