Lempel-Ziv-Welch algorithm - vb.net

Question

Heys guys,
I'm trying to implement the Lempel-Ziv-Welch algorithm on byte level in VB.NET. Here is what the Code looks like so far:
Dim FileBytes As Byte() = {Convert.ToByte(12), Convert.ToByte(13), Convert.ToByte(12), Convert.ToByte(13), Convert.ToByte(12), Convert.ToByte(13)}
Dim bw As New BitWriter(New BinaryWriter(New FileStream(output, FileMode.Create)))
Dim ByteDictionary As New Dictionary(Of Byte(), Integer)
Dim DictionaryN As Integer = 1
Dim DictionaryMax As Integer = 4048
Dim outputs As New List(Of Integer)
Dim bts As New List(Of Byte)
For index As Integer = 0 To FileBytes.Length - 1
Dim currentbyte As Byte = FileBytes(index)
If ContainsByte(ByteDictionary, AddToList(bts, currentbyte).ToArray) Then
bts.Add(currentbyte)
Else
If bts.Count > 1 Then
ByteDictionary.Add(bts.ToArray, 255 + DictionaryN)
DictionaryN += 1
Else
ByteDictionary.Add(New Byte() {currentbyte}, currentbyte)
End If
Console.WriteLine(GetByteValue(ByteDictionary, bts.ToArray))
bts.Clear()
bts.Add(currentbyte)
End If
Next
End Sub
Public Function ContainsByte(ByVal dic As Dictionary(Of Byte(), Integer), ByVal bt As Byte()) As Boolean
Dim flag = True
For Each kp As KeyValuePair(Of Byte(), Integer) In dic
If ByteArrayEquals(kp.Key, bt) Then
Return True
End If
Next
Return False
End Function
Public Function AddToList(ByVal list As List(Of Byte), ByVal bt As Byte) As List(Of Byte)
Dim newlist = New List(Of Byte)(list)
newlist.Add(bt)
Return newlist
End Function
Public Function ByteArrayEquals(ByVal first As Byte(), ByVal second As Byte()) As Boolean
If first.Length = second.Length Then
Dim flag = True
For index As Integer = 0 To first.Length - 1
If first(index) <> second(index) Then
flag = False
End If
Next
Return flag
Else
Return False
End If
End Function
Public Function GetByteValue(ByVal dic As Dictionary(Of Byte(), Integer), ByVal bt As Byte()) As Integer
For Each kp As KeyValuePair(Of Byte(), Integer) In dic
If ByteArrayEquals(kp.Key, bt) Then
Return kp.Value
End If
Next
End Function
The idea behind my implementation is from https://www.cs.cf.ac.uk/Dave/Multimedia/node214.html . But somehow it doesn't work, it simply puts out the input bytes. What is wrong with it?

Answer 1

The main problem is these two lines:
If bts.Count > 1 Then
ByteDictionary.Add(bts.ToArray, 255 + DictionaryN)
In both lines here you're using bts where you should be using bts + currentByte, i.e. you want something like
Dim currentbyte As Byte = FileBytes(index)
Dim btsPlusCurrentByte As List(Of Byte) = AddToList(bts, currentbyte)
If ContainsByte(ByteDictionary, btsPlusCurrentByte.ToArray) Then
bts.Add(currentbyte)
Else
If btsPlusCurrentByte.Count > 1 Then
ByteDictionary.Add(btsPlusCurrentByte.ToArray, 255 + DictionaryN)
The other problem is that you'll complete the loop with data left in bts that you haven't output, so I think you'll need a block to do that afterwards. It may be safe to just do
Console.WriteLine(GetByteValue(ByteDictionary, bts.ToArray))
again after the Next but I haven't thought about that very carefully.
I also think you should be able to use .NET's own built-ins rather than your four helper functions.