by Jesper Soderberg (1 Submission)
Category: Miscellaneous
Compatability: Visual Basic 3.0
Difficulty: Unknown Difficulty
Date Added: Wed 3rd February 2021
Rating: 
(5 Votes)
This is a standard LZSS compression/decompression engine. It is written in VB for learning purposes, and should be converted to C/C++ if it is to be used with large amounts of data. It uses a dictionary compression algorithm (like ZIP,ARJ and others) and works the best on data with a lot of repetitions.
Inputs
sCompData - the string to be compressed, sDecompData - the string to be decompressed
Code Returns
Should be obvious
VBcodersOption Explicit
Public Function sCompress(sCompData As String) As String
Dim lDataCount As Long
Dim lBufferStart As Long
Dim lMaxBufferSize As Long
Dim sBuffer As String
Dim lBufferOffset As Long
Dim lBufferSize As Long
Dim sDataControl As String
Dim bDataControlChar As Byte
Dim lControlCount As Long
Dim bControlPos As Byte
Dim bCompLen As Long
Dim lCompPos As Long
Dim bMaxCompLen As Long
lMaxBufferSize = 65535
bMaxCompLen = 255
lBufferStart = 0
sDataControl = ""
bDataControlChar = 0
bControlPos = 0
lControlCount = 0
If Len(sCompData) > 4 Then
sCompress = Left(sCompData, 4)
For lDataCount = 5 To Len(sCompData)
If lDataCount > lMaxBufferSize Then
lBufferSize = lMaxBufferSize
lBufferStart = lDataCount - lMaxBufferSize
Else
lBufferSize = lDataCount - 1
lBufferStart = 1
End If
sBuffer = Mid(sCompData, lBufferStart, lBufferSize)
If Len(sCompData) - lDataCount < bMaxCompLen Then bMaxCompLen = Len(sCompData) - lDataCount
lCompPos = 0
For bCompLen = 3 To bMaxCompLen Step 3
If bCompLen > bMaxCompLen Then
bCompLen = bMaxCompLen
End If
lCompPos = InStr(1, sBuffer, Mid(sCompData, lDataCount, bCompLen), 0)
If lCompPos = 0 Then
If bCompLen > 3 Then
While lCompPos = 0
lCompPos = InStr(1, sBuffer, Mid(sCompData, lDataCount, bCompLen - 1), 0)
If lCompPos = 0 Then bCompLen = bCompLen - 1
Wend
End If
bCompLen = bCompLen - 1
Exit For
End If
Next
If bCompLen > bMaxCompLen And lCompPos > 0 Then
bCompLen = bMaxCompLen
lCompPos = InStr(1, sBuffer, Mid(sCompData, lDataCount, bCompLen), 0)
End If
If lCompPos > 0 Then
lBufferOffset = lBufferSize - lCompPos + 1
sCompress = sCompress & Chr((lBufferOffset And &HFF00) / &H100) & Chr(lBufferOffset And &HFF) & Chr(bCompLen)
lDataCount = lDataCount + bCompLen - 1
bDataControlChar = bDataControlChar + 2 ^ bControlPos
Else
sCompress = sCompress & Mid(sCompData, lDataCount, 1)
End If
bControlPos = bControlPos + 1
If bControlPos = 8 Then
sDataControl = sDataControl & Chr(bDataControlChar)
bDataControlChar = 0
bControlPos = 0
End If
lControlCount = lControlCount + 1
Next
If bControlPos <> 0 Then sDataControl = sDataControl & Chr(bDataControlChar)
sCompress = Chr((lControlCount And &H8F000000) / &H1000000) & Chr((lControlCount And &HFF0000) / &H10000) & Chr((lControlCount And &HFF00) / &H100) & Chr(lControlCount And &HFF) & Chr((Len(sDataControl) And &H8F000000) / &H1000000) & Chr((Len(sDataControl) And &HFF0000) / &H10000) & Chr((Len(sDataControl) And &HFF00) / &H100) & Chr(Len(sDataControl) And &HFF) & sDataControl & sCompress
Else
sCompress = sCompData
End If
End Function
Public Function sDecompress(sDecompData As String) As String
Dim lControlCount As Long
Dim lControlPos As Long
Dim bControlBitPos As Byte
Dim lDataCount As Long
Dim lDataPos As Long
Dim lDecompStart As Long
Dim lDecompLen As Long
If Len(sDecompData) > 4 Then
lControlCount = Asc(Left(sDecompData, 1)) * &H1000000 + Asc(Mid(sDecompData, 2, 1)) * &H10000 + Asc(Mid(sDecompData, 3, 1)) * &H100 + Asc(Mid(sDecompData, 4, 1))
lDataCount = Asc(Mid(sDecompData, 5, 1)) * &H1000000 + Asc(Mid(sDecompData, 6, 1)) * &H10000 + Asc(Mid(sDecompData, 7, 1)) * &H100 + Asc(Mid(sDecompData, 8, 1)) + 9
sDecompress = Mid(sDecompData, lDataCount, 4)
lDataCount = lDataCount + 4
bControlBitPos = 0
lControlPos = 9
For lDataPos = 1 To lControlCount
If 2 ^ bControlBitPos = (Asc(Mid(sDecompData, lControlPos, 1)) And 2 ^ bControlBitPos) Then
lDecompStart = Len(sDecompress) - (CLng(Asc(Mid(sDecompData, lDataCount, 1))) * &H100 + CLng(Asc(Mid(sDecompData, lDataCount + 1, 1)))) + 1
lDecompLen = Asc(Mid(sDecompData, lDataCount + 2, 1))
sDecompress = sDecompress & Mid(sDecompress, lDecompStart, lDecompLen)
lDataCount = lDataCount + 3
Else
sDecompress = sDecompress & Mid(sDecompData, lDataCount, 1)
lDataCount = lDataCount + 1
End If
bControlBitPos = bControlBitPos + 1
If bControlBitPos = 8 Then
bControlBitPos = 0
lControlPos = lControlPos + 1
End If
Next
Else
sDecompress = sDecompData
End If
End Function
'Put a two command buttons (Command1 and Command2) on to a form and paste the following on to it as well:
Option Explicit
Private Const sFileName = "c:\compressthis.exe" ' the file to be compressed
Private Sub Command1_Click() 'Compress the file
Dim sReturn As String
Dim sFileData As String
Open sFileName For Binary As #1
sFileData = Input(LOF(1), #1)
Close #1
sReturn = sCompress(sFileData)
Debug.Print Len(sReturn), Len(sFileData)
Open Left(sFileName, Len(sFileName) - 3) & "wnc" For Output As #1
Print #1, sReturn;
Close #1
End Sub
Private Sub Command2_Click() 'Decompress the file
Dim sReturn As String
Dim sFileData As String
Open Left(sFileName, Len(sFileName) - 4) & ".wnc" For Binary As #1
sFileData = Input(LOF(1), #1)
sReturn = sDecompress(sFileData)
Close #1
Debug.Print Len(sReturn), Len(sFileData)
Open Left(sFileName, Len(sFileName) - 4) & "2" & Right(sFileName, 4) For Output As #1
Print #1, sReturn;
Close #1
End Sub