VB - How to compress a buffer with ZLIB.dll

[CVMichael]

If you want to compress data without saving to the drive, for example if you want to compress, and send it right away on a socket over the net. NOTE, you need to send/save the original size of the data, with the compressed data.

Private Declare Function Compress Lib "zlib.dll" Alias "compress" (dest As Any, destLen As Any, src As Any, ByVal srcLen As Long) As Long
Private Declare Function Uncompress Lib "zlib.dll" Alias "uncompress" (dest As Any, destLen As Any, src As Any, ByVal srcLen As Long) As Long
 
Private Sub Form_Load()
    Dim Str As String, CompStr As String
    Dim InData() As Byte, OutData() As Byte, DecompData() As Byte, OriginalSize As Long
    
    Str = "Testing... blah blah blah .....   " & String(40, "A") & String(55, "B")
    InData = StrConv(Str, vbFromUnicode)
    
    OriginalSize = UBound(InData) + 1
    
    If CompressData(InData, OutData) Then
        ' save the data or something, you have to save the OriginalSize also
        ' you need it when decompressing...
    End If
    
    Debug.Print "Original: " & UBound(InData) + 1 & " Bytes", "Compressed: " & UBound(OutData) + 1 & " Bytes"
    Debug.Print "Compression rate: " & Format((1 - (CDbl(UBound(OutData)) / UBound(InData))) * 100#, "##0.00")
    
    Erase InData
    
    ' read the data in OutData (in our case right now is actually the IN data)
    
    ReDim DecompData(OriginalSize - 1)
    Uncompress DecompData(0), OriginalSize, OutData(0), UBound(OutData) + 1
    
    Debug.Print "Decompressed string:"
    Debug.Print StrConv(DecompData, vbUnicode)
End Sub
 
Public Function CompressData(InData() As Byte, OutData() As Byte) As Boolean
    Dim BufferSize As Long
    BufferSize = (UBound(InData) + 1) * 1.01 + 12
 
    ReDim OutData(BufferSize)
 
    CompressData = Compress(OutData(0), BufferSize, InData(0), UBound(InData) + 1) = 0
 
    ReDim Preserve OutData(BufferSize - 1)
End Function

[CVMichael]

How to compress a file:

Option Explicit
 
Private Declare Function compress Lib "zlib.dll" (ByVal dest As String, destLen As Long, ByVal source As String, ByVal sourceLen As Long) As Long
 
Private Const PacketSize As Long = 1024& * 256& ' 256 KBytes buffer size
 
Private Const InFile As String = "C:\Test.txt" ' File name to compress
Private Const OutFile As String = "C:\Test.cmp" ' File name compressed
 
Private Sub cmdCompress_Click() ' Command button to compress data
    Dim InFL As Integer, OutFL As Integer
    Dim InBuff As String, OutBuff As String
    Dim InBuffSize As Long, OutBuffSize As Long
    
    InFL = FreeFile
    Open InFile For Binary Access Read Lock Read Write As InFL
    
    OutFL = FreeFile
    Open OutFile For Binary Access Write Lock Read Write As OutFL
        
        ' Do until end of file
        Do Until Loc(InFL) >= LOF(InFL)
            ' Compress PacketSize at one time (if file is very big, 1 Gig for example,
            ' you would not want to load that much in memory)
            ' or however much is left in the file
            InBuffSize = lngMIN(LOF(InFL) - Loc(InFL), PacketSize)
            
            ' read data from file
            InBuff = String(InBuffSize, 0)
            Get InFL, , InBuff
            
            ' calculate maximum size for output
            OutBuffSize = InBuffSize * 1.01 + 12
            OutBuff = String(OutBuffSize, 0)
            
            ' do the actual compression
            compress OutBuff, OutBuffSize, InBuff, Len(InBuff)
            
            ' resize the output buffer to the compressed size
            OutBuff = Left(OutBuff, OutBuffSize)
            
            ' Write the size of compressed buffer
            Put OutFL, , OutBuffSize
            ' Write the size of original (uncompressed) buffer
            Put OutFL, , InBuffSize
            ' Write the compressed data
            Put OutFL, , OutBuff
        Loop
    
    Close InFL, OutFL
End Sub
 
Private Function lngMIN(ByVal N1 As Long, ByVal N2 As Long) As Long
    If N1 < N2 Then
        lngMIN = N1
    Else
        lngMIN = N2
    End If
End Function

How to decompress the file we just compressed:

Option Explicit
 
Private Declare Function uncompress Lib "zlib.dll" (ByVal dest As String, destLen As Long, ByVal source As String, ByVal sourceLen As Long) As Long
 
Private Const InFile As String = "C:\Test.cmp" ' File name compressed
Private Const OutFile As String = "C:\Test out.txt" ' File name decompressed
 
Private Sub cmdDecompress_Click() ' Command button to decompress data
    Dim InFL As Integer, OutFL As Integer
    Dim InBuff As String, OutBuff As String
    Dim InBuffSize As Long, OutBuffSize As Long
    
    InFL = FreeFile
    Open InFile For Binary Access Read Lock Read Write As InFL
    
    OutFL = FreeFile
    Open OutFile For Binary Access Write Lock Read Write As OutFL
        
        ' Do until end of file
        Do Until Loc(InFL) >= LOF(InFL)
            Get InFL, , InBuffSize ' Compressed buffer size
            Get InFL, , OutBuffSize ' Decompressed buffer size
            
            InBuff = String(InBuffSize, 0)
            Get InFL, , InBuff ' Compressed data
            
            OutBuff = String(OutBuffSize, 0) ' allocate buffer
            
            ' Decompress data
            uncompress OutBuff, OutBuffSize, InBuff, Len(InBuff)
            
            ' Write decompressed data to file
            Put OutFL, , OutBuff
        Loop
    
    Close InFL, OutFL
End Sub

[gigel_hifi]

I have downloaded the latest version of zlib.dll from http://www.zlib.net/, and tied the example from the first post, CompressData, but I get the error "Bad DLL calling convention". Any idea how I can fix it?

[CVMichael]

Probably you did not download the correct dll for your computer.
So, form here: http://www.winimage.com/zLibDll/index.html
Download "zlib123dll.zip - pre-built zlib DLL, compiled with the Microsoft Visual studio 2003 compiler"

I don't know what version I used when I created this thread...

[gigel_hifi]

Thanks CVMichael, I also used the 123 version of the dll, but downloaded from another site, apparently they were different. It works now.

[AAraya]

Thanks for this code, it was helpful. I propose a slight "improvement" to your sample code.

In the CompressDate() routine you estimate the size needed for an output buffer using this line of code:

BufferSize = (UBound(InData) + 1) * 1.01 + 12

I'm not sure where you got this calculation from but there's no need to manually calculate this value since zLib provides a function "CompressBound" which does this work for us. Here's the VB6 declaration and a revised version of your CommitData function:

Private Declare Function CompressBound Lib "zlib.dll" Alias "compressBound" (ByVal srcLen As Long) As Long

Public Function CompressData(InData() As Byte, OutData() As Byte) As Boolean
    Dim BufferSize As Long
    BufferSize = CompressBound (UBound(InData) + 1)
 
    ReDim OutData(BufferSize)

    CompressData = Compress(OutData(0), BufferSize, InData(0), UBound(InData) + 1) = 0

    ReDim Preserve OutData(BufferSize - 1)
End Function

[Dragokas]

I'm not sure where you got this calculation from ...

http://www.gzip.org/zlib/manual.html#compress

Looks like CompressBound() is large than a previous recommendation:

uLong ZEXPORT compressBound (sourceLen)
    uLong sourceLen;
{
    return sourceLen + (sourceLen >> 12) + (sourceLen >> 14) +
           (sourceLen >> 25) + 13;
}

[AAraya]

Dragokas - thanks for sharing that.

I don't know C# so perhaps you can help me. I see that compressBound returns a uLong data type. So what data type should I used in VB to correctly handle the return from this function - a Currency? Which of these two declarations is correct?

Private Declare Function CompressBound Lib "zlib.dll" Alias "compressBound" (ByVal srcLen As Long) As Long

OR

Private Declare Function CompressBound Lib "zlib.dll" Alias "compressBound" (ByVal srcLen As Currency) As Currency

[Dragokas]

It's C, not C#.

typedef unsigned long  uLong; /* 32 bits or more */

You can cast it to/from VB6 Long (signed long) without data loss.
1-st variant is correct.

[AAraya]

Thank you!