VB.NET

Decompress Large Text File in Blocks

See more Compression Examples

Decompresses a large text file in blocks, and compares the restored (decompressed) file with the original to make sure it's correct. VB.NET

Dim success As Boolean = False

' This example requires the Chilkat API to have been previously unlocked.
' See Global Unlock Sample for sample code.

' First, let's compress a text file.
' We'll then decompress in blocks, and compare the decompressed with the original file.

' Compress a text file:
Dim compress As New Chilkat.Compression
compress.Algorithm = "deflate"

success = compress.CompressFile("qa_data/hamlet.xml","qa_data/hamlet_compressed.dat")
If (success = False) Then
    Debug.WriteLine(compress.LastErrorText)
    Exit Sub
End If


Dim fac As New Chilkat.FileAccess

' Examine the uncompressed and compressed sizes:
Dim originalPath As String = "qa_data/hamlet.xml"
' Note: The FileSize method returns a signed 32-bit integer.  If the file is potentially larger than 2GB, call FileSizeStr instead to return
' the size of the file as a string, then convert to an integer value.
Debug.WriteLine("uncompressed size: " & fac.FileSize(originalPath))
Debug.WriteLine("compressed size: " & fac.FileSize("qa_data/hamlet_compressed.dat"))

' Decompress in blocks..
Dim facSrc As New Chilkat.FileAccess
Dim facDest As New Chilkat.FileAccess

facSrc.OpenForRead("qa_data/hamlet_compressed.dat")

' If we compress in 32K chunks, find out how many blocks there will be.
Dim blockSize As Integer = 32768
Dim numBlocks As Integer = facSrc.GetNumBlocks(blockSize)

' Open an output file for the decompressed data.
Dim restoredPath As String = "qa_output/hamlet_restored.xml"
success = facDest.OpenForWrite(restoredPath)
If (success = False) Then
    Debug.WriteLine(facDest.LastErrorText)
    Exit Sub
End If


Dim decompressedStr As String
Dim compressedBytes() As Byte

' Assuming numBlocks > 1
compress.FirstChunk = True
compress.LastChunk = False

Dim i As Integer = 0
While i < numBlocks
    compressedBytes = facSrc.ReadBlock(i,blockSize)
    decompressedStr = compress.DecompressString(compressedBytes)

    facDest.AppendText(decompressedStr,"utf-8")

    i = i + 1

    compress.FirstChunk = False
    If (i = (numBlocks - 1)) Then
        compress.LastChunk = True
    End If

End While

facSrc.FileClose()
facDest.FileClose()

' Examine the size of the restored file.
Debug.WriteLine("restored size: " & fac.FileSize(restoredPath))

' Compare the contents of the original with the restored.
Dim bEqualContents As Boolean = fac.FileContentsEqual(restoredPath,originalPath)
Debug.WriteLine("Contents Equal: " & bEqualContents)