Xojo Plugin

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. Xojo Plugin

Dim success As Boolean
success = 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
    System.DebugLog(compress.LastErrorText)
    Return
End If

Dim fac As New Chilkat.FileAccess

// Examine the uncompressed and compressed sizes:
Dim originalPath As String
originalPath = "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.
System.DebugLog("uncompressed size: " + Str(fac.FileSize(originalPath)))
System.DebugLog("compressed size: " + Str(fac.FileSize("qa_data/hamlet_compressed.dat")))

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

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

// If we compress in 32K chunks, find out how many blocks there will be.
Dim blockSize As Int32
blockSize = 32768
Dim numBlocks As Int32
numBlocks = facSrc.GetNumBlocks(blockSize)

// Open an output file for the decompressed data.
Dim restoredPath As String
restoredPath = "qa_output/hamlet_restored.xml"
success = facDest.OpenForWrite(restoredPath)
If (success = False) Then
    System.DebugLog(facDest.LastErrorText)
    Return
End If

Dim decompressedStr As String
Dim compressedBytes As MemoryBlock

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

Dim i As Int32
i = 0
While i < numBlocks
    compressedBytes = facSrc.ReadBlock(i,blockSize)
    decompressedStr = compress.DecompressString(compressedBytes)

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

    i = i + 1

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

Wend

facSrc.FileClose 
facDest.FileClose 

// Examine the size of the restored file.
System.DebugLog("restored size: " + Str(fac.FileSize(restoredPath)))

// Compare the contents of the original with the restored.
Dim bEqualContents As Boolean
bEqualContents = fac.FileContentsEqual(restoredPath,originalPath)
System.DebugLog("Contents Equal: " + Str(bEqualContents))