(Visual FoxPro) Chunked Compression Using CompressBd2 with FirstChunk and LastChunk

See more Compression Examples

This example demonstrates how to compress data in multiple segments using the CompressBd2 method together with the FirstChunk and LastChunk properties. Instead of compressing all data in a single call, the input is divided into smaller chunks and processed sequentially, which is useful when handling streaming data or large inputs.

The example shows how to correctly mark the first, middle, and final chunks so that the compression stream is properly initialized and finalized. The compressed output is accumulated in a separate BinData object without modifying the input chunks. Finally, the example decompresses the combined result to verify that the original data is restored correctly.

This approach is ideal for scenarios where data is received incrementally, such as reading from a network stream, processing large files in parts, or handling real-time data feeds.

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LOCAL lnSuccess
LOCAL loCompress
LOCAL loBdOut
LOCAL lcPart1
LOCAL lcPart2
LOCAL lcPart3
LOCAL loBdIn
LOCAL lcCompressedBase64
LOCAL loBdDecompressed
LOCAL lcResultText

lnSuccess = 0

* This example assumes the Chilkat API has already been unlocked.
* See Global Unlock Sample for sample code.

loCompress = CreateObject('Chilkat.Compression')
loCompress.Algorithm = "zlib"

* This will accumulate the compressed output.
loBdOut = CreateObject('Chilkat.BinData')

* ------------------------------------------------------------------
* Simulate input arriving in chunks.
* ------------------------------------------------------------------

lcPart1 = "The quick brown fox "
lcPart2 = "jumps over the lazy dog. "
lcPart3 = "This text is split into chunks."

* ------------------------------------------------------------------
* Compress the first chunk
* ------------------------------------------------------------------

loCompress.FirstChunk = 1
loCompress.LastChunk = 0

loBdIn = CreateObject('Chilkat.BinData')
loBdIn.AppendString(lcPart1,"utf-8")

lnSuccess = loCompress.CompressBd2(loBdIn,loBdOut)
IF (lnSuccess = 0) THEN
    ? loCompress.LastErrorText
    RELEASE loCompress
    RELEASE loBdOut
    RELEASE loBdIn
    CANCEL
ENDIF

* ------------------------------------------------------------------
* Compress a middle chunk
* ------------------------------------------------------------------

loCompress.FirstChunk = 0
loCompress.LastChunk = 0

loBdIn.Clear()
loBdIn.AppendString(lcPart2,"utf-8")

lnSuccess = loCompress.CompressBd2(loBdIn,loBdOut)
IF (lnSuccess = 0) THEN
    ? loCompress.LastErrorText
    RELEASE loCompress
    RELEASE loBdOut
    RELEASE loBdIn
    CANCEL
ENDIF

* ------------------------------------------------------------------
* Compress the final chunk
* ------------------------------------------------------------------

loCompress.FirstChunk = 0
loCompress.LastChunk = 1

loBdIn.Clear()
loBdIn.AppendString(lcPart3,"utf-8")

lnSuccess = loCompress.CompressBd2(loBdIn,loBdOut)
IF (lnSuccess = 0) THEN
    ? loCompress.LastErrorText
    RELEASE loCompress
    RELEASE loBdOut
    RELEASE loBdIn
    CANCEL
ENDIF

* Get the final compressed result as base64 for display
lcCompressedBase64 = loBdOut.GetEncoded("base64")

? "Compressed data (base64):"
? lcCompressedBase64

* ------------------------------------------------------------------
* Decompress to verify correctness
* ------------------------------------------------------------------

loBdDecompressed = CreateObject('Chilkat.BinData')

loCompress.FirstChunk = 1
loCompress.LastChunk = 1

lnSuccess = loCompress.DecompressBd2(loBdOut,loBdDecompressed)
IF (lnSuccess = 0) THEN
    ? loCompress.LastErrorText
    RELEASE loCompress
    RELEASE loBdOut
    RELEASE loBdIn
    RELEASE loBdDecompressed
    CANCEL
ENDIF

* Convert decompressed bytes back to a string
lcResultText = loBdDecompressed.GetString("utf-8")

? "Decompressed text:"
? lcResultText

RELEASE loCompress
RELEASE loBdOut
RELEASE loBdIn
RELEASE loBdDecompressed