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ZLibCompress

Function ZLibCompress( const Data : TBytes) : TBytes

Example​

procedure ScriptEvent(var Value: variant);
var
  Data: TBytes;
begin
  Data := Base64DecodeBytes('RXhhbXBsZSBwYXlsb2Fk');
  Value := Base64EncodeBytes(ZLibCompress(Data));
end;

Usage​

ZLibCompress compresses a byte array into a zlib-wrapped deflate stream at Velox's default level.

Parameters

NameTypeDescription
DataTBytes, constComplete uncompressed payload. No character encoding or content schema is applied.

Returns

A new byte array containing zlib framing, deflate data and the zlib checksum.

Behaviour

The function preserves bytes, not semantic values. A successful round-trip through ZLibDecompress returns the original byte sequence.

Errors

Velox raises EZCompressionError for zlib failures. Allocation and range errors can also propagate. The RTL clears its output on its protected exception path.

Usage notes

Use this function only when the peer expects zlib framing. Protocols that require raw deflate, gzip or ZIP need their corresponding function/container rather than the same bytes with a different extension.

Additional Technical Info​

ZLibCompress compresses a complete byte array as a zlib-wrapped deflate stream at Delphi's zcDefault level. The result is not a gzip member and is not a ZIP archive.

The example compresses fictional bytes and represents the binary result as Base64. It is source-reviewed and is not executed by the documentation workflow.

Implementation

The scripting import registers vxCompression.ZLibCompress directly. The wrapper selects the System.ZLib.ZCompress(const inBuffer: TBytes; out outBuffer: TBytes; level) overload with zcDefault. In the installed Studio 37.0 RTL, Delphi allocates an estimate rounded to 256 bytes, calls DeflateInit, repeatedly finishes deflate while growing by 256 bytes, calls deflateEnd and shrinks the output to total_out.

Edge cases and quirks

  • For an empty array, the current RTL starts with a zero-length output array and takes element-zero addresses before zlib can grow the output. With ordinary production compiler settings this can produce a valid compressed empty stream, but range-check settings make those zero-index expressions an implementation boundary. Do not assume identical behaviour in a differently compiled runtime.
  • The zlib format includes its own lightweight integrity check but provides no authentication or confidentiality.
  • Input, estimates and zlib counters are bounded by 32-bit sizes in this overload. This is not a streaming large-file API.
  • zcDefault delegates the actual compression level to the embedded zlib implementation; scripts cannot select another level through this entry.

Side effects

None outside memory allocation.

Performance and concurrency

The complete input and result coexist in memory. Compression is synchronous and CPU cost varies with size and compressibility. All stream state is local to the call.

Related entries

External references

Created 2026-07-15