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Overview

========

This directory contains an update to the ZLib interface unit,

distributed by Borland as a Delphi supplemental component.

The original ZLib unit is Copyright (c) 1997,99 Borland Corp.,

and is based on zlib version 1.0.4.  There are a series of bugs

and security problems associated with that old zlib version, and

we recommend the users to update their ZLib unit.

Summary of modifications

========================

- Improved makefile, adapted to zlib version 1.2.1.

- Some field types from TZStreamRec are changed from Integer to

  Longint, for consistency with the zlib.h header, and for 64-bit

  readiness.

- The zlib_version constant is updated.

- The new Z_RLE strategy has its corresponding symbolic constant.

- The allocation and deallocation functions and function types

  (TAlloc, TFree, zlibAllocMem and zlibFreeMem) are now cdecl,

  and _malloc and _free are added as C RTL stubs.  As a result,

  the original C sources of zlib can be compiled out of the box,

  and linked to the ZLib unit.

Suggestions for improvements

============================

Currently, the ZLib unit provides only a limited wrapper around

the zlib library, and much of the original zlib functionality is

missing.  Handling compressed file formats like ZIP/GZIP or PNG

cannot be implemented without having this functionality.

Applications that handle these formats are either using their own,

duplicated code, or not using the ZLib unit at all.

Here are a few suggestions:

- Checksum class wrappers around adler32() and crc32(), similar

  to the Java classes that implement the java.util.zip.Checksum

  interface.

- The ability to read and write raw deflate streams, without the

  zlib stream header and trailer.  Raw deflate streams are used

  in the ZIP file format.

- The ability to read and write gzip streams, used in the GZIP

  file format, and normally produced by the gzip program.

- The ability to select a different compression strategy, useful

  to PNG and MNG image compression, and to multimedia compression

  in general.  Besides the compression level

    TCompressionLevel = (clNone, clFastest, clDefault, clMax);

  which, in fact, could have used the 'z' prefix and avoided

  TColor-like symbols

    TCompressionLevel = (zcNone, zcFastest, zcDefault, zcMax);

  there could be a compression strategy

    TCompressionStrategy = (zsDefault, zsFiltered, zsHuffmanOnly, zsRle);

- ZIP and GZIP stream handling via TStreams.

--

Cosmin Truta