ZLIB_Deflate

Compresses data using the ZLIB deflate algorithm.

VBA/VB6 Syntax

nRet = ZLIB_Deflate(lpOutput(0), nOutBytes, abInput(0), nInputLen) ' Note the "(0)" after the byte array parameters

C/C++ Syntax

Parameters

lpOutput

[out] abyte buffer to receive output data.

nOutBytes

[in] size of the output buffer in bytes.

lpInput

[in] input data to be compressed.

nBytes

[in] length of input array in bytes.

Returns (VBA/C)

The number of bytes successfully copied into the output buffer or the required size in bytes. If an error occurs, it returns a negative error code.

VBA Wrapper Syntax

.NET Equivalent

Zlib.Deflate Method

Remarks

To determine the required size of the output buffer, call the function with nOutBytes set to zero (or lpOutput set to NULL).

Example (VBA core function)

Dim strPlain As String
Dim strBack As String
Dim abPlain() As Byte
Dim abCompressed() As Byte
Dim nCompLen As Long
Dim nUncompLen As Long
Dim nRet As Long

' COMPRESSSION (deflation)

' Set the plaintext message
strPlain = "hello, hello, hello. This is a 'hello world' message " & _
    "for the world, repeat, for the world."
' Convert to an array of bytes
abPlain = StrConv(strPlain, vbFromUnicode)
nUncompLen = UBound(abPlain) + 1
' Find required compressed length by calling with zero length value
nCompLen = ZLIB_Deflate(0, 0, abPlain(0), nUncompLen)
Debug.Print "COMPR-LEN=" & nCompLen
ReDim abCompressed(nCompLen - 1)
' Now compress plaintext
Call ZLIB_Deflate(abCompressed(0), nCompLen, abPlain(0), nUncompLen)
' Display compressed data in hex
Debug.Print "COMPR-DATA=" & cnvHexStrFromBytes(abCompressed)

' DECOMPRESSSION (inflation)

' Uncompress the compressed data
' New in [v5.3]: we can find the uncompressed length from compressed data
nUncompLen = ZLIB_Inflate(0, 0, abCompressed(0), nCompLen)
Debug.Print "Uncompressed length = " & nUncompLen
Debug.Assert (nUncompLen > 0)
ReDim abPlain(nUncompLen - 1)
nRet = ZLIB_Inflate(abPlain(0), nUncompLen, abCompressed(0), nCompLen)

' Convert back to a string
strBack = StrConv(abPlain, vbUnicode)
Debug.Print strBack

COMPR-LEN=68
COMPR-DATA=789CCB48CDC9C9D751C840A2F4144232328B15802851411D2CA2509E5F9493A2AE909B5A5C9C989EAA90965FA45092910A11D651284A2D484D2CD14115D6030086D11F4E
Uncompressed length = 90
hello, hello, hello. This is a 'hello world' message for the world, repeat, for the world.	

Example (VBA wrapper function)

Dim strPlain As String
Dim lpToCompress() As Byte
Dim lpCompressed() As Byte
Dim lpUncompressed() As Byte
strPlain = "hello, hello, hello. This is a 'hello world' message " & _
    "for the world, repeat, for the world."
lpToCompress = StrConv(strPlain, vbFromUnicode)
lpCompressed = zlibDeflate(lpToCompress)
Debug.Print "OK=        " & "789CCB48CDC9C9D751C840A2F4144232328B15802851411D2CA2509E5F9493A2AE909B5A5C9C989EAA90965FA45092910A11D651284A2D484D2CD14115D6030086D11F4E"
Debug.Print "COMPRESSED=" & cnvHexStrFromBytes(lpCompressed)
lpUncompressed = zlibInflate(lpCompressed)
Debug.Print "'" & StrConv(lpUncompressed, vbUnicode); "'"

See Also

ZLIB_Inflate

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