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Microsoft Access VBA and VB6 Module Library

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Class: RLE in Category General VBA/VB6 : Encryption and Compression from Total Visual SourceBook

Class that implements run length encoding (RLE) to compress files and strings with VB6 and VBA.

Run Length Encoding algorithms reduce strings of recurring characters to a single character, followed by a count of how many times the character occurred.

Procedure List

Procedure Name	Type	Description
(Declarations)	Declarations	Declarations and private variables for the CRLE class.
InputFileName	Property	Get the input file name.
OutputFileName	Property	Get the output file name.
Compress	Method	Compress the input file to the output file.
CompressString	Method	Compress an input string.
Decompress	Method	Decompress the input file to the output file.
DecompressString	Method	Decompress a string that was previously compressed using the CompressString method of this class.
dblToLong	Private	Perform an unsigned conversion from a double value to a long value. This function correctly handles doubles greater than 2,147,483,647 and less than or equal to 4,294,967,295.
ExpandBufferRun	Private	Write a run of bytes to a buffer.
ExpandRun	Private	Write a run of bytes to the file.
HiByte	Private	Get the high byte of the passed integer.
HiWord	Private	Get the high integer of the passed long.
IntToLong	Private	Convert an integer value to a long integer value, treating the integer as unsigned. For example, performing the following assignment lngNumber = -1 assigns -1 to the variable lngNumber. Assigning lngNumber = IntToLong(-1) assigns 65,535 to lngNumber.
LoByte	Private	Get the low byte of the passed integer.
LongToInt	Private	Perform an unsigned conversion from a long integer value to an integer value.
LoWord	Private	Get the low integer of the passed long.
ReadFile	Private	Read the specified number of bytes from the file. Using this function significantly increases the speed of processing files. The alternative to using a file buffer is reading a byte at a time from the file.
Shll	Private	Shift a long integer value left by the specified number of bits. Left shifting is a multiplication operation. For the number of bits to shift to the left, raise two to that power, then multiply the result by the original value.
WriteBufferRun	Private	Write a run of bytes to the output buffer.
WriteRun	Private	Write a run of bytes to the file.

Example Code for Using Class: RLE

' Example of CRLE
'
' To try this example, do the following:
' 1. Create a new form
' 2. Create a command button named 'cmdTest'
' 3. Paste all the code from this example to the new form's module.
' 4. Run the form

' This example assumes that the sample files are located in the folder named by the following constant.
Private Const mcstrSamplePath As String = "C:\TVSBSamp"

Private WithEvents mclsRLE As CRLE

Private Sub cmdTest_Click()
  ' Comments: Examples of compressing and decompressing files and strings using run length encoding (RLE)
  '           Run Length Encoding algorithms reduce strings of recurring characters to a single character,
  '           followed by a count of how many times the character occurred.

  Const cstrOriginal As String = mcstrSamplePath & "\original.bmp"
  Const cstrRLE As String = mcstrSamplePath & "\compress.RLE"
  Const cstrDecompress As String = mcstrSamplePath & "\decompress-rle.bmp"

  Dim dblCompressionRatio As Double
  Dim strString As String
  Dim strCompressed As String

  ' Initialize class
  Set mclsRLE = New CRLE

  ' Example of RLE encoding a file
  mclsRLE.InputFileName = cstrOriginal
  mclsRLE.OutputFileName = cstrRLE
  mclsRLE.Compress

  ' Display the compression ratio
  dblCompressionRatio = FileLen(cstrRLE) / FileLen(cstrOriginal)
  MsgBox cstrOriginal & " file compressed to " & Format$(dblCompressionRatio, "Percent") & " of its original size"

  ' Example of RLE decoding a file
  mclsRLE.InputFileName = cstrRLE
  mclsRLE.OutputFileName = cstrDecompress
  mclsRLE.Decompress
  MsgBox cstrDecompress & " created by decompressing " & cstrRLE

  ' Example of compressing a string
  strString = "AAAABBBBCCCCDDDDDD"
  strCompressed = mclsRLE.CompressString(StrConv(strString, vbFromUnicode))

  ' Example of decompressing a string
  strString = StrConv(mclsRLE.DecompressString(strCompressed), vbUnicode)

  ' Display the compression ratio
  dblCompressionRatio = Len(strCompressed) / Len(strString)
  MsgBox "String compressed to " & Format$(dblCompressionRatio, "Percent") & " of its original size"

  ' Close
  Set mclsRLE = Nothing

End Sub

Private Sub mclsRLE_Progress(dblPercentage As Double)
  ' Comments: This procedure captures the Progress Event and reveals the percentage completion for the file that's being compressed or decoded.
  '           This example puts the information on the Immediate Window. You could show the information on a form as text or display a graphic.
  ' Params  : dblPercentage       Percent of completion between 0 and 1

  Debug.Print "Percent done: " & Format$(dblPercentage, "Percent")

End Sub

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