Huffman compression uses character frequencies to compress data. Bit codes are assigned to each character, with shorter bitcodes for more common characters, and longer bitcodes for the less common characters.
|(Declarations)||Declarations||Declarations and private variables for the CHuffman class.|
|InputFileName||Property||Get the name of the input file.|
|OutputFileName||Property||Get the name of the output file.|
|Compress||Method||Compresses the input file to the output file.|
|Decompress||Method||Decompresses the input file to the output file.|
|BuildTree||Private||Builds a Huffman tree based on the character frequency of the input data.|
|CompressByte||Private||Compresses a single byte.|
|Encode||Private||Huffman encodes a byte.|
|IntToByte||Private||Perform an unsigned conversion from an integer value to a byte value. This procedure correctly handles any integer value.
For example, lngNumber = -1 assigns -1 to the variable lngNumber. However, lngNumber = IntToLong(-1) assigns 65,535 to lngNumber.
|LongToInt||Private||Perform an unsigned conversion from a long integer to an integer value.|
|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.|
|Shlb||Private||Shift a byte 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.|
|WriteBit||Private||Write to the output file a bit at a time.|
|WriteFinish||Private||Flush any remaining data to the output file.|
' Example of CHuffman ' ' 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 mclsHuffman As CHuffman Private Sub cmdTest_Click() ' Comments: Examples of compressing and decompressing files using Huffman compression algorithms ' Huffman compression uses character frequencies to compress data. ' Bit codes are assigned to each character, with shorter bitcodes for more common characters, and ' longer bitcodes for the less common characters. Const cstrOriginal As String = mcstrSamplePath & "\sample.txt" Const cstrHuffman As String = mcstrSamplePath & "\compress.huf" Const cstrDecompress As String = mcstrSamplePath & "\decompress-huffman.txt" Dim dblCompressionRatio As Double ' Initialize class Set mclsHuffman = New CHuffman ' Example Huffman encoding a file mclsHuffman.InputFileName = cstrOriginal mclsHuffman.OutputFileName = cstrHuffman mclsHuffman.Compress ' Display the compression achieved dblCompressionRatio = FileLen(cstrHuffman) / FileLen(cstrOriginal) MsgBox "File compressed to " & Format$(dblCompressionRatio, "Percent") & " of its original size" ' Example Huffman decoding a file mclsHuffman.InputFileName = cstrHuffman mclsHuffman.OutputFileName = cstrDecompress mclsHuffman.Decompress MsgBox cstrDecompress & " created by decompressing " & cstrHuffman ' Close class Set mclsHuffman = New CHuffman End Sub Private Sub mclsHuffman_FileProgress(dblPercentage As Double) ' Comments: This procedure captures the Progress Event and reveals the percentage completion for the file that's being compressed ' This example puts the information on the Immediate Window. You could show the information on a form as text or display a graphic. ' Params : dblPercentage Percent of completion between 0 and 1 Debug.Print "Percent done: " & Format$(dblPercentage, "Percent") End Sub
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