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Huffman Compression for MS Access

Uses Fredrik Qvarfort's Huffman Encoding/Decoding Class in an MS Access database as a module.

Assumes
Encoding Time: Inputfile: 3,114,776 Outputfile: 1,781,030 (57%) Time: 16.92 seconds Decoding (same file) Time: 15.60 seconds Scond File (small text file) Encoding: Inputfile: 18548 Outputfile: 11608 Time: .121 seconds Decoded file: .119 seconds Using an MS Access .MDE database: File 1: Encoded: 15.362 Seconds Decoded: 13.719 Seconds File 2: Encoded: .11 Seconds Decoded: .009 Seconds So performance is improved a bit using the .MDE file rather than the .MDB file. I have not attempted to create a .DLL or Class object and link to it or reference to it from MS Access yet. I would think a .DLL would be even faster - however, still not attaining the speeds of a pure VB compiled class. The test machine is a PIII 700 with 512 Megs of Ram running Windows 2000 SP2.

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'Huffman Encoding/Decoding Class
'-------------------------------
'
'(c) 2000, Fredrik Qvarfort
'
'Modified for MS Access by Chad M. Kovac
'http://www.TechKnowledgeyInc.com
'03/13/2002
Option Explicit
'Progress Values for the encoding routine
Private Const PROGRESS_CALCFREQUENCY = 7
Private Const PROGRESS_CALCCRC = 5
Private Const PROGRESS_ENCODING = 88
'Progress Values for the decoding routine
Private Const PROGRESS_DECODING = 89
Private Const PROGRESS_CHECKCRC = 11
'Events
'Event Progress(Procent As Integer)
Private Type HUFFMANTREE
 ParentNode As Integer
 RightNode As Integer
 LeftNode As Integer
 Value As Integer
 Weight As Long
End Type
Private Type ByteArray
 Count As Byte
 Data() As Byte
End Type
Private Declare Sub CopyMem Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
Public Sub EncodeFile(SourceFile As String, DestFile As String)
 Dim ByteArray() As Byte
 Dim Filenr As Integer
 
 'Make sure the source file exists
 If (Not FileExist(SourceFile)) Then
 Err.Raise vbObjectError, "clsHuffman.EncodeFile()", "Source file does not exist"
 End If
 
 'Read the data from the sourcefile
 Filenr = FreeFile
 Open SourceFile For Binary As #Filenr
 ReDim ByteArray(0 To LOF(Filenr) - 1)
 Get #Filenr, , ByteArray()
 Close #Filenr
 
 'Compress the data
 Call EncodeByte(ByteArray(), UBound(ByteArray) + 1)
 
 'If the destination file exist we need to
 'destroy it because opening it as binary
 'will not clear the old data
 If (FileExist(DestFile)) Then Kill DestFile
 
 'Save the destination string
 Open DestFile For Binary As #Filenr
 Put #Filenr, , ByteArray()
 Close #Filenr
End Sub
Public Sub DecodeFile(SourceFile As String, DestFile As String)
 Dim ByteArray() As Byte
 Dim Filenr As Integer
 
 'Make sure the source file exists
 If (Not FileExist(SourceFile)) Then
 Err.Raise vbObjectError, "clsHuffman.DecodeFile()", "Source file does not exist"
 End If
 
 'Read the data from the sourcefile
 Filenr = FreeFile
 Open SourceFile For Binary As #Filenr
 ReDim ByteArray(0 To LOF(Filenr) - 1)
 Get #Filenr, , ByteArray()
 Close #Filenr
 
 'Uncompress the data
 Call DecodeByte(ByteArray(), UBound(ByteArray) + 1)
 
 'If the destination file exist we need to
 'destroy it because opening it as binary
 'will not clear the old data
 If (FileExist(DestFile)) Then Kill DestFile
 
 'Save the destination string
 Open DestFile For Binary As #Filenr
 Put #Filenr, , ByteArray()
 Close #Filenr
End Sub
Private Sub CreateTree(Nodes() As HUFFMANTREE, NodesCount As Long, Char As Long, Bytes As ByteArray)
 Dim a As Integer
 Dim NodeIndex As Long
 
 NodeIndex = 0
 For a = 0 To (Bytes.Count - 1)
 If (Bytes.Data(a) = 0) Then
  'Left node
  If (Nodes(NodeIndex).LeftNode = -1) Then
  Nodes(NodeIndex).LeftNode = NodesCount
  Nodes(NodesCount).ParentNode = NodeIndex
  Nodes(NodesCount).LeftNode = -1
  Nodes(NodesCount).RightNode = -1
  Nodes(NodesCount).Value = -1
  NodesCount = NodesCount + 1
  End If
  NodeIndex = Nodes(NodeIndex).LeftNode
 ElseIf (Bytes.Data(a) = 1) Then
  'Right node
  If (Nodes(NodeIndex).RightNode = -1) Then
  Nodes(NodeIndex).RightNode = NodesCount
  Nodes(NodesCount).ParentNode = NodeIndex
  Nodes(NodesCount).LeftNode = -1
  Nodes(NodesCount).RightNode = -1
  Nodes(NodesCount).Value = -1
  NodesCount = NodesCount + 1
  End If
  NodeIndex = Nodes(NodeIndex).RightNode
 Else
  Stop
 End If
 Next
 
 Nodes(NodeIndex).Value = Char
End Sub
Public Sub EncodeByte(ByteArray() As Byte, ByteLen As Long)
Application.SysCmd acSysCmdInitMeter, "Encoding...", 100
 Dim i As Long
 Dim j As Long
 Dim Char As Byte
 Dim BitPos As Byte
 Dim lNode1 As Long
 Dim lNode2 As Long
 Dim lNodes As Long
 Dim lLength As Long
 Dim Count As Integer
 Dim lWeight1 As Long
 Dim lWeight2 As Long
 Dim Result() As Byte
 Dim ByteValue As Byte
 Dim ResultLen As Long
 Dim Bytes As ByteArray
 Dim NodesCount As Integer
 Dim NewProgress As Integer
 Dim CurrProgress As Integer
 Dim BitValue(0 To 7) As Byte
 Dim CharCount(0 To 255) As Long
 Dim Nodes(0 To 511) As HUFFMANTREE
 Dim CharValue(0 To 255) As ByteArray
 
 'If the source string is empty or contains
 'only one character we return it uncompressed
 'with the prefix string "HEO" & vbCr
 If (ByteLen = 0) Then
 ReDim Preserve ByteArray(0 To ByteLen + 3)
 If (ByteLen > 0) Then
  Call CopyMem(ByteArray(4), ByteArray(0), ByteLen)
 End If
 ByteArray(0) = 72 '"H"
 ByteArray(1) = 69 '"E"
 ByteArray(2) = 48 '"0"
 ByteArray(3) = 13 'vbCr
 Exit Sub
 End If
 
 'Create the temporary result array and make
 'space for identifier, checksum, textlen and
 'the ASCII values inside the Huffman Tree
 ReDim Result(0 To 522)
 
 'Prefix the destination string with the
 '"HE3" & vbCr identification string
 Result(0) = 72
 Result(1) = 69
 Result(2) = 51
 Result(3) = 13
 ResultLen = 4
 
 'Count the frequency of each ASCII code
 For i = 0 To (ByteLen - 1)
 CharCount(ByteArray(i)) = CharCount(ByteArray(i)) + 1
 If (i Mod 1000 = 0) Then
  NewProgress = i / ByteLen * PROGRESS_CALCFREQUENCY
  If (NewProgress <> CurrProgress) Then
  CurrProgress = NewProgress
  Application.SysCmd acSysCmdUpdateMeter, CurrProgress
  End If
 End If
 Next
 
 'Create a leaf for each character
 For i = 0 To 255
 If (CharCount(i) > 0) Then
  With Nodes(NodesCount)
  .Weight = CharCount(i)
  .Value = i
  .LeftNode = -1
  .RightNode = -1
  .ParentNode = -1
  End With
  NodesCount = NodesCount + 1
 End If
 Next
 
 'Create the Huffman Tree
 For lNodes = NodesCount To 2 Step -1
 'Get the two leafs with the smallest weights
 lNode1 = -1: lNode2 = -1
 For i = 0 To (NodesCount - 1)
  If (Nodes(i).ParentNode = -1) Then
  If (lNode1 = -1) Then
   lWeight1 = Nodes(i).Weight
   lNode1 = i
  ElseIf (lNode2 = -1) Then
   lWeight2 = Nodes(i).Weight
   lNode2 = i
  ElseIf (Nodes(i).Weight < lWeight1) Then
   If (Nodes(i).Weight < lWeight2) Then
   If (lWeight1 < lWeight2) Then
    lWeight2 = Nodes(i).Weight
    lNode2 = i
   Else
    lWeight1 = Nodes(i).Weight
    lNode1 = i
   End If
   Else
   lWeight1 = Nodes(i).Weight
   lNode1 = i
   End If
  ElseIf (Nodes(i).Weight < lWeight2) Then
   lWeight2 = Nodes(i).Weight
   lNode2 = i
  End If
  End If
 Next
 
 'Create a new leaf
 With Nodes(NodesCount)
  .Weight = lWeight1 + lWeight2
  .LeftNode = lNode1
  .RightNode = lNode2
  .ParentNode = -1
  .Value = -1
 End With
 
 'Set the parentnodes of the two leafs
 Nodes(lNode1).ParentNode = NodesCount
 Nodes(lNode2).ParentNode = NodesCount
 
 'Increase the node counter
 NodesCount = NodesCount + 1
 Next
 'Traverse the tree to get the bit sequence
 'for each character, make temporary room in
 'the data array to hold max theoretical size
 ReDim Bytes.Data(0 To 255)
 Call CreateBitSequences(Nodes(), NodesCount - 1, Bytes, CharValue)
 
 'Calculate the length of the destination
 'string after encoding
 For i = 0 To 255
 If (CharCount(i) > 0) Then
  lLength = lLength + CharValue(i).Count * CharCount(i)
 End If
 Next
 lLength = IIf(lLength Mod 8 = 0, lLength \ 8, lLength \ 8 + 1)
 
 'If the destination is larger than the source
 'string we leave it uncompressed and prefix
 'it with a 4 byte header ("HE0" & vbCr)
 If ((lLength = 0) Or (lLength > ByteLen)) Then
 ReDim Preserve ByteArray(0 To ByteLen + 3)
 Call CopyMem(ByteArray(4), ByteArray(0), ByteLen)
 ByteArray(0) = 72
 ByteArray(1) = 69
 ByteArray(2) = 48
 ByteArray(3) = 13
 Exit Sub
 End If
 
 'Add a simple checksum value to the result
 'header for corruption identification
 Char = 0
 For i = 0 To (ByteLen - 1)
 Char = Char Xor ByteArray(i)
 If (i Mod 10000 = 0) Then
  NewProgress = i / ByteLen * PROGRESS_CALCCRC + PROGRESS_CALCFREQUENCY
  If (NewProgress <> CurrProgress) Then
  CurrProgress = NewProgress
  Application.SysCmd acSysCmdUpdateMeter, CurrProgress
  End If
 End If
 Next
 Result(ResultLen) = Char
 ResultLen = ResultLen + 1
 
 'Add the length of the source string to the
 'header for corruption identification
 Call CopyMem(Result(ResultLen), ByteLen, 4)
 ResultLen = ResultLen + 4
 
 'Create a small array to hold the bit values,
 'this is faster than calculating on-fly
 For i = 0 To 7
 BitValue(i) = 2 ^ i
 Next
 
 'Store the number of characters used
 Count = 0
 For i = 0 To 255
 If (CharValue(i).Count > 0) Then
  Count = Count + 1
 End If
 Next
 Call CopyMem(Result(ResultLen), Count, 2)
 ResultLen = ResultLen + 2
 
 'Store the used characters and the length
 'of their respective bit sequences
 Count = 0
 For i = 0 To 255
 If (CharValue(i).Count > 0) Then
  Result(ResultLen) = i
  ResultLen = ResultLen + 1
  Result(ResultLen) = CharValue(i).Count
  ResultLen = ResultLen + 1
  Count = Count + 16 + CharValue(i).Count
 End If
 Next
 
 'Make room for the Huffman Tree in the
 'destination byte array
 ReDim Preserve Result(0 To ResultLen + Count \ 8)
 
 'Store the Huffman Tree into the result
 'converting the bit sequences into bytes
 BitPos = 0
 ByteValue = 0
 For i = 0 To 255
 With CharValue(i)
  If (.Count > 0) Then
  For j = 0 To (.Count - 1)
   If (.Data(j)) Then ByteValue = ByteValue + BitValue(BitPos)
   BitPos = BitPos + 1
   If (BitPos = 8) Then
   Result(ResultLen) = ByteValue
   ResultLen = ResultLen + 1
   ByteValue = 0
   BitPos = 0
   End If
  Next
  End If
 End With
 Next
 If (BitPos > 0) Then
 Result(ResultLen) = ByteValue
 ResultLen = ResultLen + 1
 End If
 
 'Resize the destination string to be able to
 'contain the encoded string
 ReDim Preserve Result(0 To ResultLen - 1 + lLength)
 
 'Now we can encode the data by exchanging each
 'ASCII byte for its appropriate bit string.
 Char = 0
 BitPos = 0
 For i = 0 To (ByteLen - 1)
 With CharValue(ByteArray(i))
  For j = 0 To (.Count - 1)
  If (.Data(j) = 1) Then Char = Char + BitValue(BitPos)
  BitPos = BitPos + 1
  If (BitPos = 8) Then
   Result(ResultLen) = Char
   ResultLen = ResultLen + 1
   BitPos = 0
   Char = 0
  End If
  Next
 End With
 If (i Mod 10000 = 0) Then
  NewProgress = i / ByteLen * PROGRESS_ENCODING + PROGRESS_CALCCRC + PROGRESS_CALCFREQUENCY
  If (NewProgress <> CurrProgress) Then
  CurrProgress = NewProgress
  Application.SysCmd acSysCmdUpdateMeter, CurrProgress
  End If
 End If
 Next
 'Add the last byte
 If (BitPos > 0) Then
 Result(ResultLen) = Char
 ResultLen = ResultLen + 1
 End If
 
 'Return the destination in string format
 ReDim ByteArray(0 To ResultLen - 1)
 Call CopyMem(ByteArray(0), Result(0), ResultLen)
 'Make sure we get a "100%" progress message
 If (CurrProgress <> 100) Then
 Application.SysCmd acSysCmdUpdateMeter, 100
 End If
Application.SysCmd acSysCmdClearStatus
End Sub
Public Function DecodeString(Text As String) As String
 
 Dim ByteArray() As Byte
 
 'Convert the string to a byte array
 ByteArray() = StrConv(Text, vbFromUnicode)
 
 'Compress the byte array
 Call DecodeByte(ByteArray, Len(Text))
 
 'Convert the compressed byte array to a string
 DecodeString = StrConv(ByteArray(), vbUnicode)
 
End Function
Public Function EncodeString(Text As String) As String
 
 Dim ByteArray() As Byte
 
 'Convert the string to a byte array
 ByteArray() = StrConv(Text, vbFromUnicode)
 
 'Compress the byte array
 Call EncodeByte(ByteArray, Len(Text))
 
 'Convert the compressed byte array to a string
 EncodeString = StrConv(ByteArray(), vbUnicode)
 
End Function
Public Sub DecodeByte(ByteArray() As Byte, ByteLen As Long)
Application.SysCmd acSysCmdInitMeter, "Decoding...", 100
 Dim i As Long
 Dim j As Long
 Dim Pos As Long
 Dim Char As Byte
 Dim CurrPos As Long
 Dim Count As Integer
 Dim CheckSum As Byte
 Dim Result() As Byte
 Dim BitPos As Integer
 Dim NodeIndex As Long
 Dim ByteValue As Byte
 Dim ResultLen As Long
 Dim NodesCount As Long
 Dim lResultLen As Long
 Dim NewProgress As Integer
 Dim CurrProgress As Integer
 Dim BitValue(0 To 7) As Byte
 Dim Nodes(0 To 511) As HUFFMANTREE
 Dim CharValue(0 To 255) As ByteArray
 
 If (ByteArray(0) <> 72) Or (ByteArray(1) <> 69) Or (ByteArray(3) <> 13) Then
 'The source did not contain the identification
 'string "HE?" & vbCr where ? is undefined at
 'the moment (does not matter)
 ElseIf (ByteArray(2) = 48) Then
 'The text is uncompressed, return the substring
 'Decode = Mid$(Text, 5)
 Call CopyMem(ByteArray(0), ByteArray(4), ByteLen - 4)
 ReDim Preserve ByteArray(0 To ByteLen - 5)
 Exit Sub
 ElseIf (ByteArray(2) <> 51) Then
 'This is not a Huffman encoded string
 Err.Raise vbObjectError, "HuffmanDecode()", "The data either was not compressed with HE3 or is corrupt (identification string not found)"
 Exit Sub
 End If
 
 CurrPos = 5
 
 'Extract the checksum
 CheckSum = ByteArray(CurrPos - 1)
 CurrPos = CurrPos + 1
 
 'Extract the length of the original string
 Call CopyMem(ResultLen, ByteArray(CurrPos - 1), 4)
 CurrPos = CurrPos + 4
 lResultLen = ResultLen
 
 'If the compressed string is empty we can
 'skip the function right here
 If (ResultLen = 0) Then Exit Sub
 
 'Create the result array
 ReDim Result(0 To ResultLen - 1)
 
 'Get the number of characters used
 Call CopyMem(Count, ByteArray(CurrPos - 1), 2)
 CurrPos = CurrPos + 2
 
 'Get the used characters and their
 'respective bit sequence lengths
 For i = 1 To Count
 With CharValue(ByteArray(CurrPos - 1))
  CurrPos = CurrPos + 1
  .Count = ByteArray(CurrPos - 1)
  CurrPos = CurrPos + 1
  ReDim .Data(0 To .Count - 1)
 End With
 Next
 
 'Create a small array to hold the bit values,
 'this is (still) faster than calculating on-fly
 For i = 0 To 7
 BitValue(i) = 2 ^ i
 Next
 
 'Extract the Huffman Tree, converting the
 'byte sequence to bit sequences
 ByteValue = ByteArray(CurrPos - 1)
 CurrPos = CurrPos + 1
 BitPos = 0
 For i = 0 To 255
 With CharValue(i)
  If (.Count > 0) Then
  For j = 0 To (.Count - 1)
   If (ByteValue And BitValue(BitPos)) Then .Data(j) = 1
   BitPos = BitPos + 1
   If (BitPos = 8) Then
   ByteValue = ByteArray(CurrPos - 1)
   CurrPos = CurrPos + 1
   BitPos = 0
   End If
  Next
  End If
 End With
 Next
 If (BitPos = 0) Then CurrPos = CurrPos - 1
 
 'Create the Huffman Tree
 NodesCount = 1
 Nodes(0).LeftNode = -1
 Nodes(0).RightNode = -1
 Nodes(0).ParentNode = -1
 Nodes(0).Value = -1
 For i = 0 To 255
 Call CreateTree(Nodes(), NodesCount, i, CharValue(i))
 Next
 
 'Decode the actual data
 ResultLen = 0
 For CurrPos = CurrPos To ByteLen
 ByteValue = ByteArray(CurrPos - 1)
 For BitPos = 0 To 7
  If (ByteValue And BitValue(BitPos)) Then
  NodeIndex = Nodes(NodeIndex).RightNode
  Else
  NodeIndex = Nodes(NodeIndex).LeftNode
  End If
  If (Nodes(NodeIndex).Value > -1) Then
  Result(ResultLen) = Nodes(NodeIndex).Value
  ResultLen = ResultLen + 1
  If (ResultLen = lResultLen) Then GoTo DecodeFinished
  NodeIndex = 0
  End If
 Next
 If (CurrPos Mod 10000 = 0) Then
  NewProgress = CurrPos / ByteLen * PROGRESS_DECODING
  If (NewProgress <> CurrProgress) Then
  CurrProgress = NewProgress
  Application.SysCmd acSysCmdUpdateMeter, CurrProgress
    Application.SysCmd acSysCmdUpdateMeter, CurrProgress
  End If
 End If
 Next
DecodeFinished:
 'Verify data to check for corruption.
 Char = 0
 For i = 0 To (ResultLen - 1)
 Char = Char Xor Result(i)
 If (i Mod 10000 = 0) Then
  NewProgress = i / ResultLen * PROGRESS_CHECKCRC + PROGRESS_DECODING
  If (NewProgress <> CurrProgress) Then
  CurrProgress = NewProgress
  Application.SysCmd acSysCmdUpdateMeter, CurrProgress
  End If
 End If
 Next
 If (Char <> CheckSum) Then
 Err.Raise vbObjectError, "clsHuffman.Decode()", "The data might be corrupted (checksum did not match expected value)"
 End If
 'Return the uncompressed string
 ReDim ByteArray(0 To ResultLen - 1)
 Call CopyMem(ByteArray(0), Result(0), ResultLen)
 
 'Make sure we get a "100%" progress message
 If (CurrProgress <> 100) Then
  Application.SysCmd acSysCmdUpdateMeter, 100
 End If
Application.SysCmd acSysCmdClearStatus
End Sub
Private Sub CreateBitSequences(Nodes() As HUFFMANTREE, ByVal NodeIndex As Integer, Bytes As ByteArray, CharValue() As ByteArray)
 Dim NewBytes As ByteArray
 
 'If this is a leaf we set the characters bit
 'sequence in the CharValue array
 If (Nodes(NodeIndex).Value > -1) Then
 CharValue(Nodes(NodeIndex).Value) = Bytes
 Exit Sub
 End If
 
 'Traverse the left child
 If (Nodes(NodeIndex).LeftNode > -1) Then
 NewBytes = Bytes
 NewBytes.Data(NewBytes.Count) = 0
 NewBytes.Count = NewBytes.Count + 1
 Call CreateBitSequences(Nodes(), Nodes(NodeIndex).LeftNode, NewBytes, CharValue)
 End If
 
 'Traverse the right child
 If (Nodes(NodeIndex).RightNode > -1) Then
 NewBytes = Bytes
 NewBytes.Data(NewBytes.Count) = 1
 NewBytes.Count = NewBytes.Count + 1
 Call CreateBitSequences(Nodes(), Nodes(NodeIndex).RightNode, NewBytes, CharValue)
 End If
 
End Sub
Private Function FileExist(Filename As String) As Boolean
 On Error GoTo FileDoesNotExist
 
 Call FileLen(Filename)
 FileExist = True
 Exit Function
 
FileDoesNotExist:
 FileExist = False
 
End Function