This is a BASE85 Encoding Visual Basic class that expands the text to be encoded by 4:5 i.e. uses 5 ASCII characters to represent 4 bytes with 80% accuracy. Used in Postscript and PDF documents. Useful if binary data needs to be persisted in ASCII text.
Currently it’s main uses are in Adobe PDF and PostScript file formats. It is also used by git to produce path of binary files i.e. used to encode the patch files. Here is their C implementation of Base85 encoding source code. This code extensively uses Modulus Operator to do the calculations and if-else statements. You can find more details on Modulus operator and some sample usage in this article. There is an article on conditional operations i.e. if-else in C.
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Used in Postscript ' and PDF documents. Useful if binary data needs to be ' persisted in ASCII text, see also base 64 for email. ' --------------------------------------------------------------- ' Public properties Public bolHeader As Boolean Public bolPreserve As Boolean Private intCount As Integer Private intWidth As Integer Private intPossition As Integer Private dblCurValue As Double Private strBuffer As String Private bBuffer() As Byte Private dblBufferPos As Double ' ---------------------------- ' Public Methods ' ---------------------------- ' Creates base85 encoded sections, if bolHeader is true then the ' "<~" is written to the buffer immediately before the base85 ' encoded data. Public Function Encode(ByRef bArray() As Byte) As String ' Encode here Dim d As Double ' Clear and reset current buffer strBuffer = vbNullString dblCurValue = 0 intCount = 0 intPossition = 0 If bolHeader Then init85 For d = 0 To UBound(bArray) put85 Val(bArray(d)) Next ' Purge partial value if exits and add footer. cleanup85 ' Return the buffer Encode = strBuffer End Function ' Decodes byte array, if bolHeader is true then the routine will look ' for the "<~" headers and decode the sections. If bolHeader is false ' then the function assumes it has been passed a single bose85 section ' terminated with "~>" Public Function Decode(ByVal strIn As String) As Byte() ' Decode here Dim x As Double Dim c As Integer Dim l As Double ' Get the length of the string l = Len(strIn) ' Clear current buffer dblBufferPos = 0 ReDim bBuffer(l - 1) If bolHeader Then ' Cycle through the characters looking for a starting point. For x = 0 To l - 1 c = Asc(Mid$(strIn, x + 1, 1)) If c = Asc("<") Then ' We may have found one unless it is the last character If x <> l - 1 Then ' It is a starting point if followed by a ~ character If Asc(Mid$(strIn, x + 2, 1)) = Asc("~") And (x <> l - 2) Then ' Found a starting point x = decode85(strIn, x + 2, l) Else ' False alarm If bolPreserve Then putbyte c End If Else ' Last character If bolPreserve Then putbyte c End If Else ' Havent found starting point If bolPreserve Then putbyte c End If Next x Else ' We have been given a single base85 stream that should end with "~>" x = decode85(strIn, 0, l) End If ' Return the buffer Decode = bBuffer End Function ' ---------------------------- ' Private Methods ' ---------------------------- Private Sub Class_Initialize() ' Write header (Default) bolHeader = True ' Preserve non Base85 sections bolPreserve = True intCount = 0 ' 72 characters seems like a nice round number intWidth = 72 strBuffer = vbNullString intPossition = 0 dblCurValue = 0 dblBufferPos = 0 End Sub ' ---------------------------- ' Support Routines ' ---------------------------- Private Function decode85(ByVal strIn As String, ByVal x As Double, ByVal l As Double) As Double Dim c As Integer Dim y As Double Dim dblValue As Double Dim intBytes As Integer ' Start decoding characters at possition x For y = x To l - 1 c = Asc(Mid$(strIn, y + 1, 1)) Select Case c Case 0 ' Asc(vbNullChar) Case 9 ' Asc(vbTab) Case 10 ' Asc(vbNewLine) Case 12 ' Asc(vbFormFeed) Case 13 ' Asc(vbCr) Case 32 ' Asc(vbBack) ' Whitespace formating characters, strip and forget Case Asc("z") ' Zero value found If intBytes <> 0 Then ' Corrupt file? ' z found inside ascii85 structure Else putbyte (0) putbyte (0) putbyte (0) putbyte (0) End If Case Asc("~") If Asc(Mid$(strIn, y + 2, 1)) = Asc(">") Then ' Skip over footer y = y + 2 Else ' Did not find the second character of the footer y = y + 1 End If If intBytes > 0 Then intBytes = intBytes - 1 dblValue = dblValue + (85 ^ (4 - intBytes)) wput dblValue, intBytes End If Exit For Case Else If c < Asc("!") Then ' Corrupt file? ' Bad character in ascii85 region Err.Raise vbObjectError + 1, , "Bad character in ascii85 region" ElseIf c > Asc("u") Then ' Corrupt file? ' Bad character in ascii85 region Err.Raise vbObjectError + 1, , "Bad character in ascii85 region" Else dblValue = dblValue + (c - 33) * (85 ^ (4 - intBytes)) intBytes = intBytes + 1 If intBytes = 5 Then wput dblValue, 4 intBytes = 0 dblValue = 0 End If End If End Select Next y ' Return the position at which we stopped decoding so the the ' main decode routine can continue to find other Base85 sections decode85 = y End Function ' Break value down by shifting to the right. We want the lower byte. Private Sub wput(ByVal dblValue As Double, ByVal intBytes As Integer) Select Case intBytes Case 4 putbyte (RShift(dblValue, 24) Mod 256) putbyte (RShift(dblValue, 16) Mod 256) putbyte (RShift(dblValue, 8) Mod 256) putbyte (dblValue Mod 256) Case 3 putbyte (RShift(dblValue, 24) Mod 256) putbyte (RShift(dblValue, 16) Mod 256) putbyte (RShift(dblValue, 8) Mod 256) Case 2 putbyte (RShift(dblValue, 24) Mod 256) putbyte (RShift(dblValue, 16) Mod 256) Case 1 putbyte (RShift(dblValue, 24) Mod 256) End Select End Sub Private Sub encode85() Dim i As Integer Dim y As Integer Dim strArray(5) As Byte i = 5 y = 0 Do ' get the the modulus of the current value when devided by 85, ' rounding floating-point numbers to integers strArray(y) = dblCurValue Mod 85 y = y + 1 ' divide current value by 85 returning an integer result dblCurValue = dblCurValue \ 85 i = i - 1 Loop While i > 0 ' Number of elements in array that have useful data i = intCount Do y = y - 1 ' Add 33 or ASC value of ! character to make sure it is in the ' correct range. putchar Chr$(strArray(y) + 33) intPossition = intPossition + 1 If (intPossition) >= intWidth Then ' At the end of the specified width intPossition = 0 putchar vbNewLine End If i = i - 1 Loop While i > -1 End Sub ' Cycle through the values shifting them, on the 4th value encode and ' write to output. Private Sub put85(ByVal c As Double) intCount = intCount + 1 Select Case intCount - 1 Case 0 dblCurValue = dblCurValue Or LShift(c, 24) Case 1 dblCurValue = dblCurValue Or LShift(c, 16) Case 2 dblCurValue = dblCurValue Or LShift(c, 8) Case 3 dblCurValue = dblCurValue Or c If (dblCurValue = 0) Then putchar ("z") intPossition = intPossition + 1 If (intPossition) >= intWidth Then ' At the end of the specified width intPossition = 0 putchar vbNewLine End If Else encode85 End If dblCurValue = 0 intCount = 0 End Select End Sub ' Create the header (Optional) Private Sub init85() putchar "<~" ' Header intPossition = 2 End Sub ' Write any values still left to encode and attach footer Private Sub cleanup85() If (intCount > 0) Then encode85 If (intPossition + 2) > intWidth Then putchar vbNewLine putchar "~>" ' Footer End Sub ' Append characters to the string buffer Private Sub putchar(ByVal strIn As String) ' Debug.Print "Putting character " & strIn strBuffer = strBuffer & strIn End Sub ' Append bytes to the byte buffer Private Sub putbyte(ByVal bIn As Byte) ' Debug.Print "Putting byte " & Chr(bIn) bBuffer(dblBufferPos) = bIn dblBufferPos = dblBufferPos + 1 End Sub ' Thank you Lewis Moten. Why doen't VB support this? Private Function LShift(ByVal pnValue As Double, ByVal pnShift As Double) As Double ' Equivilant to C's Bitwise << operator LShift = pnValue * (2 ^ pnShift) End Function Private Function RShift(ByVal pnValue As Double, ByVal pnShift As Double) As Double ' Equivilant to C's Bitwise >> operator RShift = pnValue \ (2 ^ pnShift) End Function |
Similarly there are Base64 encoding and decoding schemes which are commonly used to encode binary data.
