by Derek Haas (4 Submissions)
Category: Files/File Controls/Input/Output
Compatability: Visual Basic 3.0
Difficulty: Unknown Difficulty
Date Added: Wed 3rd February 2021
Rating: 
(2 Votes)
This module allows you to view a file as a collection of bits rather than as a collection of bytes. It allows you to read/write a single bit at a time or read/write up to 32 bits at once.
Inputs
It's all explained in the code
Code Returns
Same as above
Side Effects
Don't try writing to a file opened for reading, and don't try reading from a file opened for writing - there is no error checking for that and the results are unpredictable.
Don't try to read or write more than 32 bits at a time with the InputBits and OutputBits functions.
If you try to write a value with less bits than that value requires, the correct value will not be written. For example, don't try to write the value 32 into a file using only 4 bits.
After every call to inputbits and inputbit, you should check for eof on the input file using this code:
'inputbit/inputbits call here
if eof(bitfile.filenum) = True then 'replace bitfile with the name of the variable
'put code to exit loop or leave function here
end if
VBcodersType BitFile
FileNum As Integer 'File handle
holder As Byte 'holds a byte from file
mask As Byte 'used to read bits
End Type
Public Function OpenOBitFile(FileName As String) As BitFile
'Parameters - Filename
'Returns - Bitfile
'What it does - Opens a file for output a single bit at a time
'Example - dim OutputFile as bitfile
' OutputFile = OpenOBitFile("C:\test.bit")
Dim bitfilename As BitFile
FileNum = FreeFile 'get lowest available file handle
Open FileName For Binary As FileNum 'open it
bitfilename.FileNum = FileNum 'assign file number to structure
bitfilename.holder = 0 'bit holder = 0
bitfilename.mask = 128 'used to read individual bits
OpenOBitFile = bitfilename
End Function
Public Function OpenIBitFile(FileName As String) As BitFile
'Parameters - Filename
'Returns - Bitfile
'What it does - Opens a file for input a single bit at a time
'Example - dim InputFile as bitfile
' InputFile = OpenIBitFile("C:\command.com")
Dim bitfilename As BitFile
FileNum = FreeFile 'get lowest available file handle
Open FileName For Binary As FileNum 'open it
bitfilename.FileNum = FileNum 'assign file number to structure
bitfilename.holder = 0 'bit holder = 0
bitfilename.mask = 128 'used to read individual bits
OpenIBitFile = bitfilename
End Function
Public Sub CloseIBitFile(bitfilename As BitFile)
'Parameters - bitfile
'Returns - Nothing
'What it does - Closes the file associated with a bitfile
'Example - CloseIBitFile(InputFile)
Close bitfilename.FileNum 'Close the file associated with the bitfile
End Sub
Public Sub CloseOBitFile(bitfilename As BitFile)
'Parameters - bitfile
'Returns - Nothing
'What it does - Closes the file associated with a bitfile
'Example - CloseOBitFile(OutputFile)
If bitfilename.mask <> 128 Then 'If there is unwritten data...
Put bitfilename.FileNum, , bitfilename.holder 'Write it now
End If
Close bitfilename.FileNum 'Close the file
End Sub
Public Sub OutputBit(ByRef bitfilename As BitFile, bit As Byte)
'Parameters - bitfile, bit to write
'Returns - nothing
'What it does - Writes the specified bit to the file
'Example - OutputBit(OutputFile, 1)
If bit <> 0 Then
bitfilename.holder = bitfilename.holder Or bitfilename.mask
'the holder stores up written bits until there are 8
'At that point vb's normal file handling facilities can write it
End If
bitfilename.mask = bitfilename.mask \ 2 'decrease mask by power of 2
If bitfilename.mask = 0 Then 'if mask is empty
Put bitfilename.FileNum, , bitfilename.holder 'write the byte
bitfilename.holder = 0 'reset holder and mask
bitfilename.mask = 128
End If
End Sub
Public Sub OutputBits(ByRef bitfilename As BitFile, ByVal code As Long, ByVal count As Integer)
'Parameters - bitfile, data to write, number of bits to use
'Returns - nothing
'What it does - Writes the specified info using the specified number of bits
'Example - OutputBits(OutputFile, 28, 7)
Dim mask As Long
mask = 2 ^ (count - 1)
Do While mask <> 0
If (mask And code) <> 0 Then 'if the bits match up...
bitfilename.holder = bitfilename.holder Or bitfilename.mask 'put the bit in the holder
End If
bitfilename.mask = bitfilename.mask \ 2
mask = mask \ 2
If bitfilename.mask = 0 Then 'when there are 8 bits, write the holder to the file
Put bitfilename.FileNum, , bitfilename.holder
bitfilename.holder = 0 'and reset the holder and mask
bitfilename.mask = 128
End If
Loop
End Sub
Public Function InputBit(ByRef bitfilename As BitFile) As Byte
'Parameters - bitfile
'returns - the next bit from the file
'Example: bit = InputBit(InputBitFile)
Dim value As Byte
If bitfilename.mask = 128 Then 'if at end of previous byte
Get bitfilename.FileNum, , bitfilename.holder 'get a new byte from file
End If
value = bitfilename.holder And bitfilename.mask 'get the bit
bitfilename.mask = bitfilename.mask \ 2 'move the mask bit down one
If bitfilename.mask = 0 Then
bitfilename.mask = 128
End If
If value <> 0 Then 'return 0 or 1 depending on value
InputBit = 1
Else
InputBit = 0
End If
End Function
Public Function InputBits(ByRef bitfilename As BitFile, count As Integer) As Long
'Parameters - bitfile, number of bits to read
'returns - the value of the next count bits in the bitfile
'Example: byte = InputBits(InputBitFile, 8)
'This function works just like inputbit except that it loops through and reads the specified
'number of bits and puts them into a temporary holder
Dim holder As Long
Dim longmask As Long
longmask = 2 ^ (count - 1)
Do While (longmask <> 0)
If bitfilename.mask = 128 Then
Get bitfilename.FileNum, , bitfilename.holder
End If
If (bitfilename.holder And bitfilename.mask) <> 0 Then
holder = holder Or longmask
End If
bitfilename.mask = bitfilename.mask \ 2
longmask = longmask \ 2
If bitfilename.mask = 0 Then
bitfilename.mask = 128
End If
Loop
InputBits = holder
End Function
