VB.NET - How to calculate the parity bit of a byte array - vb.net

Question

What is the most efficient way calculate the parity bit (if the number of active bits are odd or even) in a byte array? I have though about iterating through all the bits and summing up the active bits, but that would be very impractical purely based on the number of iterations required on larger byte arrays/files.

Answer 1

For your convenience (and my curiosity), I have done some timing tests with a parity lookup table compared to the other two methods suggested so far:
Module Module1
Dim rand As New Random
Dim parityLookup(255) As Integer
Sub SetUpParityLookup()
' setBitsCount data from http://stackoverflow.com/questions/109023/how-to-count-the-number-of-set-bits-in-a-32-bit-integer
Dim setBitsCount = {
0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
}
For i = 0 To 255
parityLookup(i) = setBitsCount(i) And 1
Next
End Sub
' Method using lookup table
Function ParityOfArray(a() As Byte) As Integer
Dim parity As Integer = 0 ' use an Integer because they are faster
For i = 0 To a.Length - 1
parity = parity Xor parityLookup(a(i))
Next
Return parity
End Function
' Method by Alireza
Function ComputeParity(bytes() As Byte) As Byte
Dim parity As Boolean = False
For i As Integer = 0 To bytes.Length - 1
Dim b As Byte = bytes(i)
While b <> 0
parity = Not parity
b = CByte(b And (b - 1))
End While
Next
Return Convert.ToByte(parity)
End Function
' Method by dbasnett
Function CountBits(byteArray As Byte()) As Integer
Dim rv As Integer = 0
For Each b As Byte In byteArray
Dim count As Integer = b
count = ((count >> 1) And &H55) + (count And &H55)
count = ((count >> 2) And &H33) + (count And &H33)
count = ((count >> 4) And &HF) + (count And &HF)
rv += count
Next
Return rv
End Function
Sub FillWithRandomBytes(ByRef a() As Byte)
rand.NextBytes(a)
End Sub
Sub Main()
SetUpParityLookup()
Dim nBytes = 10000
Dim a(nBytes - 1) As Byte
FillWithRandomBytes(a)
Dim p As Integer
Dim sw As New Stopwatch
sw.Start()
p = ParityOfArray(a)
sw.Stop()
Console.WriteLine("ParityOfArray - Parity: {0} Time: {1}", p, sw.ElapsedTicks)
sw.Restart()
p = ComputeParity(a)
sw.Stop()
Console.WriteLine("ComputeParity - Parity: {0} Time: {1}", p, sw.ElapsedTicks)
sw.Restart()
p = CountBits(a)
sw.Stop()
' Note that the value returned from CountBits should be And-ed with 1.
Console.WriteLine("CountBits - Parity: {0} Time: {1}", p And 1, sw.ElapsedTicks)
Console.ReadLine()
End Sub
End Module
Typical ouput:
ParityOfArray - Parity: 0 Time: 386
ComputeParity - Parity: 0 Time: 1014
CountBits - Parity: 0 Time: 695

Answer 2

An efficient way to do this is to use the x & (x - 1) operation in a loop, until x becomes zero. This way you will loop only by the number of bits set to 1.
In VB.NET for a byte array:
Function ComputeParity(bytes() As Byte) As Byte
Dim parity As Boolean = False
For i As Integer = 0 To bytes.Length - 1
Dim b As Byte = bytes(i)
While b <> 0
parity = Not parity
b = b And (b - 1)
End While
Next
Return Convert.ToByte(parity)
End Function

Answer 3

Here is a function that counts bits.
Private Function CountBits(byteArray As Byte()) As Integer
Dim rv As Integer = 0
For x As Integer = 0 To byteArray.Length - 1
Dim b As Byte = byteArray(x)
Dim count As Integer = b
count = ((count >> 1) And &H55) + (count And &H55)
count = ((count >> 2) And &H33) + (count And &H33)
count = ((count >> 4) And &HF) + (count And &HF)
rv += count
Next
Return rv
End Function
Note: this code came from a collection of bit twiddling hacks I found some years ago. I converted it to VB.