SSpoke - 1 year ago 155
Vb.net Question

How do I reverse the Mod operator

I have a simple function that handles overflows with modulus operator

``````   Private Function RandomizeBlock(seed As Integer, ByVal block() As Byte) As Byte()
Dim Generator As System.Random = New System.Random(seed)
Dim newblock(255) As Byte
Dim i As Integer = 0
For i = 0 To block.Length - 1
newblock(i) = (block(i) + Generator.Next(0, 256)) Mod 256
Next

Return newblock
End Function
``````

How do I undo the randomization done to a block?

I know mod works like this:

`253,254,255,0,1,2,3,4`
overwraps on
`0`
.

Do can I find the inverse of
`reverse`
here?

``````rndValue = Generator.Next(0, 256)
reverse_1 = ((256 - rndValue) + block(i)) Mod 256
reverse_2 = ((256 + rndValue) - block(i)) Mod 256
``````

You just have to keep in mind that working modulo `p`, you don't have actual numbers but remainder classes. The first `p` natural numbers are usually used as the representatives of these classes. Luckily, subtraction and addition are perfectly compatible with remainder classes.
The `Mod` implementation of VB converts any positive number to the representative of its remainder class. However, it can not do it on negative numbers. You have to do it on your own.
``````Dim reverse As Integer = block(i) - rndValue;