Java hashCode method in VB.NET

Question

The point of this method is to emulate Java hashCode().

In order for this to work the project must have arithmetic overflow allowed under:

project --> compiler options --> advanced.

    Public Shared Function GetHashCodeStr(value As String) As Integer
        Dim finalCode As Integer = 0

        For i As Integer = 0 To value.Length - 1
           Dim powerValue = (Convert.ToInt32(value(i)) * Math.Pow(31, value.Length - 1 - i))

        If powerValue > Integer.MaxValue Then
            Dim timesGreaterThanInt = Math.Floor(powerValue / Integer.MaxValue)
            powerValue = powerValue - timesGreaterThanInt - (timesGreaterThanInt) * Integer.MaxValue
        End If

        finalCode += CInt(powerValue)
    Next

    Return finalCode
End Function

Answer 1

I know little about this language, but dealing with powers of 31 is surely no good idea. Most probably, it'll overflow or lose precession and you get pure garbage.

If there's something like long (64 bit integer), you can rewrite the Java implementation easily. The only relevant part is the loop doing

h = 31*h + val[off++];

Just reduce h after every iteration to the int range, so it doesn't crash your Windows overflow.

Actually, you need no long, a double (floating point with 56 bits of mantissa) would do. Simple float (24 bits mantissa) is unusable here.

Answer 2

My initial implementation was flawed in that the exponent used to compute the hash could result (with a sufficiently long string) in a potential overflow of long. Using the official JAVA implementation prevents that from happening and ultimately executes faster. Allowing arithmetic overflow (in your VB.NET project) is still important to not receive an error when overflowing the hash integer value which is expected behavior.

Public Shared Function GetHashCodeStr(ByVal input As String) As Integer

    Dim h As Integer = 0
    Dim hash As Integer = 0
    If h = 0 Then
        Dim off As Integer = 0
        Dim val() As Char = Input.ToCharArray()
        Dim len As Integer = val.Length

        Dim i As Integer
        For i = 0 To len - 1 Step 1
            h = 31 * h + Convert.ToInt32(val(off))
            off = off + 1
        Next
        hash = h
    End If
    Return h
End Function