Why should the "prime-based" hashcode implmentation be used instead of the "naive" one?
- by Wilhelm
I have seen that a prime number implmentation of the GetHashCode function is being recommend, for example here. However using the following code (in VB, sorry), it seems as if that implementation gives the same hash density as a "naive" xor implementation. If the density is the same, I would suppose there is the same probability of cllision in both implementations. Am I missing anything on why is the prime approach preferred?
I am supossing that if the hash code is a byte I do not lose generality for the integer case.
Sub Main()
Dim XorHashes(255) As Integer
Dim PrimeHashes(255) As Integer
For i = 0 To 255
For j = 0 To 255
For k = 0 To 255
XorHashes(GetXorHash(i, j, k)) += 1
PrimeHashes(GetPrimeHash(i, j, k)) += 1
Next
Next
Next
For i = 0 To 255
Console.WriteLine("{0}: {1}, {2}", i, XorHashes(i), PrimeHashes(i))
Next
Console.ReadKey()
End Sub
Public Function GetXorHash(ByVal valueOne As Integer, ByVal valueTwo As Integer, ByVal valueThree As Integer) As Byte
Return CByte((valueOne Xor valueTwo Xor valueThree) Mod 256)
End Function
Public Function GetPrimeHash(ByVal valueOne As Integer, ByVal valueTwo As Integer, ByVal valueThree As Integer) As Byte
Dim TempHash = 17
TempHash = 31 * TempHash + valueOne
TempHash = 31 * TempHash + valueTwo
TempHash = 31 * TempHash + valueThree
Return CByte(TempHash Mod 256)
End Function
