How to implement a hash function for an array in VB.net

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The classic function in C# to create a hash value from an array looks like this or similar:

C#

public int GetHashCode(T[] array)
    {
        unchecked
        {
            if (array == null)
            {
                return 0;
            }
            int hash = 17;
            foreach (T item in array)
            {
                hash = hash * 31 + item.GetHashCode;
            }
            return hash;
        }
    }

But since unchecked is not available in VB.Net I will quickly get an exception when it overflows.
I thought I could solve it using bitshifting, but in VB its only arithmetic and not circular so every bit that is shifted out is dropped.

And while I can't believe this has never been solved in VB my google foo is failing me.

What is the standard solution for getting a hash value for an array in VB?

Posted 19-May-14 4:41am

Jörgen Andersson

Updated 19-May-14 4:49am

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CPallini · Accepted Answer · 2014-05-19T05:39:00

Solution 2

Couldn't you just declare hash as Int64 and clear the upper part when needed?

Posted 19-May-14 5:39am

CPallini

Comments

Jörgen Andersson 19-May-14 12:27pm

That would mean that the first items influence on the hash would be shifted out after just seven more items added.
This is the reason I wanted to use a circular bitshift, but that is also not available in VB.

CPallini 19-May-14 12:33pm

However that's exactly what happens in the C# code, right?

Jörgen Andersson 19-May-14 12:37pm

No, that's why you want the 'Unchecked' operator, it overflows without an exception.

CPallini 19-May-14 15:43pm

Well, 'overflows without an exceptions' doesn't prevent it to discard the bits.

Jörgen Andersson 19-May-14 16:33pm

Dammit, you're right.
That means I've tried to fix something that wasn't as broken as I thought. Either that, or the implementation is faulty.

Jörgen Andersson 20-May-14 4:15am

I will go by another solution (a variant of the shift and wrap as described here http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx).
But since your suggestion is actually solving the problem as it's stated I will accept your solution.

CPallini 20-May-14 4:23am

Thank you. Actually the Int64 solution has a gotcha, since you have to explicitely handle the sign bit (if you want to fully comply with the C# code), hence, I guess the other solution you found is more practical. Good luck.

Jörgen Andersson 21-May-14 5:20am

As a future reference I've posted my solution

Sergey Alexandrovich Kryukov 19-May-14 15:33pm

I guess not. Many algorithms like that one are fundamentally based on uncheched integer operations, which are default operations implemented on the CPU instruction set. Checked is adding some extra instructions. If some language does not have "unchecked" (having no "checked" would be fine) simply would mean that the authors of the language does not consider their users as real engineers, just treat them like coding plankton. Maybe this is the case with VB.NET, but I'm not sure.

Even if this is not the case, I never understood why developing anything in any kind of Basic at all. VB.NET is never taken by Microsoft seriously; they standardize C# and even C++/CLI, but will hardly be daring enough to standardize such thing as VB.NET; no wonder...

—SA

Jörgen Andersson · Accepted Answer · 2014-05-20T23:19:00

For future reference, this is how my solution looks like:

VB

Public Overrides Function GetHashCode() As Integer
     Dim hash As Int64 = 19
     For Each Item As T In Me
         hash = hash * 31 + Item.GetHashCode
         hash = ((hash << 32) >> 32) + hash >> 32
     Next
     hash = ((hash << 32) >> 32)
     Return CInt(hash)
 End Function

There is some room for improving performance, the overflow only needs to be handled every 6 iterations for example