How can I improve this 31-bit SuperFastHash implementation in VBA?
This is quite possibly the most clever VBA code I have ever seen.

There's not much room for improvement here, except a couple naming nitpicks:


If that's what they stand for, functions shr and shl could afford to be called ShiftRight and ShiftLeft.
I don't see a reason for shr and shl to not follow the PascalCasing naming convention.
Given the almost-flawlessly consistent camelCasing convention for locals &amp; parameters, I don't see a reason for Value and Shift parameters to not stick to it. (except perhaps VBA/VB6's stubornness with casing?)


As for string handling, as per this source AscW is faster than Asc so again, good call:


  AscW(Mid$(..)) considerations
  
  Don't copy too many characters with Mid$. AscW only examines the first
  character anyway. AscW(Mid$(s, x, 1)) is the best call. Note that if
  you call AscW(Mid$(s, x)) without the third parameter, Mid$ executes
  slowly when s is a long string.
  
  Example of potentially slow code:

For x = 1 To Len(s)
    If AscW(Mid$(s, x)) = ... Then ... 
Next

  
  The above is better written as:

For x = 1 To Len(s)
    If AscW(Mid$(s, x, 1)) = ... Then ... 
Next