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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 & 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