closed form solution?

[Gabriel Zachmann]

Dear all,

does anybody know a closed form solution for the following equation?

    floor( a * 2^i/U ) = floor( b * 2^i/U )

where

    U = 2^d
    a,b in [0,U-1]

I am looking for a closed form solution which would return the largest i
solving the above equation.
I know of 2 ways which both use some kind of lookup table of hash table,
but I would prefer the closed form.

(Remark: you probably have already seen that i is a common ancestor
of a and b in a binary tree.)

Thanks a lot in advance for any thoughts, hints, and pointers.
Gabriel Zachmann.

-- 
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| Wovon man nicht sprechen kann,                                      |
| darueber muss man schweigen.     (Ludwig Wittgenstein)              |
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| zach at cs.uni-bonn.de                 __@/'  Gabriel.Zachmann at gmx.net |
| web.informatik.uni-bonn.de/~zach    __@/'   www.gabrielzachmann.org |
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