In my problem '01' is different than '10', and I need to count them separately. Still, your point of not needing to count the last is well taken, as I can count 3, and thereby know the 4th. However, by doing this, I'm not seeing any significant performance gains, and it may be a bit slower. 
              Rate browser+LR  browserUK
browser+LR 19055/s         --        -2%
browserUK  19509/s         2%         --

[download]
Code for benchmarking...
<Reveal this spoiler or all in this thread>

In reply to Re^3: Speeding permutation counting by albert
in thread Speeding permutation counting by albert

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