Robert,
I've implemented your tweaked solution and indeed it runs significantly 
faster! In retrospect it looks so obvious, almost embarrasing!

Thanks a million, your effort is appreciated!

Ralf,
yes I am aware of NArray, and probably that will be the optimal 
solution. At some stage I will probably use it, however, at least for 
now, I want to avoid external dependencies and just go with a pure Ruby 
solution.

Thanks,
Chris

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