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copied!<p>Presumably you are desperately interested in speed, or else you would just suck up the conditional test with every RNG call. Any other alternative is probably going to be slower than the branch anyway...</p> <p>...unless you know exactly what the internal structure of the RNG is. Particularly, what are its return values? If they're not IEEE-754 floats or doubles, you have my sympathies. If they are, how many real bits of randomness are in them? You would expect 24 for floats and 53 for doubles (the number of mantissa bits). If those are naively generated, you may be able to use shifts and masks to hack together a plain old random integer generator out of them, and then use that in your function (depending on the size of your range, you may be able to use more shifts and masks to avoid any branching if you have such a generator). If you have a high-quality generator that produces full quality 24- or 53-bit random numbers, then with a single multiply you can convert them from [0,1] to [0,1): just multiply by the largest generatable floating-point number that is less than 1, and your range problem is gone. This trick will still work if the mantissas aren't fully populated with random bits, but you'll need to do a bit more work to find the right multiplier.</p> <p>You may want to look at the <a href="http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/MT2002/CODES/mt19937ar.c" rel="nofollow noreferrer">C source to the Mersenne Twister</a> to see their treatment of similar problems.</p>

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