I said last time that binary-searching the rationals (WOLOG between zero and one) for a particular fraction that is very close to a given double does not really work, because we end up with only fractions that have powers of two in the denominators. We already know what fraction with a power of two in the denominator is closest to our given double; itself!

But there is a way to binary-search the rationals between zero and one, as it turns out. Before we get into it, a quick refresher on how binary search works:
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All right, we have an arbitrary-precision rational arithmetic type now, so we can do arithmetic on fractions with confidence. Remember the problem I set out to explore here was: a double is actually a fraction whose denominator is a large power of two, so fractions which do not have powers of two in their denominators cannot be represented with full fidelity by a double. Now that we have this machinery we can see exactly what the representation error is:
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A couple of years ago I developed my own arbitrary precision natural number and integer mathematics types, just for fun and to illustrate how it could be done. I’m going to do the same for rational numbers here, but rather than using my integer type, I’ll just use the existing BigInteger type to represent the numerator and denominator. Either would do just fine.

Of course I could have used some existing BigRational types but for various reasons that I might go into in a future blog article, I decided it would be more interesting to simply write my own. Let’s get right to it; I’ll annotate the code as I go.
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