The fast path in David Gay’s decimal to floating-point conversion routine relies on this property of the first twenty-three nonnegative powers of ten: they have exact representations in double-precision floating-point. While it’s easy to see why powers of ten up to 1015 are exact, it’s less clear why the powers of ten from 1016 to 1022 are. To see why, you have to look at their binary representations.
Producing correctly rounded decimal to floating-point conversions is hard, but only because it is made to be done efficiently. There is a simple algorithm that produces correct conversions, but it’s too slow — it’s based entirely on arbitrary-precision integer arithmetic. Nonetheless, you should know this algorithm, because it will help you understand the highly-optimized conversion routines used in practice, like David Gay’s strtod() function. I will outline the algorithm, which is easily implemented in a language like C, using a “big integer” library like GMP.
Ratio of Big Integers (2119/1020) Producing the 53-Bit Significand of 1e-20
