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Showing posts from August, 2014

Deconstructing Floats: frexp() and ldexp() in JavaScript

While working on my  SqueakJS VM, it became necessary to deconstruct floating point numbers into their mantissa and exponent parts, and assembling them again. Peeking into the C sources of the regular VM, I saw they use the  frexp ()   and ldexp () functions found in the standard C math library. Unfortunately, JavaScript does not provide these two functions. But surely there must have been someone who needed these before me, right? Sure enough, a Google search came up with a few implementations. However, an hour later I was convinced none of them actually are fully equivalent to the C functions. They were imprecise, that is, deconstructing a float using frexp() and reconstructing it with ldexp() did not result in the original value. But that is the basic use case: for all float values, if [ mantissa , exponent ] = frexp (value) then value = ldexp ( mantissa , exponent ) even if the value is subnormal . None of the implementations (even the complex ones) really worked. I