I've used various HP scientific and graphing calculators that have functions to convert a floating point number to a symbolic representation of a rational numbe

I've used various HP scientific and graphing calculators that have functions to convert a floating point number to a symbolic representation of a rational number, or a fraction of pi. These functions are approximate, and their tolerance for how large the denominator of the fraction may get is determined by the current setting for how many decimal digits the display of numbers is rounded to. In practice, this works pretty well, and they implemented this capability long before they had a full CAS on any of their calculators.