My understanding is that fixed-length conversion to string is more or less trivial, the tricky part is writing a fast algorithm to compute the shortest round-trippable conversion to string, correct?
"Correct" conversion, where the length doesn't matter (which could be called "fixed length" for a certain length that is big enough) is easier, because one can avoid a lot of rounding and imprecision issue: using the right technique, just produce enough digits until the imprecisions don't matter anymore.
However, fixed length is as difficult as the shortest, if the length is limited, since the last digit sometimes lies on the boundary and thus runs into the same difficulties as the shortest digit. Think of it this way: needing to decide whether 6 digits is enough, is often similar to asking whether the 6th digit is a 0 or a 9 (roughly speaking). This means that producing the best fixed-length representation (for length 6) runs into the exactly same question.
