> I implemented float-to-string for my compiler's printf back sometime before 1985. It's a medium difficult problem.

It might have been medium difficult back in 1985, but the state of the art has progressed in the intervening 40 years. The constraint of correctness makes things trickier because it further imposes three requirements: completeness, uniqueness, and determinism.

Unless the implementer has the skill to prove these properties about the algorithm they've implemented, such an implementation is likely not to achieve those requirements.