I spent a while this year writing a small zero-dependency library of health and fitness formulas — BMI, Mifflin-St Jeor, the US Navy body-fat equation, FFMI, Epley and Brzycki, Karvonen. Thirty-odd functions, no dependencies, ESM, one file.
It should have been a boring job. It mostly was. But three things came out of it that I think generalise well beyond fitness maths, and one of them genuinely changed how I think about what "correct" means in a library.
The boring part: these equations get reimplemented constantly, and quietly wrong
Search for any of these formulas and you will find dozens of implementations. A lot of them are subtly broken, in ways that never throw and never look wrong:
The wrong coefficient set. The Navy body-fat equation has separate male and female forms, and separate metric and imperial constants. Four combinations, and three of them are wrong for any given call.
