package system

// DFUsedPercent is the used percentage the way `df` prints it: used / (used + available). The blocks reserved for root
// (5 % on ext4 by default) are in neither term, so a full disk reads 100 % here. Every disk percent the box shows or
// alarms on is computed by this one function (R-889, operator ruling 2026-10-06 — `09` §3 decision 138): until then
// the box divided by the whole filesystem and read ~5 points LOW on a full disk (61.8 of 68.7 GB = 90 % where `df`
// said 95 %). `df` rounds up; this does not, so the two agree within one point. Units only need to match.
// Pinned by TestDFUsedPercent_MatchesDFOnRealNumbers and TestR889_EveryDiskPercentIsTheDFOne.
func DFUsedPercent(used, avail float64) float64 {
	if used <= 0 && avail <= 0 {
		return 0
	}
	if used < 0 {
		used = 0
	}
	if avail < 0 {
		avail = 0
	}
	return used / (used + avail) * 100
}