pub fn quantile_from_sorted(sorted: &[f64], q: f64) -> f64 {
let n = sorted.len();
if n == 0 {
return f64::NAN;
}
if n == 1 {
return sorted[0];
}
let pos = q.clamp(0.0, 1.0) * (n - 1) as f64;
let lo = pos.floor() as usize;
let hi = (lo + 1).min(n - 1);
let frac = pos - lo as f64;
sorted[lo] * (1.0 - frac) + sorted[hi] * frac
}