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export const sum  = (xs: readonly number[]): number => xs.reduce((a, b) => a + b, 0);
export const mean = (xs: readonly number[]): number => xs.length ? sum(xs) / xs.length : 0;

export function median(xs: readonly number[]): number {
  if (xs.length === 0) return 0;
  const s = [...xs].sort((a, b) => a - b);
  const m = Math.floor(s.length / 2);
  return s.length % 2 ? s[m]! : (s[m - 1]! + s[m]!) / 2;
}

sum([1, 2, 3, 4]);       // 10
mean([1, 2, 3, 4]);      // 2.5
median([1, 3, 8, 4, 2]); // 3