/**
 * Computes the arithmetic mean of an array of numbers.
 *
 * @param {number[]} numbers - Array of numeric values.
 * @returns {number} The mean of the numbers, or NaN for empty array or invalid inputs.
 */
export function mean(numbers) {
  if (!Array.isArray(numbers) || numbers.length === 0) return NaN;

  let sum = 0;
  for (const n of numbers) {
    if (typeof n !== "number" || !Number.isFinite(n)) return NaN;
    sum += n;
  }
  return sum / numbers.length;
}