solution.ts

/**
 * Convert a non-negative millisecond duration into a compact human-friendly string
 * using units d, h, m, s.
 * Rules applied:
 * - Use floor division
 * - Omit zero-valued units
 * - Show at most two largest non-zero units (e.g. "2d 3h", "1h 2m", "45s")
 * - For ms <= 0 or invalid input return '0s'
 */

export function formatTimeRemaining(ms: number): string {
  // Validate input: must be a finite number greater than 0
  if (typeof ms !== 'number' || !Number.isFinite(ms) || ms <= 0) return '0s';

  // Convert to whole seconds using floor division
  const totalSeconds = Math.floor(ms / 1000);
  if (totalSeconds <= 0) return '0s';

  // Seconds per unit
  const SECONDS_PER_DAY = 86400; // 24 * 3600
  const SECONDS_PER_HOUR = 3600;
  const SECONDS_PER_MINUTE = 60;

  let remaining = totalSeconds;
  const days = Math.floor(remaining / SECONDS_PER_DAY);
  remaining -= days * SECONDS_PER_DAY;

  const hours = Math.floor(remaining / SECONDS_PER_HOUR);
  remaining -= hours * SECONDS_PER_HOUR;

  const minutes = Math.floor(remaining / SECONDS_PER_MINUTE);
  remaining -= minutes * SECONDS_PER_MINUTE;

  const seconds = remaining;

  // Ordered from largest to smallest
  const units: Array<[number, string]> = [
    [days, 'd'],
    [hours, 'h'],
    [minutes, 'm'],
    [seconds, 's'],
  ];

  // Keep only non-zero units
  const nonZero = units.filter(([value]) => value > 0);

  if (nonZero.length === 0) {
    // No non-zero unit (e.g., ms < 1000 but > 0)
    return '0s';
  }

  // Show at most two largest non-zero units
  const shown = nonZero.slice(0, 2);

  return shown.map(([value, label]) => `${value}${label}`).join(' ');
}