Rounding for display and for money

/** Half away from zero, tolerant of binary noise (1.005 → 1.01, 2.675 → 2.68). */
export function roundTo(v, digits = 0) {
  const f = 10 ** digits;
  const y = Math.abs(v) * f;
  const r = Math.floor(y + 0.5 + 1e-9 * Math.max(1, y));
  return (v < 0 ? -r : r) / f;
}

/** Grouped fixed-point number: 1234.5 → "1,234.50". */
export function fixed(v, digits = 2) {
  if (!Number.isFinite(v)) return '';
  const r = roundTo(v, digits);
  const [i, f] = Math.abs(r).toFixed(digits).split('.');
  return (r < 0 ? '-' : '') + i.replace(/\B(?=(\d{3})+(?!\d))/g, ',') + (f ? '.' + f : '');
}

/** Feet-inches to the nearest 1/denom inch: 12'-6 1/2", 0'-3/4" is written 0'-0 3/4". */
export function formatFtIn(ft, denom = 16) {
  if (!Number.isFinite(ft)) return '';
  const neg = ft < 0;
  const n = Math.round(Math.abs(ft) * 12 * denom + 1e-9);   // whole 1/denom inches
  const perFoot = 12 * denom;
  const feet = Math.floor(n / perFoot);
  const rem = n - feet * perFoot;
  const inch = Math.floor(rem / denom);
  let num = rem - inch * denom, den = denom;
  while (num && num % 2 === 0 && den % 2 === 0) { num /= 2; den /= 2; }
  const zero = feet === 0 && inch === 0 && num === 0;
  return (neg && !zero ? '-' : '') + feet + "'-" + inch + (num ? ' ' + num + '/' + den : '') + '"';
}

/** A length for display in a chosen unit: 'ftin', 'ft' (decimal), 'in', 'm', 'mm'. */
export function formatLength(ft, unit = 'ftin', digits = 2) {
  if (!Number.isFinite(ft)) return '';
  switch (unit) {
    case 'ftin': return formatFtIn(ft);
    case 'ft': return fixed(ft, digits) + "'";
    case 'in': return fixed(ft * 12, digits) + '"';
    case 'm': return fixed(ft * M_PER_FT, 3) + ' m';
    case 'mm': return fixed(ft * M_PER_FT * 1000, 0) + ' mm';
    default: return fixed(ft, digits);
  }
}