Jaro-Winkler String Similarity

export function jaroSimilarity(a: string, b: string): number {
  if (a === b) return 1;
  if (a.length === 0 || b.length === 0) return 0;
  const window = Math.max(0, Math.floor(Math.max(a.length, b.length) / 2) - 1);
  const aMatches = new Array<boolean>(a.length).fill(false);
  const bMatches = new Array<boolean>(b.length).fill(false);
  let matches = 0;
  for (let i = 0; i < a.length; i += 1) {
    const start = Math.max(0, i - window);
    const end = Math.min(i + window + 1, b.length);
    for (let j = start; j < end; j += 1) {
      if (bMatches[j] || a[i] !== b[j]) continue;
      aMatches[i] = true;
      bMatches[j] = true;
      matches += 1;
      break;
    }
  }
  if (matches === 0) return 0;
  let transpositions = 0;
  let cursor = 0;
  for (let i = 0; i < a.length; i += 1) {
    if (!aMatches[i]) continue;
    while (!bMatches[cursor]) cursor += 1;
    if (a[i] !== b[cursor]) transpositions += 1;
    cursor += 1;
  }
  return (matches / a.length + matches / b.length + (matches - transpositions / 2) / matches) / 3;
}

export function jaroWinkler(a: string, b: string, prefixScale = 0.1): number {
  if (prefixScale < 0 || prefixScale > 0.25) {
    throw new RangeError("prefixScale must be between 0 and 0.25");
  }
  const jaro = jaroSimilarity(a, b);
  let prefix = 0;
  const limit = Math.min(4, a.length, b.length);
  while (prefix < limit && a[prefix] === b[prefix]) prefix += 1;
  return jaro + prefix * prefixScale * (1 - jaro);
}