/**
 * Compute the Sørensen–Dice coefficient between two strings.
 * Used for move detection (rename detection by content similarity).
 *
 * Returns a value between 0 (no similarity) and 1 (identical).
 */
export function stringSimilarity(a: string, b: string): number {
  if (a === b) return 1;
  if (a.length < 2 || b.length < 2) return 0;

  const bigramsA = getBigrams(a);
  const bigramsB = getBigrams(b);

  let intersection = 0;
  for (const [bigram, countA] of bigramsA) {
    const countB = bigramsB.get(bigram) ?? 0;
    intersection += Math.min(countA, countB);
  }

  const total = bigramsA.size + bigramsB.size;
  if (total === 0) return 0;

  return (2 * intersection) / total;
}

function getBigrams(str: string): Map<string, number> {
  const bigrams = new Map<string, number>();
  for (let i = 0; i < str.length - 1; i++) {
    const bigram = str.slice(i, i + 2);
    bigrams.set(bigram, (bigrams.get(bigram) ?? 0) + 1);
  }
  return bigrams;
}