/** Cosine similarity in [-1, 1]; 0 when either vector is empty or zero or the lengths differ. */
export function cosineSimilarity(left: readonly number[], right: readonly number[]): number {
  if (left.length === 0 || left.length !== right.length) return 0;
  let dot = 0;
  let leftNorm = 0;
  let rightNorm = 0;
  for (let index = 0; index < left.length; index += 1) {
    const a = left[index]!;
    const b = right[index]!;
    dot += a * b;
    leftNorm += a * a;
    rightNorm += b * b;
  }
  return leftNorm === 0 || rightNorm === 0 ? 0 : dot / Math.sqrt(leftNorm * rightNorm);
}