triangles.js

// https://github.com/yankouskia/love-triangle
module.exports = function getLoveTrianglesCount(preferences = []) {
  return (
    preferences.map((x, i) => getTriangle(preferences, i)).filter(x => x)
      .length / 3
  );
};

const getTriangle = (preferences, index) => {
  if (preferences[index] - 1 == index) {
    return null; // narcissus
  }
  const b = preferences[index] - 1;
  const c = preferences[b] - 1;
  const d = preferences[c] - 1;
  return d == index ? [b, c, d] : null;
};