/**
 * The `computed` utility creates a function that will cache its output until
 * any of the dependent values are dirty.
 *
 * @param {...String} dependentKeys The keys of the dependent values.
 * @param {function} compute The function which computes the value using the
 *     dependent values.
 * @return {Function}
 */
export default function computed(...dependentKeys) {
  const keys = dependentKeys.slice(0, -1);
  const compute = dependentKeys.slice(-1)[0];

  const dependentValues = {};
  let computedValue;

  return function() {
    let recompute = false;

    // Read all of the dependent values. If any of them have changed since last
    // time, then we'll want to recompute our output.
    keys.forEach(key => {
      const value = typeof this[key] === 'function' ? this[key]() : this[key];

      if (dependentValues[key] !== value) {
        recompute = true;
        dependentValues[key] = value;
      }
    });

    if (recompute) {
      computedValue = compute.apply(this, keys.map(key => dependentValues[key]));
    }

    return computedValue;
  };
}