// @flow

module.exports = function shallowEqual<T, U>(
  objA?: ?T,
  objB?: ?U,
  compare?: ?(objValue: any, otherValue: any, key?: string) => boolean | void,
  compareContext?: ?any
): boolean {
  var ret = compare ? compare.call(compareContext, objA, objB) : void 0;

  if (ret !== void 0) {
    return !!ret;
  }

  if (objA === objB) {
    return true;
  }

  if (typeof objA !== "object" || !objA || typeof objB !== "object" || !objB) {
    return false;
  }

  var keysA = Object.keys(objA);
  var keysB = Object.keys(objB);

  if (keysA.length !== keysB.length) {
    return false;
  }

  var bHasOwnProperty = Object.prototype.hasOwnProperty.bind(objB);

  // Test for A's keys different from B.
  for (var idx = 0; idx < keysA.length; idx++) {
    var key = keysA[idx];

    if (!bHasOwnProperty(key)) {
      return false;
    }

    var valueA = objA[key];
    var valueB = objB[key];

    ret = compare ? compare.call(compareContext, valueA, valueB, key) : void 0;

    if (ret === false || (ret === void 0 && valueA !== valueB)) {
      return false;
    }
  }

  return true;
};