const isArray = Array.isArray;
const keyList = Object.keys;
const hasProp = Object.prototype.hasOwnProperty;

export function isDeeplyEqual<T extends any>(a: T, b: T): boolean {
  if (a === b) { return true; }


  if (a && b && typeof a === 'object' && typeof b === 'object') {
    const arrA = isArray(a)
      , arrB = isArray(b);
    let i
      , length
      , key;

    if (arrA && arrB) {
      length = a.length;
      if (length !== b.length) { return false; }
      for (i = length; i-- !== 0;) {
        if (!isDeeplyEqual(a[i], b[i])) { return false;
      } }
      return true;
    }

    if (arrA !== arrB) { return false; }

    const dateA = a instanceof Date
      , dateB = b instanceof Date;
    if (dateA !== dateB) { return false; }
    if (dateA && dateB) { return a.getTime() === b.getTime(); }

    const regexpA = a instanceof RegExp
      , regexpB = b instanceof RegExp;
    if (regexpA !== regexpB) { return false; }
    if (regexpA && regexpB) { return a.toString() === b.toString(); }

    const keys = keyList(a);
    length = keys.length;

    if (length !== keyList(b).length) {
      return false;
    }

    for (i = length; i-- !== 0;) {
      if (!hasProp.call(b, keys[i])) { return false;
    } }

    for (i = length; i-- !== 0;) {
      key = keys[i];
      if (!isDeeplyEqual(a[key], b[key])) { return false; }
    }

    return true;
  }

  return a !== a && b !== b;
}