/**
 * 将字符串编译为 DOM 结构
 * @deprecated 请注意，不要用该函数，因为 compile('<div></div><p></p>') 会编译成嵌套结构：<p><div></div></p>
 * @public
 * @param  {String} domString
 * @return {HTMLElement}
 * @throws {Error} 如果输入不是合法的 DOM 字符串
 *
 * @example
 * compile('<div><p><a><em></em></a></p></div>')
 * // =>
 * <div>
 *   <p>
 *     <a>
 *       <em></em>
 *     </a>
 *   </p>
 * </div>
 *
 * @example
 * compile('<div></p>')
 * // => Uncaught Error: DOM string invalid
 */
function compile(domString) {
  var validated = validate(domString);
  if (!validated) {
    throw new Error('DOM string invalid');
  }

  const compiled = assemble(validated);

  return compiled;
}

/**
 * 检测是否是合法的 DOM 字符串
 * @private
 *
 * @param  {String} domString
 * @return {String[]|false} 若合法则返回标签名数组，否则返回 false
 *
 * @example
 * validate('<div></div>')
 * // => ['div']
 *
 * @example
 * validate('<div></p>')
 * // => false
 *
 * @example
 * validate('<div><p><a><em></em></a></p></div>')
 * // => ['div', 'p', 'a', 'em']
 */
function validate(domString) {
  const beginTagPattern = /^<(\w+)>/;
  const closeTagPattern = /^<\/(\w+)>/;
  const stack = [];
  const tags = [];

  let testString = domString;
  let delta = 1;

  while (testString.length > 0) {
    const beginTagMatches = testString.match(beginTagPattern);
    const closeTagMatches = testString.match(closeTagPattern);
    let matches;

    if (beginTagMatches) {
      matches = beginTagMatches;
      stack.push(matches[1]);
      // console.log('after push stack:', stack);
    } else if (closeTagMatches) {
      matches = closeTagMatches;

      if (stack[stack.length - 1] === matches[1]) {
        const tag = stack.pop();

        tags.push(tag);

        // console.log('after pop stack:', stack);
      }
    } else {
      break;
    }

    delta = matches[0].length + matches.index;
    testString = testString.slice(delta);
  }

  // console.log('final stack:', stack);

  return stack.length === 0 ? tags.reverse() : false;
}

/**
 * 将标签组装为嵌套的 DOM 结构
 * @private
 * @param  {String[]} tags
 * @return {HTMLElement}
 *
 * @example
 * assemble(['div', 'p', 'a', 'em'])
 * // => <div><p><a><em></em></a></p></div>
 */
function assemble(tags) {
  const elements = tags.map((tag) => document.createElement(tag));

  elements.forEach((element, index, elements) => {
    // console.log('element:', element);
    if (index !== elements.length - 1) {
      element.appendChild(elements[index + 1]);
    }
  });

  return elements[0];
}