text: /**
   * @author Toru Nagashima <https://github.com/mysticatea>
   * See LICENSE file in root directory for full license.
   */
  "use strict";
  
  const KEYS = require("./visitor-keys.json");
  
  // Types.
  const NODE_TYPES = Object.freeze(Object.keys(KEYS));
  
  // Freeze the keys.
  for (const type of NODE_TYPES) {
      Object.freeze(KEYS[type]);
  }
  Object.freeze(KEYS);
  
  // List to ignore keys.
  const KEY_BLACKLIST = new Set([
      "parent",
      "leadingComments",
      "trailingComments"
  ]);
  
  /**
   * Check whether a given key should be used or not.
   * @param {string} key The key to check.
   * @returns {boolean} `true` if the key should be used.
   */
  function filterKey(key) {
      return !KEY_BLACKLIST.has(key) && key[0] !== "_";
  }
  
  //------------------------------------------------------------------------------
  // Public interfaces
  //------------------------------------------------------------------------------
  
  module.exports = Object.freeze({
  
      /**
       * Visitor keys.
       * @type {{ [type: string]: string[] | undefined }}
       */
      KEYS,
  
      /**
       * Get visitor keys of a given node.
       * @param {Object} node The AST node to get keys.
       * @returns {string[]} Visitor keys of the node.
       */
      getKeys(node) {
          return Object.keys(node).filter(filterKey);
      },
  
      // Disable valid-jsdoc rule because it reports syntax error on the type of @returns.
      // eslint-disable-next-line valid-jsdoc
      /**
       * Make the union set with `KEYS` and given keys.
       * @param {Object} additionalKeys The additional keys.
       * @returns {{ [type: string]: string[] | undefined }} The union set.
       */
      unionWith(additionalKeys) {
          const retv = Object.assign({}, KEYS);
  
          for (const type of Object.keys(additionalKeys)) {
              if (retv.hasOwnProperty(type)) {
                  const keys = new Set(additionalKeys[type]);
  
                  for (const key of retv[type]) {
                      keys.add(key);
                  }
  
                  retv[type] = Object.freeze(Array.from(keys));
              } else {
                  retv[type] = Object.freeze(Array.from(additionalKeys[type]));
              }
          }
  
          return Object.freeze(retv);
      }
  });
  