/**
 * @fileoverview Rule to flag the use of empty character classes in regular expressions
 * @author Ian Christian Myers
 */

'use strict';

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const { RegExpParser, visitRegExpAST } = require('@eslint-community/regexpp');

//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------

const parser = new RegExpParser();
const QUICK_TEST_REGEX = /\[\]/u;

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

/** @type {import('../types').Rule.RuleModule} */
module.exports = {
  meta: {
    type: 'problem',

    docs: {
      description: 'Disallow empty character classes in regular expressions',
      recommended: true,
      url: 'https://eslint.org/docs/latest/rules/no-empty-character-class',
    },

    schema: [],

    messages: {
      unexpected: 'Empty class.',
    },
  },

  create(context) {
    return {
      'Literal[regex]'(node) {
        const { pattern, flags } = node.regex;

        if (!QUICK_TEST_REGEX.test(pattern)) {
          return;
        }

        let regExpAST;

        try {
          regExpAST = parser.parsePattern(pattern, 0, pattern.length, {
            unicode: flags.includes('u'),
            unicodeSets: flags.includes('v'),
          });
        } catch {
          // Ignore regular expressions that regexpp cannot parse
          return;
        }

        visitRegExpAST(regExpAST, {
          onCharacterClassEnter(characterClass) {
            if (
              !characterClass.negate &&
              characterClass.elements.length === 0
            ) {
              context.report({ node, messageId: 'unexpected' });
            }
          },
        });
      },
    };
  },
};