text: /**
   * @fileoverview Look for useless escapes in strings and regexes
   * @author Onur Temizkan
   */
  
  "use strict";
  
  //------------------------------------------------------------------------------
  // Rule Definition
  //------------------------------------------------------------------------------
  
  /**
  * Returns the union of two sets.
  * @param {Set} setA The first set
  * @param {Set} setB The second set
  * @returns {Set} The union of the two sets
  */
  function union(setA, setB) {
      return new Set(function *() {
          yield* setA;
          yield* setB;
      }());
  }
  
  const VALID_STRING_ESCAPES = new Set("\\nrvtbfux\n\r");
  const REGEX_GENERAL_ESCAPES = new Set("\\bcdDfnrsStvwWxu0123456789");
  const REGEX_CHARCLASS_ESCAPES = union(REGEX_GENERAL_ESCAPES, new Set("]"));
  const REGEX_NON_CHARCLASS_ESCAPES = union(REGEX_GENERAL_ESCAPES, new Set("^/.$*+?[{}|()B"));
  
  /**
  * Parses a regular expression into a list of characters with character class info.
  * @param {string} regExpText The raw text used to create the regular expression
  * @returns {Object[]} A list of characters, each with info on escaping and whether they're in a character class.
  * @example
  *
  * parseRegExp('a\\b[cd-]')
  *
  * returns:
  * [
  *   {text: 'a', index: 0, escaped: false, inCharClass: false, startsCharClass: false, endsCharClass: false},
  *   {text: 'b', index: 2, escaped: true, inCharClass: false, startsCharClass: false, endsCharClass: false},
  *   {text: 'c', index: 4, escaped: false, inCharClass: true, startsCharClass: true, endsCharClass: false},
  *   {text: 'd', index: 5, escaped: false, inCharClass: true, startsCharClass: false, endsCharClass: false},
  *   {text: '-', index: 6, escaped: false, inCharClass: true, startsCharClass: false, endsCharClass: false}
  * ]
  */
  function parseRegExp(regExpText) {
      const charList = [];
  
      regExpText.split("").reduce((state, char, index) => {
          if (!state.escapeNextChar) {
              if (char === "\\") {
                  return Object.assign(state, {escapeNextChar: true});
              }
              if (char === "[" && !state.inCharClass) {
                  return Object.assign(state, {inCharClass: true, startingCharClass: true});
              }
              if (char === "]" && state.inCharClass) {
                  if (charList.length && charList[charList.length - 1].inCharClass) {
                      charList[charList.length - 1].endsCharClass = true;
                  }
                  return Object.assign(state, {inCharClass: false, startingCharClass: false});
              }
          }
          charList.push({text: char, index, escaped: state.escapeNextChar, inCharClass: state.inCharClass, startsCharClass: state.startingCharClass, endsCharClass: false});
          return Object.assign(state, {escapeNextChar: false, startingCharClass: false});
      }, {escapeNextChar: false, inCharClass: false, startingCharClass: false});
  
      return charList;
  }
  
  module.exports = {
      meta: {
          docs: {
              description: "disallow unnecessary escape characters",
              category: "Best Practices",
              recommended: false
          },
  
          schema: []
      },
  
      create(context) {
  
          /**
           * Reports a node
           * @param {ASTNode} node The node to report
           * @param {number} startOffset The backslash's offset from the start of the node
           * @param {string} character The uselessly escaped character (not including the backslash)
           * @returns {void}
           */
          function report(node, startOffset, character) {
              context.report({
                  node,
                  loc: {
                      line: node.loc.start.line,
                      column: node.loc.start.column + startOffset
                  },
                  message: "Unnecessary escape character: \\{{character}}.",
                  data: {character}
              });
          }
  
          /**
           * Checks if the escape character in given string slice is unnecessary.
           *
           * @private
           * @param {ASTNode} node - node to validate.
           * @param {string} match - string slice to validate.
           * @returns {void}
           */
          function validateString(node, match) {
              const isTemplateElement = node.type === "TemplateElement";
              const escapedChar = match[0][1];
              let isUnnecessaryEscape = !VALID_STRING_ESCAPES.has(escapedChar);
              let isQuoteEscape;
  
              if (isTemplateElement) {
                  isQuoteEscape = escapedChar === "`";
  
                  if (escapedChar === "$") {
  
                      // Warn if `\$` is not followed by `{`
                      isUnnecessaryEscape = match.input[match.index + 2] !== "{";
                  } else if (escapedChar === "{") {
  
                      /* Warn if `\{` is not preceded by `$`. If preceded by `$`, escaping
                       * is necessary and the rule should not warn. If preceded by `/$`, the rule
                       * will warn for the `/$` instead, as it is the first unnecessarily escaped character.
                       */
                      isUnnecessaryEscape = match.input[match.index - 1] !== "$";
                  }
              } else {
                  isQuoteEscape = escapedChar === node.raw[0];
              }
  
              if (isUnnecessaryEscape && !isQuoteEscape) {
                  report(node, match.index, match[0].slice(1));
              }
          }
  
          /**
           * Checks if a node has an escape.
           *
           * @param {ASTNode} node - node to check.
           * @returns {void}
           */
          function check(node) {
              const isTemplateElement = node.type === "TemplateElement";
              const value = isTemplateElement ? node.value.raw : node.raw;
              const pattern = /\\[^\d]/g;
  
              if (isTemplateElement && node.parent && node.parent.parent && node.parent.parent.type === "TaggedTemplateExpression") {
  
                  // Don't report tagged template literals, because the backslash character is accessible to the tag function.
                  return;
              }
  
              if (typeof node.value === "string" || isTemplateElement) {
  
                  /*
                   * JSXAttribute doesn't have any escape sequence: https://facebook.github.io/jsx/.
                   * In addition, backticks are not supported by JSX yet: https://github.com/facebook/jsx/issues/25.
                   */
                  if (node.parent.type === "JSXAttribute") {
                      return;
                  }
  
                  let match;
  
                  while ((match = pattern.exec(value))) {
                      validateString(node, match);
                  }
              } else if (node.regex) {
                  parseRegExp(node.regex.pattern)
  
                      /*
                       * The '-' character is a special case, because it's only valid to escape it if it's in a character
                       * class, and is not at either edge of the character class. To account for this, don't consider '-'
                       * characters to be valid in general, and filter out '-' characters that appear in the middle of a
                       * character class.
                       */
                      .filter(charInfo => !(charInfo.text === "-" && charInfo.inCharClass && !charInfo.startsCharClass && !charInfo.endsCharClass))
  
                      /*
                       * The '^' character is also a special case; it must always be escaped outside of character classes, but
                       * it only needs to be escaped in character classes if it's at the beginning of the character class. To
                       * account for this, consider it to be a valid escape character outside of character classes, and filter
                       * out '^' characters that appear at the start of a character class.
                       */
                      .filter(charInfo => !(charInfo.text === "^" && charInfo.startsCharClass))
  
                      // Filter out characters that aren't escaped.
                      .filter(charInfo => charInfo.escaped)
  
                      // Filter out characters that are valid to escape, based on their position in the regular expression.
                      .filter(charInfo => !(charInfo.inCharClass ? REGEX_CHARCLASS_ESCAPES : REGEX_NON_CHARCLASS_ESCAPES).has(charInfo.text))
  
                      // Report all the remaining characters.
                      .forEach(charInfo => report(node, charInfo.index, charInfo.text));
              }
  
          }
  
          return {
              Literal: check,
              TemplateElement: check
          };
      }
  };
  