/**
   * @fileoverview Rule to flag references to the undefined variable.
   * @author Michael Ficarra
   */
  "use strict";
  
  //------------------------------------------------------------------------------
  // Rule Definition
  //------------------------------------------------------------------------------
  
  module.exports = {
      meta: {
          docs: {
              description: "disallow the use of `undefined` as an identifier",
              category: "Variables",
              recommended: false
          },
  
          schema: []
      },
  
      create(context) {
  
          /**
           * Report an invalid "undefined" identifier node.
           * @param {ASTNode} node The node to report.
           * @returns {void}
           */
          function report(node) {
              context.report({
                  node,
                  message: "Unexpected use of undefined."
              });
          }
  
          /**
           * Checks the given scope for references to `undefined` and reports
           * all references found.
           * @param {escope.Scope} scope The scope to check.
           * @returns {void}
           */
          function checkScope(scope) {
              const undefinedVar = scope.set.get("undefined");
  
              if (!undefinedVar) {
                  return;
              }
  
              const references = undefinedVar.references;
  
              const defs = undefinedVar.defs;
  
              // Report non-initializing references (those are covered in defs below)
              references
                  .filter(ref => !ref.init)
                  .forEach(ref => report(ref.identifier));
  
              defs.forEach(def => report(def.name));
          }
  
          return {
              "Program:exit"() {
                  const globalScope = context.getScope();
  
                  const stack = [globalScope];
  
                  while (stack.length) {
                      const scope = stack.pop();
  
                      stack.push.apply(stack, scope.childScopes);
                      checkScope(scope);
                  }
              }
          };
  
      }
  };