/**
   * @fileoverview Rule to flag the generator functions that does not have yield.
   * @author Toru Nagashima
   */
  
  "use strict";
  
  //------------------------------------------------------------------------------
  // Rule Definition
  //------------------------------------------------------------------------------
  
  module.exports = {
      meta: {
          docs: {
              description: "require generator functions to contain `yield`",
              category: "ECMAScript 6",
              recommended: true
          },
  
          schema: []
      },
  
      create(context) {
          const stack = [];
  
          /**
           * If the node is a generator function, start counting `yield` keywords.
           * @param {Node} node - A function node to check.
           * @returns {void}
           */
          function beginChecking(node) {
              if (node.generator) {
                  stack.push(0);
              }
          }
  
          /**
           * If the node is a generator function, end counting `yield` keywords, then
           * reports result.
           * @param {Node} node - A function node to check.
           * @returns {void}
           */
          function endChecking(node) {
              if (!node.generator) {
                  return;
              }
  
              const countYield = stack.pop();
  
              if (countYield === 0 && node.body.body.length > 0) {
                  context.report({ node, message: "This generator function does not have 'yield'." });
              }
          }
  
          return {
              FunctionDeclaration: beginChecking,
              "FunctionDeclaration:exit": endChecking,
              FunctionExpression: beginChecking,
              "FunctionExpression:exit": endChecking,
  
              // Increases the count of `yield` keyword.
              YieldExpression() {
  
                  /* istanbul ignore else */
                  if (stack.length > 0) {
                      stack[stack.length - 1] += 1;
                  }
              }
          };
      }
  };