var arrays = require("../../utils/arrays"), GrammarError = require("../../grammar-error"), asts = require("../asts"), visitor = require("../visitor"); /* * Reports left recursion in the grammar, which prevents infinite recursion in * the generated parser. * * Both direct and indirect recursion is detected. The pass also correctly * reports cases like this: * * start = "a"? start * * In general, if a rule reference can be reached without consuming any input, * it can lead to left recursion. */ function reportLeftRecursion(ast) { var check = visitor.build({ rule: function(node, visitedRules) { check(node.expression, visitedRules.concat(node.name)); }, sequence: function(node, visitedRules) { arrays.every(node.elements, function(element) { if (element.type === "rule_ref") { check(element, visitedRules); } return asts.matchesEmpty(ast, element); }); }, rule_ref: function(node, visitedRules) { if (arrays.contains(visitedRules, node.name)) { throw new GrammarError( "Left recursion detected for rule \"" + node.name + "\".", node.location ); } check(asts.findRule(ast, node.name), visitedRules); } }); check(ast, []); } module.exports = reportLeftRecursion;