Classify grammars with cycles as not LL(1)

Author: mdaines

src/grammar/calculations/parsing/ll/ll1_classification.js

@@ -2,7 +2,7 @@ import * as SetOperations from "../../../../set_operations.js";
 import { getFirst } from "../helpers.js";
 
 export default function(calculations) {
-  const { nullAmbiguity } = calculations;
+  const { nullAmbiguity, cycle } = calculations;
 
   var i, k, l, s;
   var head, body, bodyFirst;
@@ -13,6 +13,12 @@ export default function(calculations) {
     return { member: false, reason: "it contains a null ambiguity" };
   }
 
+  // Also, return immediately if the grammar contains a cycle.
+
+  if (typeof cycle !== "undefined") {
+    return { member: false, reason: "it contains a cycle" };
+  }
+
   const { productions, first, follow, terminals, nonterminals, nullable } = calculations;
 
   // Check for first set clashes. Instead of checking intersections of

test/fixtures/example_grammars.js

@@ -501,5 +501,9 @@ export default {
     ["D", "E"],
     ["T", "X"],
     ["X"]
+  ],
+  "issue-41": [
+    ["S", "E"],
+    ["E", "E"]
   ]
 };