Another infinite loop grammar detection. Fix #133

peglib.h

@@ -2872,7 +2872,9 @@ private:
     };
 
     g["Sequence"] = [&](const SemanticValues &vs) {
-      if (vs.size() == 1) {
+      if (vs.empty()) {
+        return npd(dot());
+      } else if (vs.size() == 1) {
         return std::any_cast<std::shared_ptr<Ope>>(vs[0]);
       } else {
         std::vector<std::shared_ptr<Ope>> opes;

test/test2.cc

@@ -159,6 +159,14 @@ TEST_CASE("Infinite loop 7", "[infinite loop]") {
   REQUIRE(!pg);
 }
 
+TEST_CASE("Infinite loop 8", "[infinite loop]") {
+    parser pg(R"(
+        ROOT <- ('A' /)*
+    )");
+
+    REQUIRE(!pg);
+}
+
 TEST_CASE("Not infinite 1", "[infinite loop]") {
   parser pg(R"(
         Numbers <- Number* EOF

test/test3.cc

@@ -42,6 +42,11 @@ TEST_CASE("PEG Expression", "[peg]")
     REQUIRE(exact(g, "Expression", "") == true);
     REQUIRE(exact(g, "Expression", " ") == false);
     REQUIRE(exact(g, "Expression", " a b ") == false);
+
+    // NOTE: The followings are actually allowed in the original Ford's paper...
+    REQUIRE(exact(g, "Expression", "a//b ") == true);
+    REQUIRE(exact(g, "Expression", "a // b ") == true);
+    REQUIRE(exact(g, "Expression", "a / / b ") == true);
 }
 
 TEST_CASE("PEG Sequence", "[peg]")