Fixed backtracking problem with the prioritized choice.

peglib.h

@@ -245,11 +245,16 @@ public:
     PrioritizedChoice(std::vector<std::shared_ptr<Rule>>&& rules) : rules_(std::move(rules)) {}
 
     Match parse(const char* s, size_t l, SemanticValues& sv) const {
+        auto sz = sv.values.size();
         for (const auto& rule : rules_) {
             auto m = rule->parse(s, l, sv);
             if (m.ret) {
                 return success(m.len);
             }
+            while (sv.values.size() > sz) {
+                sv.values.pop_back();
+                sv.names.pop_back();
+            }
         }
         return fail();
     }

test/test.cc

@@ -5,6 +5,11 @@
 #include <peglib.h>
 #include <iostream>
 
+TEST_CASE("Empty syntax test", "[general]")
+{
+    REQUIRE_THROWS(peglib::make_parser(""));
+}
+
 TEST_CASE("String capture test", "[general]")
 {
     auto parser = peglib::make_parser(
@@ -95,6 +100,35 @@ TEST_CASE("Lambda action test", "[general]")
     REQUIRE(ss == "hello");
 }
 
+TEST_CASE("Simple calculator test", "[general]")
+{
+    auto syntax =
+        " Additive  <- Multitive '+' Additive / Multitive "
+        " Multitive <- Primary '*' Multitive / Primary "
+        " Primary   <- '(' Additive ')' / Number "
+        " Number    <- [0-9]+ ";
+
+    auto parser = make_parser(syntax);
+
+    parser["Additive"] = [](const vector<Any>& v) {
+        return v.size() == 1 ? v[0] : v[0].get<int>() + v[1].get<int>();
+    };
+    parser["Multitive"] = [](const vector<Any>& v) {
+        return v.size() == 1 ? v[0] : v[0].get<int>() * v[1].get<int>();
+    };
+    parser["Primary"] = [](const vector<Any>& v) {
+        return v.size() == 1 ? v[0] : v[1];        
+    };
+    parser["Number"] = [](const char* s, size_t l) {
+        return atoi(s);
+    };
+
+    int val;
+    parser.parse("1+2*3", val);
+
+    REQUIRE(val == 7);
+}
+
 TEST_CASE("Calculator test", "[general]")
 {
     // Construct grammer