[parser] Fix the branch restore mechanism of parse()

Author: LuisHsu

src/lib/parser/parser.cpp

@@ -363,15 +363,21 @@ void Pargen::Parser::generate_source(std::ostream& os){
         "        }\n"
         "        auto& saved = branches.top();\n"
         "        auto buf_top = buffer.begin();\n"
-        "        for(auto it = stack.begin(); it != saved; it = std::next(it)){\n"
+        "        Stack::iterator new_head = stack.begin();\n"
+        "        for(auto it = stack.begin(); (it != stack.end()) && (it != saved); it = std::next(it)){\n"
+        "            new_head = it;\n"
         "            if(std::holds_alternative<token_t>(it->elem)){\n"
         "                buffer.emplace(buf_top, it->term, std::get<token_t>(it->elem));\n"
         "            }else{\n"
         "                auto flattened = std::get<Node>(it->elem).flatten();\n"
         "                buffer.insert(buf_top, flattened.begin(), flattened.end());\n"
         "            }\n"
         "        }\n"
-        "        saved->branch += 1;\n"
+        "        new_head->branch += 1;\n"
+        "        new_head->term = buffer.front().first;\n"
+        "        new_head->elem = buffer.front().second;\n"
+        "        buffer.pop_front();\n"
+        "        stack.erase(stack.begin(), new_head);\n"
         "    };\n"
         "    // Parse\n"
         "    while(stack.front().state != End){\n"
@@ -381,7 +387,7 @@ void Pargen::Parser::generate_source(std::ostream& os){
         "            std::vector<Act>& acts = state[entry.term];\n"
         "            if(entry.branch == 0){\n"
         "                if(acts.size() > 1){\n"
-        "                    branches.emplace(stack.begin());\n"
+        "                    branches.emplace(std::next(stack.begin()));\n"
         "                }\n"
         "            }else if(entry.branch == acts.size() - 1){\n"
         "                branches.pop();\n"