2 files changed, 35 insertions, 4 deletions

diff --git a/src/generator.cpp b/src/generator.cpp
index 6198aba..d63b859 100644
--- a/src/generator.cpp
+++ b/src/generator.cpp
@@ -97,9 +97,9 @@ void generator_t::generate_itemsets() {
 			auto added = add_set(entry.second);
 
 			if (entry.first.get_type() == SYMBOL_TYPE_NONTERM)
-				gotos.emplace(std::make_pair(cur.second, entry.first.get_value()), added.first->second);
+				add_goto(cur.second, entry.first.get_value(), added.first->second);
 			else
-				shifts.emplace(std::make_pair(cur.second, entry.first), added.first->second);
+				add_shift(cur.second, entry.first, added.first->second);
 
 			if (added.second)
 				queue.push(*added.first);
@@ -109,9 +109,9 @@ void generator_t::generate_itemsets() {
 			auto it = rule_ids.find(item);
 			if (it != rule_ids.end()) {
 				if (it->second)
-					reductions.emplace(cur.second, it->second);
+					add_reduction(cur.second, it->second);
 				else
-					shifts.emplace(std::make_pair(cur.second, symbol_t::make_nonterm("")), 0);
+					add_shift(cur.second, symbol_t::make_nonterm(""), 0);
 			}
 		}
 	}
diff --git a/src/generator.hpp b/src/generator.hpp
index 8178bbd..47ef6bf 100644
--- a/src/generator.hpp
+++ b/src/generator.hpp
@@ -28,6 +28,7 @@
 
 #include "item.hpp"
 
+#include <stdexcept>
 #include <set>
 #include <map>
 
@@ -35,6 +36,13 @@
 namespace solar {
 
 class generator_t {
+public:
+	class conflict_error : public std::runtime_error {
+	public:
+		explicit conflict_error(const std::string& what_arg) : std::runtime_error(what_arg) {}
+		explicit conflict_error(const char* what_arg) : std::runtime_error(what_arg) {}
+	};
+
 private:
 	std::vector<std::pair<item_t, std::string>> rules;
 	std::map<item_t, size_t> rule_ids;
@@ -49,6 +57,8 @@ private:
 	std::map<size_t, size_t> reductions;
 	std::map<std::pair<size_t, std::string>, size_t> gotos;
 
+	std::set<size_t> shift_conflicts;
+
 	void close_set(std::set<item_t> *set);
 	std::set<item_t> get_set(const std::string &nonterm);
 
@@ -56,6 +66,27 @@ private:
 		return itemsets.emplace(set, itemsets.size());
 	}
 
+	void add_reduction(size_t from, size_t to) {
+		if (reductions.count(from))
+			throw conflict_error("reduce/reduce conflict");
+		if (shift_conflicts.count(from))
+			throw conflict_error("shift/reduce conflict");
+
+		reductions.emplace(from, to);
+	}
+
+	void add_shift(size_t from, const symbol_t &sym, size_t to) {
+		if (reductions.count(from))
+			throw conflict_error("shift/reduce conflict");
+
+		shift_conflicts.insert(from);
+		shifts.emplace(std::make_pair(from, sym), to);
+	}
+
+	void add_goto(size_t from, const std::string &nonterm, size_t to) {
+		gotos.emplace(std::make_pair(from, nonterm), to);
+	}
+
 	void generate_itemsets();
 
 public: