1 files changed, 21 insertions, 0 deletions
diff --git a/bnf.cpp b/bnf.cpp
new file mode 100644
index 0000000..1efb459
--- /dev/null
+++ b/bnf.cpp
@@ -0,0 +1,21 @@
+#include "bnf.h"
+
+std::map<std::string, std::set<std::string>> Reverse(BNF bnf){
+ std::map<std::string, std::set<std::string>> result;
+
+ for (const auto& [from, to] : bnf) {
+  for (const auto& list : to) {
+   for (const auto& element : list) {
+    auto i{result.find(element)};
+    if (i != result.end()) // already present
+     i->second.insert(from);
+    else // new element
+     result.emplace(element, std::set{from});
+   }
+  }
+ }
+
+ return result;
+}
+
+