1 files changed, 20 insertions, 0 deletions
diff --git a/bnf.cpp b/bnf.cpp
index f240f29..7290962 100644
--- a/bnf.cpp
+++ b/bnf.cpp
@@ -19,6 +19,26 @@ std::map<std::string, std::set<std::string>> Reverse(BNF bnf)
  return result;
 }
 
+std::map<std::string, std::set<std::string>> reverseFirst(BNF bnf)
+{
+ std::map<std::string, std::set<std::string>> result;
+
+ for (const auto& [from, to] : bnf) {
+  for (const auto& list : to) {
+   if (list.size() > 0) {
+    const auto& element{list[0]};
+    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;
+}
+
 BNF SubBNF(const BNF& bnf, const std::string& top)
 {
  BNF result;