1 files changed, 30 insertions, 1 deletions

diff --git a/bnf.cpp b/bnf.cpp
index 1efb459..f240f29 100644
--- a/bnf.cpp
+++ b/bnf.cpp
@@ -1,6 +1,7 @@
 #include "bnf.h"
 
-std::map<std::string, std::set<std::string>> Reverse(BNF bnf){
+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) {
@@ -18,4 +19,32 @@ std::map<std::string, std::set<std::string>> Reverse(BNF bnf){
  return result;
 }
 
+BNF SubBNF(const BNF& bnf, const std::string& top)
+{
+ BNF result;
+
+ std::vector<std::string> todo{top};
+
+ while (!todo.empty()) {
+  std::string current{todo.back()};
+  todo.pop_back();
+
+  auto it = bnf.find(current);
+  if (it != bnf.end()) {
+   // add current value
+   result[it->first] = it->second;
+
+   // add sub-tree values if not present yet, but available in original bnf
+   for (auto& variant: it->second) {
+    for (auto& child: variant) {
+     if (result.find(child) == result.end() && bnf.find(child) != bnf.end()) {
+      todo.push_back(child);
+     }
+    }
+   }
+  }
+ }
+
+ return result;
+}