From 6fcfe0a5cf8f53658e50e346076768eec229e695 Mon Sep 17 00:00:00 2001
From: Roland Reichwein <mail@reichwein.it>
Date: Sun, 1 Mar 2020 22:34:35 +0100
Subject: Vector invalidation fix

---
 bnf.cpp | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

(limited to 'bnf.cpp')

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;
-- 
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