From 48da2e2805dd56932674e709f28d70836aac499d Mon Sep 17 00:00:00 2001
From: Eric Mertens <emertens@galois.com>
Date: Wed, 9 Nov 2022 15:45:55 -0800
Subject: [PATCH] fewer lookups in kahn's algorithm

---
 2019/14.cpp | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/2019/14.cpp b/2019/14.cpp
index 571dd91..74779e2 100644
--- a/2019/14.cpp
+++ b/2019/14.cpp
@@ -45,27 +45,30 @@ auto Parse(std::istream & in) -> Recipes {
 auto Topsort(Recipes const& recipes) -> std::vector<std::string>
 {
   // Kahn's algorithm
-  std::set<std::string> perm;
-  std::set<std::string> temp;
   std::vector<std::string> result;
+  result.reserve(recipes.size());
 
-  auto visit_ = [&](auto& rec, std::string name) -> void {
-    if (!perm.contains(name)) {
-      if (!temp.insert(name).second) throw std::runtime_error{"cyclic recipes"};
-      if (recipes.contains(name)) {
-        for (auto && [_, c] : recipes.at(name).second) {
-          rec(rec, c);
+  std::map<std::string, bool> marks; // false temp, true done
+  auto const visit = [&](auto const& visit, auto const& name) -> void {
+    auto [mark_iterator, is_new] = marks.try_emplace(name, false);
+    auto& [_, is_done] = *mark_iterator;
+
+    if (is_new) {
+      auto const rit = recipes.find(name);
+      if (rit != recipes.end()) {
+        for (auto const& [_, c] : rit->second.second) {
+          visit(visit, c);
         }
       }
-      temp.erase(name);
-      perm.insert(name);
+      is_done = true;
       result.push_back(name);
+    } else if (!is_done) {
+      throw std::runtime_error{"cyclic graph"};
     }
   };
-  auto visit = [&](auto name) { visit_(visit_, name); };
 
-  for (auto && [k,_] : recipes) {
-    visit(k);
+  for (auto const& [c,_] : recipes) {
+    visit(visit, c);
   }
 
   std::reverse(result.begin(), result.end());