From a655436ac6ec4470d1322721400b3cafe2157ab0 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Sat, 14 Dec 2019 15:52:55 +0100
Subject: [PATCH] Make topological sort generic.

---
 2019/src/day14.cpp | 26 +++-----------------------
 2019/src/utils.hpp | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 23 deletions(-)

diff --git a/2019/src/day14.cpp b/2019/src/day14.cpp
index 0f5339a..2abebfd 100644
--- a/2019/src/day14.cpp
+++ b/2019/src/day14.cpp
@@ -7,6 +7,7 @@
 #include <regex>
 #include <charconv>
 #include "days.hpp"
+#include "utils.hpp"
 
 namespace {
     typedef std::pair<std::string, std::int64_t> requirement_t;
@@ -68,7 +69,7 @@ namespace {
     std::vector<std::string> topological_order(const std::map<reqlist_t, reqlist_t> &recipes) {
         std::vector<std::string> order;
 
-        std::unordered_map<std::string_view, std::vector<std::string>> edges;
+        std::unordered_map<std::string, std::vector<std::string>> edges;
         for (auto &entry : recipes) {
             for (auto &production : entry.first) {
                 std::transform(entry.second.begin(), entry.second.end(), std::back_inserter(edges[production.first]),
@@ -78,28 +79,7 @@ namespace {
             }
         }
 
-        std::unordered_map<std::string_view, int> incoming_edge_count;
-        for (const auto &entry : edges) {
-            for (const auto &parent : entry.second) {
-                incoming_edge_count[parent]++;
-            }
-        }
-
-        std::deque<std::string_view> childless{"FUEL"};
-
-        while (!childless.empty()) {
-            auto current = childless.front();
-            childless.pop_front();
-            order.emplace_back(current);
-
-            for (const auto &parent : edges[current]) {
-                if (--incoming_edge_count[parent] == 0) {
-                    childless.push_back(parent);
-                }
-            }
-        }
-
-        return order;
+        return aoc2019::topological_sort(edges);
     }
 
     std::int64_t ore_to_fuel(const std::map<reqlist_t, reqlist_t> &recipes, std::int64_t amount = 1) {
diff --git a/2019/src/utils.hpp b/2019/src/utils.hpp
index 7ae5f2b..a7f04d0 100644
--- a/2019/src/utils.hpp
+++ b/2019/src/utils.hpp
@@ -27,6 +27,49 @@ namespace aoc2019 {
 
     std::vector<int> run_intcode(std::vector<int> &program, std::deque<int> inputs = {});
 
+    template<class Node>
+    std::vector<Node> topological_sort(const std::unordered_map<Node, std::vector<Node>> &edge_list) {
+        std::unordered_map<Node, int> incoming_edges;
+
+        for (auto &entry : edge_list) {
+            // Ensure entry for parent exist
+            incoming_edges[entry.first] += 0;
+
+            for (auto &node : entry.second) {
+                incoming_edges[node]++;
+            }
+        }
+
+        std::vector<Node> order;
+        std::deque<Node> childless;
+
+        for (auto &entry : incoming_edges) {
+            if (!entry.second) {
+                childless.push_back(entry.first);
+            }
+        }
+
+        while (!childless.empty()) {
+            auto current = childless.front();
+            childless.pop_front();
+            order.emplace_back(current);
+
+            if (auto it = edge_list.find(current); it != edge_list.end()) {
+                for (const auto &parent : it->second) {
+                    if (--incoming_edges[parent] == 0) {
+                        childless.push_back(parent);
+                    }
+                }
+            }
+        }
+
+        if (order.size() != incoming_edges.size()) {
+            throw std::domain_error("Not a DAG.");
+        }
+
+        return order;
+    }
+
     class IntCodeComputer {
     public:
         typedef std::int64_t value_t;