From bc3edfd93cfdca71ea684c36c72a4193764bc7b2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20M=C3=BCller?= <stefan.mueller@aksdb.de>
Date: Thu, 5 Dec 2024 20:24:53 +0100
Subject: [PATCH] Add solution for "Day 5: Print Queue", part 2

---
 AdventOfCode2024/PrintQueue.cpp | 17 ++++++++++++-----
 README.md                       |  6 ++++++
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/AdventOfCode2024/PrintQueue.cpp b/AdventOfCode2024/PrintQueue.cpp
index 65bdf9d..dee3eba 100644
--- a/AdventOfCode2024/PrintQueue.cpp
+++ b/AdventOfCode2024/PrintQueue.cpp
@@ -13,6 +13,7 @@
 // You should have received a copy of the GNU General Public License along with
 // this program.  If not, see <http://www.gnu.org/licenses/>.
 
+#include <algorithm>
 #include <sstream>
 #include <vector>
 
@@ -77,7 +78,8 @@ void PrintQueue::processUpdatePages(const std::string& line)
     std::stringstream stream{ line };
     std::string token;
     auto isCorrectOrder{ true };
-    while (isCorrectOrder && std::getline(stream, token, ','))
+    // We completely construct 'pages' for part 2, even if the ordering is not correct.
+    while (std::getline(stream, token, ','))
     {
         size_t page = std::stoi(token);
         size_t i{ 0 };
@@ -86,14 +88,19 @@ void PrintQueue::processUpdatePages(const std::string& line)
             isCorrectOrder = !orderingRules_[page][pages[i]];
             i++;
         }
-        if (isCorrectOrder)
-        {
-            pages.push_back(page);
-        }
+        pages.push_back(page);
     }
 
     if (isCorrectOrder)
     {
         part1 += pages[pages.size() / 2];
     }
+    else
+    {
+        // This works because the input defines a complete ordering on the occurring
+        // page numbers.
+        std::sort(pages.begin(), pages.end(),
+            [&](int const& a, int const& b) { return orderingRules_[a][b]; });
+        part2 += pages[pages.size() / 2];
+    }
 }
diff --git a/README.md b/README.md
index c6fe225..a67202d 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,12 @@ A simple [finite state machine](AdventOfCode2024/StringStateMachine.h) crawling
 
 For this puzzle I added a class for [points in two-dimensional space](AdventOfCode2024/Point2.h), so I can use these for simplifying directional computations. With that, the algorithm looks for all `X` and `A` for part 1 and 2, respectively, and tries to find the remaining characters, starting from that `X` or `A`.
 
+### Day 5: Print Queue
+
+:mag_right: Puzzle: <https://adventofcode.com/2024/day/5>, :white_check_mark: Solver: [`PrintQueue.cpp`](AdventOfCode2024/PrintQueue.cpp)
+
+My implementation uses an ordering matrix (a two-dimensional boolean array) to track which page combinations are ordered, and then queries that matrix for each ordered combination of pages in a single line. The same matrix can then also be used for a custom sort function for part 2.
+
 ## License
 
 Copyright (C) 2024 Stefan Müller