From 57b89409cee9def1b15f27c09163d3cde1582be0 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Fri, 5 Dec 2025 09:59:22 +0100
Subject: aoc 2025, day 5, part 2

---
 2025/src/day05.cpp | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

(limited to '2025/src/day05.cpp')

diff --git a/2025/src/day05.cpp b/2025/src/day05.cpp
index 2322948..ee565a3 100644
--- a/2025/src/day05.cpp
+++ b/2025/src/day05.cpp
@@ -26,10 +26,10 @@ read_file(const filesystem::path& filename)
 
 	for ( string line; getline(file, line); ) {
 		smatch match;
-		if ( regex_search(line, match, range) ) {
+		if ( regex_match(line, match, range) ) {
 			ranges.emplace_back(stol(match[1]), stol(match[2]));
 		}
-		else if ( regex_search(line, match, single) ) {
+		else if ( regex_match(line, match, single) ) {
 			numbers.emplace_back(stol(match[1]));
 		}
 	}
@@ -43,7 +43,7 @@ part1(const Ranges& ranges, const vector<long>& numbers)
 	long count = 0;
 	for ( const auto number: numbers ) {
 		for ( const auto [start, end]: ranges ) {
-			if ( number >= start && number <= end ) {
+			if ( start <= number && number <= end ) {
 				++count;
 				break;
 			}
@@ -52,6 +52,33 @@ part1(const Ranges& ranges, const vector<long>& numbers)
 	cout << "Part 1: " << count << '\n';
 }
 
+void
+part2(Ranges ranges)
+{
+	ranges::sort(ranges);
+
+	Ranges merged;
+	merged.emplace_back(ranges[0]);
+
+	for ( size_t i = 1; i < ranges.size(); ++i ) {
+		auto& [last_start, last_end] = merged.back();
+		auto& [curr_start, curr_end] = ranges[i];
+
+		if ( last_end < curr_start ) {
+			merged.emplace_back(ranges[i]);
+		}
+		else if ( last_end < curr_end ) {
+			last_end = curr_end;
+		}
+	}
+
+	long count = 0;
+	for ( const auto [start, end]: merged ) {
+		count += end - start + 1;
+	}
+	cout << "Part 2: " << count << '\n';
+}
+
 } // namespace
 
 int
@@ -59,4 +86,5 @@ main()
 {
 	const auto [ranges, numbers] = read_file("data/day05.txt");
 	part1(ranges, numbers);
+	part2(ranges);
 }
-- 
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