From 8c96639ec1f6757570510fc27f1c5fabece35eaf Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Sun, 16 Nov 2025 13:23:34 +0100
Subject: aoc 2018, days 1-11

---
 2018/src/day04.cpp | 135 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 135 insertions(+)
 create mode 100644 2018/src/day04.cpp

(limited to '2018/src/day04.cpp')

diff --git a/2018/src/day04.cpp b/2018/src/day04.cpp
new file mode 100644
index 0000000..ea3c69f
--- /dev/null
+++ b/2018/src/day04.cpp
@@ -0,0 +1,135 @@
+#include <algorithm>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+struct Record {
+	int  guard_id;
+	long start_sleep;
+	long stop_sleep;
+};
+
+long
+to_timestamp(const string& line)
+{
+	long year = stol(line.substr(1, 4));
+	long mon  = stol(line.substr(6, 2));
+	long day  = stol(line.substr(9, 2));
+	long hour = stol(line.substr(12, 2));
+	long min  = stol(line.substr(15, 2));
+
+	auto timestamp = (year * 100'000'000) + (mon * 1'000'000) + (day * 10'000) + (hour * 100) + min; // NOLINT
+
+	return timestamp;
+}
+
+vector<Record>
+read_file(const filesystem::path& filename)
+{
+	ifstream       file{ filename };
+	vector<string> lines;
+
+	for ( string line; getline(file, line); ) {
+		lines.emplace_back(line);
+	}
+
+	ranges::sort(lines);
+
+	vector<Record> records;
+
+	int  guard       = -1;
+	long start_sleep = -1;
+	long stop_sleep  = -1;
+	for ( const auto& line: lines ) {
+		if ( line.find(" Guard #") != string::npos ) {
+			guard = stoi(line.substr(26));
+		}
+		else if ( line.find(" falls asleep") != string::npos ) {
+			start_sleep = to_timestamp(line);
+		}
+		else if ( line.find(" wakes up") != string::npos ) {
+			stop_sleep = to_timestamp(line);
+			records.emplace_back(guard, start_sleep, stop_sleep);
+		}
+	}
+
+	return records;
+}
+
+void
+part1(const vector<Record>& records)
+{
+	map<int, long> sleep_amounts;
+
+	for ( const auto& record: records ) {
+		sleep_amounts[record.guard_id] += record.stop_sleep - record.start_sleep;
+	}
+
+	auto max_amount = ranges::max_element(sleep_amounts,
+	                                      [](const auto& lhs, const auto& rhs) {
+		                                      return lhs.second < rhs.second;
+	                                      });
+
+	map<long, unsigned> sleep_counts;
+
+	for ( const auto& record: records ) {
+		if ( record.guard_id != max_amount->first ) {
+			continue;
+		}
+
+		for ( auto timestamp = record.start_sleep; timestamp != record.stop_sleep; ++timestamp ) {
+			sleep_counts[timestamp % 10'000]++;
+		}
+	}
+
+	auto max_count = ranges::max_element(sleep_counts,
+	                                     [](const auto& lhs, const auto& rhs) {
+		                                     return lhs.second < rhs.second;
+	                                     });
+
+	cout << "Part 1: " << max_amount->first * (max_count->first % 100) << '\n';
+}
+
+void
+part2(const vector<Record>& records)
+{
+	map<int, map<long, unsigned>> sleep_counts;
+
+	for ( const auto& record: records ) {
+		for ( auto timestamp = record.start_sleep; timestamp != record.stop_sleep; ++timestamp ) {
+			sleep_counts[record.guard_id][timestamp % 10'000]++;
+		}
+	}
+
+	int      found_guard = -1;
+	long     found_min   = -1;
+	unsigned max_count   = 0;
+	for ( const auto& [id, times]: sleep_counts ) {
+		for ( const auto& [min, count]: times ) {
+			if ( count > max_count ) {
+				max_count   = count;
+				found_guard = id;
+				found_min   = min % 100;
+			}
+		}
+	}
+
+	cout << "Part 2: " << found_guard * found_min << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto records = read_file("data/day04.txt");
+	part1(records);
+	part2(records);
+}
-- 
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