From 22f96da42562d1f9ab3e5fd1715a6158ebebf3a2 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Sat, 18 Oct 2025 10:38:00 +0200
Subject: day 15, aoc 2016

---
 2016/src/day15.cpp | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100644 2016/src/day15.cpp

(limited to '2016/src')

diff --git a/2016/src/day15.cpp b/2016/src/day15.cpp
new file mode 100644
index 0000000..ced6102
--- /dev/null
+++ b/2016/src/day15.cpp
@@ -0,0 +1,89 @@
+#include <algorithm>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <regex>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+struct Disc {
+	[[nodiscard]]
+	bool zeroAtTime(int time) const
+	{
+		return (time + num + phase) % positions == 0;
+	}
+
+	int num;
+	int phase;
+	int positions;
+};
+
+vector<int>
+extractNumbers(const string& text)
+{
+	static const regex number_regex("\\d+");
+
+	vector<int> numbers;
+	smatch      match;
+
+	auto searchStart = text.cbegin();
+	while ( regex_search(searchStart, text.cend(), match, number_regex) ) {
+		numbers.emplace_back(stoi(match.str()));
+		searchStart = match.suffix().first;
+	}
+
+	return numbers;
+}
+
+vector<Disc>
+readFile(const filesystem::path& filename)
+{
+	ifstream     file{ filename };
+	vector<Disc> discs;
+
+	for ( string line; getline(file, line); ) {
+		auto numbers = extractNumbers(line);
+		discs.emplace_back(numbers.at(0), numbers.at(3), numbers.at(1));
+	}
+
+	return discs;
+}
+
+void
+solve(const vector<Disc>& discs)
+{
+	for ( int time = 0;; ++time ) {
+		auto found = ranges::all_of(discs, [&time](const auto& disc) { return disc.zeroAtTime(time); });
+		if ( found ) {
+			cout << time << '\n';
+			return;
+		}
+	}
+}
+
+void
+part1(const vector<Disc>& discs)
+{
+	solve(discs);
+}
+
+void
+part2(vector<Disc> discs)
+{
+	discs.emplace_back(discs.size() + 1, 0, 11);
+	solve(discs);
+}
+
+} // namespace
+
+int
+main()
+{
+	auto discs = readFile("data/day15.txt");
+	part1(discs);
+	part2(discs);
+}
-- 
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