From 0450ecd6a67226ddbaeedde64ef45dfdd72cddd1 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Sun, 19 Oct 2025 22:03:39 +0200
Subject: day 18, aoc 2016

---
 2016/src/day18.cpp | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)
 create mode 100644 2016/src/day18.cpp

(limited to '2016')

diff --git a/2016/src/day18.cpp b/2016/src/day18.cpp
new file mode 100644
index 0000000..66d1bf0
--- /dev/null
+++ b/2016/src/day18.cpp
@@ -0,0 +1,80 @@
+#include <algorithm>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <string>
+
+using namespace std;
+
+namespace {
+
+string
+readFile(const filesystem::path& filename)
+{
+	ifstream file{ filename };
+	string   line;
+	getline(file, line);
+	return line;
+}
+
+void
+solve(string line, size_t lines)
+{
+	long count = 0;
+	while ( lines-- > 0 ) {
+		string current_line = "." + line + ".";
+
+		string new_line;
+
+		for ( size_t i = 1; i < current_line.size() - 1; ++i ) {
+			auto next_chr = '.';
+
+			const auto left   = current_line[i - 1];
+			const auto middle = current_line[i];
+			const auto right  = current_line[i + 1];
+
+			if ( left == '^' && middle == '^' && right == '.' ) {
+				next_chr = '^';
+			}
+			else if ( left == '.' && middle == '^' && right == '^' ) {
+				next_chr = '^';
+			}
+			else if ( left == '^' && middle == '.' && right == '.' ) {
+				next_chr = '^';
+			}
+			else if ( left == '.' && middle == '.' && right == '^' ) {
+				next_chr = '^';
+			}
+
+			new_line += next_chr;
+		}
+
+		count += ranges::count(line, '.');
+
+		line = std::move(new_line);
+	}
+
+	cout << count << '\n';
+}
+
+void
+part1(const string& line)
+{
+	solve(line, 40);
+}
+
+void
+part2(const string& line)
+{
+	solve(line, 400000);
+}
+
+} // namespace
+
+int
+main()
+{
+	auto line = readFile("data/day18.txt");
+	part1(line);
+	part2(line);
+}
-- 
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