From f29300ee1b3a3bada6297aa9b017435a5da9b17d Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Sun, 16 Nov 2025 22:56:16 +0100
Subject: aoc 2018, days 12

---
 2018/src/day12.cpp | 127 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)
 create mode 100644 2018/src/day12.cpp

(limited to '2018/src')

diff --git a/2018/src/day12.cpp b/2018/src/day12.cpp
new file mode 100644
index 0000000..21d74b2
--- /dev/null
+++ b/2018/src/day12.cpp
@@ -0,0 +1,127 @@
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <string>
+#include <unordered_set>
+
+using namespace std;
+
+namespace {
+
+using Plants = unordered_set<long>;
+using Rules  = map<string, char>;
+using Puzzle = tuple<Plants, Rules>;
+
+Puzzle
+read_file(const filesystem::path& filename)
+{
+	fstream file{ filename };
+
+	string line;
+	getline(file, line);
+	line.erase(0, line.find(": ") + 2);
+
+	Plants plants;
+	for ( size_t idx = 0; idx != line.length(); ++idx ) {
+		if ( line.at(idx) == '#' ) {
+			plants.insert(static_cast<long>(idx));
+		}
+	}
+
+	getline(file, line);
+
+	Rules rules;
+	while ( getline(file, line) ) {
+		auto lhs = line.substr(0, 5);
+		auto rhs = line.substr(9);
+
+		rules[lhs] = rhs.at(0);
+	}
+
+	return { plants, rules };
+}
+
+Plants
+update(const Plants& current, const Rules& rules)
+{
+	auto [min, max] = ranges::minmax(current);
+
+	Plants next;
+
+	for ( auto i = min - 2; i <= max + 2; ++i ) {
+		string pattern;
+		pattern += current.contains(i - 2) ? '#' : '.';
+		pattern += current.contains(i - 1) ? '#' : '.';
+		pattern += current.contains(i) ? '#' : '.';
+		pattern += current.contains(i + 1) ? '#' : '.';
+		pattern += current.contains(i + 2) ? '#' : '.';
+
+		if ( rules.contains(pattern) && rules.at(pattern) == '#' ) {
+			next.insert(i);
+		}
+	}
+
+	return next;
+}
+
+void
+part1(const Puzzle& puzzle)
+{
+	auto [state, rules] = puzzle;
+
+	for ( int i = 0; i != 20; ++i ) {
+		state = update(state, rules);
+	}
+
+	long sum = 0;
+	for ( const auto pos: state ) {
+		sum += pos;
+	}
+	cout << "Part 1: " << sum << '\n';
+}
+
+void
+part2(const Puzzle& puzzle)
+{
+	auto [state, rules] = puzzle;
+
+	auto to_string = [&] {
+		string result;
+
+		auto [min, max] = ranges::minmax(state);
+		for ( auto pos = min; pos <= max; ++pos ) {
+			result += (state.contains(pos) ? '#' : '.');
+		}
+
+		return result;
+	};
+
+	unordered_set<string> seen;
+	for ( long shift = 50000000000;; --shift ) {
+		auto pattern = to_string();
+
+		if ( seen.contains(pattern) ) {
+			long sum = 0;
+			for ( const auto pos: state ) {
+				sum += pos + shift;
+			}
+			cout << "Part 2: " << sum << '\n';
+			return;
+		}
+
+		seen.insert(pattern);
+
+		state = update(state, rules);
+	}
+}
+
+} // namespace
+
+int
+main()
+{
+	auto puzzle = read_file("data/day12.txt");
+	part1(puzzle);
+	part2(puzzle);
+}
-- 
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