From 9af62d7c591b7893f47f111784224eb0d5c5e4ff Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Fri, 6 Dec 2024 22:05:54 +0100
Subject: aoc 2024, day6

---
 2024/src/day06.cpp | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 2024/src/day06.cpp

(limited to '2024/src')

diff --git a/2024/src/day06.cpp b/2024/src/day06.cpp
new file mode 100644
index 0000000..1198d54
--- /dev/null
+++ b/2024/src/day06.cpp
@@ -0,0 +1,139 @@
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <set>
+#include <string>
+#include <tuple>
+using namespace std;
+
+using pos_type = tuple<size_t, size_t>;
+
+map<pos_type, char>
+read_file(string_view filename)
+{
+	fstream             input{ filename };
+	map<pos_type, char> data;
+
+	size_t yPos = 0;
+	for ( string line; getline(input, line); ) {
+		for ( size_t xPos = 0; xPos != line.size(); ++xPos ) {
+			data[{ xPos, yPos }] = line[xPos];
+		}
+		++yPos;
+	}
+
+	return data;
+}
+
+set<pos_type>
+distinct_positions(const map<pos_type, char>& data)
+{
+	static const array<pos_type, 4> deltas = {
+		make_tuple(0, -1),
+		make_tuple(1, 0),
+		make_tuple(0, 1),
+		make_tuple(-1, 0)
+	};
+
+	size_t direction = 0;
+
+	auto position = get<0>(*ranges::find_if(data, [](auto position) {
+		return get<1>(position) == '^';
+	}));
+
+	set<pos_type> positions;
+	while ( true ) {
+		positions.emplace(position);
+
+		auto next_position = make_tuple(get<0>(position) + get<0>(deltas.at(direction)),
+		                                get<1>(position) + get<1>(deltas.at(direction)));
+
+		if ( !data.contains(next_position) ) {
+			break;
+		}
+
+		if ( data.at(next_position) == '#' ) {
+			direction = (direction + 1) % deltas.size();
+		}
+		else {
+			position = next_position;
+		}
+	}
+
+	return positions;
+}
+
+void
+part1(const map<pos_type, char>& data)
+{
+	auto positions = distinct_positions(data);
+
+	cout << positions.size() << endl;
+}
+
+bool
+would_loop(const map<pos_type, char>& data)
+{
+	static const array<pos_type, 4> deltas = {
+		make_tuple(0, -1),
+		make_tuple(1, 0),
+		make_tuple(0, 1),
+		make_tuple(-1, 0)
+	};
+
+	size_t direction = 0;
+
+	auto position = get<0>(*ranges::find_if(data, [](auto position) {
+		return get<1>(position) == '^';
+	}));
+
+	set<tuple<pos_type, size_t>> positions;
+
+	while ( true ) {
+		positions.emplace(position, direction);
+
+		auto next_position = make_tuple(get<0>(position) + get<0>(deltas.at(direction)),
+		                                get<1>(position) + get<1>(deltas.at(direction)));
+
+		if ( !data.contains(next_position) ) {
+			return false;
+		}
+
+		if ( data.at(next_position) == '#' ) {
+			direction = (direction + 1) % deltas.size();
+		}
+		else {
+			position = next_position;
+		}
+
+		if ( positions.contains({ position, direction }) ) {
+			return true;
+		}
+	}
+}
+
+void
+part2(map<pos_type, char> data)
+{
+	const auto positions = distinct_positions(data);
+
+	long sum = 0;
+	for ( const auto& position: positions ) {
+		if ( data.at(position) == '.' ) {
+			data.at(position) = '#';
+			if ( would_loop(data) ) {
+				++sum;
+			}
+			data.at(position) = '.';
+		}
+	}
+	cout << sum << endl;
+}
+
+int
+main()
+{
+	auto data = read_file("data/day06.txt");
+	part1(data);
+	part2(data);
+}
-- 
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