From 56e890cec0a28c0a485212ccebfaf774235a79a2 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Wed, 3 Jan 2024 23:35:54 +0100
Subject: prepare for more puzzles ... :)

---
 2023/src/day21.cpp | 148 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 2023/src/day21.cpp

(limited to '2023/src/day21.cpp')

diff --git a/2023/src/day21.cpp b/2023/src/day21.cpp
new file mode 100644
index 0000000..c633155
--- /dev/null
+++ b/2023/src/day21.cpp
@@ -0,0 +1,148 @@
+#include <cstddef>
+#include <fstream>
+#include <iostream>
+#include <queue>
+#include <set>
+#include <string>
+#include <vector>
+using namespace std;
+
+using position = tuple<size_t, size_t>;
+
+vector<string>
+read_file(string_view filename)
+{
+	fstream        input{ filename };
+	vector<string> data;
+
+	for ( string line; getline(input, line); ) {
+		data.emplace_back(line);
+	}
+
+	return data;
+}
+
+position
+find_start_position(const vector<string>& lines)
+{
+	for ( size_t row = 0; row != lines.size(); ++row ) {
+		for ( size_t col = 0; col != lines[row].size(); ++col ) {
+			if ( lines[row][col] == 'S' ) {
+				return { row, col };
+			}
+		}
+	}
+	return {};
+}
+
+set<position>
+find_neighbours(position pos, const vector<string>& lines)
+{
+	static const vector<position> movements = { { -1, 0 }, { 0, 1 }, { 1, 0 }, { 0, -1 } };
+
+	set<position> neighbours;
+	const auto [row, col] = pos;
+
+	for ( const auto& [drow, dcol]: movements ) {
+		const auto nrow = row + drow;
+		const auto ncol = col + dcol;
+
+		if ( nrow < lines.size() && ncol < lines[0].size() && (lines[nrow][ncol] == '.' || lines[nrow][ncol] == 'S') ) {
+			neighbours.emplace(nrow, ncol);
+		}
+	}
+
+	return neighbours;
+}
+
+size_t
+count(vector<string> lines, position start, size_t rounds)
+{
+	queue<position> positions;
+	positions.emplace(start);
+
+	size_t sum = 0;
+	for ( size_t round = 0; round != rounds; ++round ) {
+		set<position> next_positions;
+
+		sum = 0;
+		while ( !positions.empty() ) {
+			const auto [curr_row, curr_col] = positions.front();
+			lines[curr_row][curr_col]       = '.';
+
+			const auto neighbours = find_neighbours(positions.front(), lines);
+			positions.pop();
+
+			for ( const auto& [row, col]: neighbours ) {
+				lines[row][col] = 'O';
+				next_positions.emplace(row, col);
+				++sum;
+			}
+		}
+
+		for ( const auto& position: next_positions ) {
+			positions.emplace(position);
+		}
+	}
+	return sum;
+}
+
+void
+part1(const vector<string>& lines)
+{
+	const auto start = find_start_position(lines);
+	cout << count(lines, start, 64) << endl;
+}
+
+void
+part2(const vector<string>& lines)
+{
+	const auto start      = find_start_position(lines);
+	const auto [row, col] = start;
+
+	const auto pow2 = [](size_t val) -> size_t {
+		return val * val;
+	};
+
+	const auto   size       = lines.size();
+	const size_t steps      = 26501365;
+	const auto   grid_width = steps / size - 1;
+
+	const auto num_odd_tiles   = pow2(grid_width / 2 * 2 + 1);
+	const auto num_even_tiles  = pow2((grid_width + 1) / 2 * 2);
+	const auto num_odd_points  = count(lines, start, size * 2 + 1);
+	const auto num_even_points = count(lines, start, size * 2);
+
+	auto sum = num_odd_tiles * num_odd_points + num_even_tiles * num_even_points;
+
+	const auto corner_top    = count(lines, { size - 1, col }, size - 1);
+	const auto corner_right  = count(lines, { row, 0 }, size - 1);
+	const auto corner_bottom = count(lines, { 0, col }, size - 1);
+	const auto corner_left   = count(lines, { row, size - 1 }, size - 1);
+
+	sum += corner_top + corner_right + corner_bottom + corner_left;
+
+	const auto small_top_right    = count(lines, { size - 1, 0 }, size / 2 - 1);
+	const auto small_top_left     = count(lines, { size - 1, size - 1 }, size / 2 - 1);
+	const auto small_bottom_right = count(lines, { 0, 0 }, size / 2 - 1);
+	const auto small_bottom_left  = count(lines, { 0, size - 1 }, size / 2 - 1);
+
+	sum += (grid_width + 1) * (small_top_right + small_top_left + small_bottom_right + small_bottom_left);
+
+	const auto large_top_right    = count(lines, { size - 1, 0 }, size * 3 / 2 - 1);
+	const auto large_top_left     = count(lines, { size - 1, size - 1 }, size * 3 / 2 - 1);
+	const auto large_bottom_right = count(lines, { 0, 0 }, size * 3 / 2 - 1);
+	const auto large_bottom_left  = count(lines, { 0, size - 1 }, size * 3 / 2 - 1);
+
+	sum += grid_width * (large_top_right + large_top_left + large_bottom_right + large_bottom_left);
+
+	cout << sum << endl;
+}
+
+int
+main()
+{
+	auto lines = read_file("data/day21.txt");
+	part1(lines);
+	part2(lines);
+}
-- 
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