From bdd35a7bee7f23c912d0c453abc263805d8d8ccf Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Fri, 7 Nov 2025 12:04:46 +0100
Subject: aoc 2017, days 21-25

---
 2017/src/day21.cpp | 186 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 186 insertions(+)
 create mode 100644 2017/src/day21.cpp

(limited to '2017/src/day21.cpp')

diff --git a/2017/src/day21.cpp b/2017/src/day21.cpp
new file mode 100644
index 0000000..64af5b6
--- /dev/null
+++ b/2017/src/day21.cpp
@@ -0,0 +1,186 @@
+#include <algorithm>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+vector<string>
+split(const string& line, const string& delimiters)
+{
+	vector<string> result;
+
+	size_t start = 0;
+	size_t end   = 0;
+
+	while ( (end = line.find_first_of(delimiters, start)) != string::npos ) {
+		if ( end != start ) {
+			result.emplace_back(line.substr(start, end - start));
+		}
+
+		start = end + 1;
+	}
+
+	if ( start != line.size() ) {
+		result.emplace_back(line.substr(start));
+	}
+
+	return result;
+}
+
+vector<string>
+rotate90(const vector<string>& input)
+{
+	if ( input.empty() ) {
+		return {};
+	}
+
+	const auto rows = input.size();
+	const auto cols = input[0].size();
+
+	vector<string> rotated(cols, string(rows, ' '));
+
+	for ( size_t row = 0; row != rows; ++row ) {
+		for ( size_t col = 0; col != cols; ++col ) {
+			rotated[col][rows - 1 - row] = input[row][col];
+		}
+	}
+
+	return rotated;
+}
+
+vector<string>
+flip(const vector<string>& input)
+{
+	vector<string> result = input;
+
+	// oder: ranges::reverse(result);
+	for ( auto& row: result ) {
+		ranges::reverse(row);
+	}
+
+	return result;
+}
+
+vector<vector<string>>
+generate_variants(vector<string> pattern)
+{
+	vector<vector<string>> variants;
+
+	for ( int flip_count = 0; flip_count < 2; ++flip_count ) {
+		for ( int round = 0; round < 4; ++round ) {
+			variants.push_back(pattern);
+			pattern = rotate90(pattern);
+		}
+		pattern = flip(pattern);
+	}
+
+	return variants;
+}
+
+map<vector<string>, vector<string>>
+read_file(const filesystem::path& filename)
+{
+	ifstream                            file{ filename };
+	map<vector<string>, vector<string>> data;
+
+	for ( string line; getline(file, line); ) {
+		auto parts = split(line, " ");
+		auto lhs   = split(parts.at(0), "/");
+		auto rhs   = split(parts.at(2), "/");
+
+		for ( const auto& variant: generate_variants(lhs) ) {
+			data[variant] = rhs;
+		}
+	}
+
+	return data;
+}
+
+vector<string>
+enhance(const vector<string>& image, const map<vector<string>, vector<string>>& rules)
+{
+	const auto size           = image.size();
+	const auto block_size     = (size % 2 == 0) ? 2U : 3U;
+	const auto new_block_size = block_size + 1;
+	const auto blocks_per_row = size / block_size;
+	const auto new_size       = blocks_per_row * new_block_size;
+
+	vector<string> new_image(new_size, string(new_size, '.'));
+
+	// Für jeden Block...
+	for ( size_t block_row = 0; block_row != blocks_per_row; ++block_row ) {
+		for ( size_t block_col = 0; block_col != blocks_per_row; ++block_col ) {
+			// ...Extrahiere den Block als vector<string>
+			vector<string> block;
+			block.reserve(block_size);
+			for ( size_t row = 0; row != block_size; ++row ) {
+				block.push_back(image[(block_row * block_size) + row].substr(block_col * block_size, block_size));
+			}
+
+			const auto& new_block = rules.at(block);
+
+			// Füge neuen Block ins Bild ein
+			for ( size_t row = 0; row != new_block_size; ++row ) {
+				for ( size_t col = 0; col != new_block_size; ++col ) {
+					new_image[(block_row * new_block_size) + row][(block_col * new_block_size) + col] = new_block[row][col];
+				}
+			}
+		}
+	}
+
+	return new_image;
+}
+
+int
+count(const vector<string>& image)
+{
+	int count = 0;
+	for ( const auto& row: image ) {
+		for ( char chr: row ) {
+			if ( chr == '#' ) {
+				++count;
+			}
+		}
+	}
+	return count;
+}
+
+int
+solve(const map<vector<string>, vector<string>>& rules, size_t iterations)
+{
+	vector<string> image = { ".#.", "..#", "###" };
+
+	while ( iterations-- > 0 ) {
+		image = enhance(image, rules);
+	}
+
+	return count(image);
+}
+
+void
+part1(const map<vector<string>, vector<string>>& rules)
+{
+	cout << "Part1: " << solve(rules, 5) << '\n';
+}
+
+void
+part2(const map<vector<string>, vector<string>>& rules)
+{
+	cout << "Part2: " << solve(rules, 18) << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto rules = read_file("data/day21.txt");
+	part1(rules);
+	part2(rules);
+}
-- 
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