From 8c96639ec1f6757570510fc27f1c5fabece35eaf Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Sun, 16 Nov 2025 13:23:34 +0100
Subject: aoc 2018, days 1-11

---
 2018/src/day11.cpp | 143 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 143 insertions(+)
 create mode 100644 2018/src/day11.cpp

(limited to '2018/src/day11.cpp')

diff --git a/2018/src/day11.cpp b/2018/src/day11.cpp
new file mode 100644
index 0000000..1f8f9e2
--- /dev/null
+++ b/2018/src/day11.cpp
@@ -0,0 +1,143 @@
+#include <iostream>
+#include <map>
+
+using namespace std;
+
+namespace {
+
+const int WIDTH  = 300;
+const int HEIGHT = 300;
+
+using Grid = map<tuple<int, int>, int>;
+
+int
+power_level(int x, int y, int serial)
+{
+	auto rack_id = x + 10;
+	auto level   = rack_id * y;
+	level += serial;
+	level *= rack_id;
+	level /= 100;
+	level %= 10;
+	level -= 5;
+	return level;
+}
+
+Grid
+build_grid(int serial)
+{
+	Grid grid;
+	for ( int x = 1; x <= WIDTH; ++x ) {
+		for ( int y = 1; y <= HEIGHT; ++y ) {
+			grid[{ x, y }] = power_level(x, y, serial);
+		}
+	}
+	return grid;
+}
+
+int
+sum3x3(const Grid& grid, int x, int y)
+{
+	auto sum = 0;
+	for ( int dx = 0; dx != 3; ++dx ) {
+		for ( int dy = 0; dy != 3; ++dy ) {
+			sum += grid.at({ x + dx, y + dy });
+		}
+	}
+	return sum;
+}
+
+void
+part1(const int serial)
+{
+	auto grid = build_grid(serial);
+
+	auto best_sum = numeric_limits<int>::min();
+	auto best_x   = 0;
+	auto best_y   = 0;
+
+	for ( int x = 1; x <= WIDTH - 2; ++x ) {
+		for ( int y = 1; y <= HEIGHT - 2; ++y ) {
+			if ( auto sum = sum3x3(grid, x, y); sum > best_sum ) {
+				best_sum = sum;
+				best_x   = x;
+				best_y   = y;
+			}
+		}
+	}
+
+	cout << "Part 1: " << best_x << ',' << best_y << '\n';
+}
+
+Grid
+pre_process(const Grid& grid)
+{
+	Grid sat;
+
+	for ( int y = 1; y <= HEIGHT; ++y ) {
+		for ( int x = 1; x <= WIDTH; ++x ) {
+			const auto a = grid.at({ x, y });
+			const auto b = (x > 1) ? sat.at({ x - 1, y }) : 0;
+			const auto c = (y > 1) ? sat.at({ x, y - 1 }) : 0;
+			const auto d = (x > 1 && y > 1) ? sat.at({ x - 1, y - 1 }) : 0;
+
+			sat[{ x, y }] = a + b + c - d;
+		}
+	}
+
+	return sat;
+}
+
+int
+get_safe(const Grid& grid, int x, int y)
+{
+	return (x < 1 || y < 1) ? 0 : grid.at({ x, y });
+}
+
+int
+square_sum(const Grid& grid, int x, int y, int s)
+{
+	int x2 = x + s - 1;
+	int y2 = y + s - 1;
+
+	return get_safe(grid, x2, y2) - get_safe(grid, x - 1, y2) - get_safe(grid, x2, y - 1) + get_safe(grid, x - 1, y - 1);
+}
+
+void
+part2(const int serial)
+{
+	static const int MAX_SIZE = 100;
+
+	auto grid = pre_process(build_grid(serial));
+
+	auto best_sum = numeric_limits<int>::min();
+	auto best_x   = 0;
+	auto best_y   = 0;
+	auto best_s   = 0;
+
+	for ( int s = 1; s <= MAX_SIZE; ++s ) {
+		for ( int x = 1; x <= WIDTH - s + 1; ++x ) {
+			for ( int y = 1; y <= HEIGHT - s + 1; ++y ) {
+				if ( auto sum = square_sum(grid, x, y, s); sum > best_sum ) {
+					best_sum = sum;
+					best_x   = x;
+					best_y   = y;
+					best_s   = s;
+				}
+			}
+		}
+	}
+
+	cout << "Part 2: " << best_x << ',' << best_y << "," << best_s << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	static const int SERIAL = 7989;
+
+	part1(SERIAL);
+	part2(SERIAL);
+}
-- 
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