1 files changed, 101 insertions, 0 deletions
diff --git a/2021/day09/day09.cpp b/2021/day09/day09.cpp
new file mode 100644
index 0000000..a034659
--- /dev/null
+++ b/2021/day09/day09.cpp
@@ -0,0 +1,101 @@
+#include <iostream>
+#include <fstream>
+#include <string>
+#include <vector>
+#include <algorithm>
+
+#include "../utils.h"
+
+int xytoi (int size, int x, int y) { return size*y + x; }
+int itox (int size, int i) { return i % size; }
+int itoy (int size, int i) { return i / size; }
+bool oob (int sizex, int sizey, int x, int y) {
+	if (x < 0 || y < 0) return true;
+	if (x >= sizex || y >= sizey) return true;
+	return false;
+}
+
+std::vector<std::pair<int, int>> get_dirs() {
+	std::vector<std::pair<int, int>> dirs {
+		{ 1, 0 }, { 0, 1 }, { -1, 0 }, { 0, -1 }
+	};
+	return dirs;
+}
+
+bool is_min (std::vector<int> grid, int size, int i) {
+	int x = itox(size, i);
+	int y = itoy(size, i);
+	for (auto dir : get_dirs()) {
+		int j = xytoi(size, dir.first+x, dir.second+y);
+		if (!oob(size, grid.size()/size, dir.first+x, dir.second+y))
+			if (grid[j] <= grid[i])	
+				return false;
+	}
+	return true;
+}
+
+std::vector<int> flood (std::vector<int> grid, int size, int i) {
+	std::vector<int> visit ;
+	std::vector<int> q { i };
+	while (q.size() > 0) {
+		int x = itox(size, q[0]);
+		int y = itoy(size, q[0]);
+		if (std::find(std::begin(visit), std::end(visit), q[0]) == std::end(visit)) 
+			visit.push_back(q[0]);
+		q.erase(std::begin(q));
+		for (auto dir : get_dirs()) {
+			int j = xytoi(size, dir.first+x, dir.second+y);
+			if (std::find(std::begin(visit), std::end(visit), j) != std::end(visit)) 
+				continue;
+			if (!oob(size, grid.size()/size, dir.first+x, dir.second+y)) {
+				if (grid[j] < 9) {
+					q.push_back(j);
+				}
+			}
+		}
+	}
+	return visit;
+}
+
+int main (int argc, char * argv[]) {
+	std::string raw;
+	std::getline(std::ifstream(argv[1]), raw, '\0');
+	std::vector<std::string> lines;
+	split(lines, raw, "\n");
+
+	int size = 0;
+	std::vector<int> grid;
+	for (std::string line : lines) {
+		if (line.size() == 0) continue;
+		size = 0;
+		for (char c : line) {
+			grid.push_back(std::stoi(std::string { c } ));
+			size++;
+		}
+	}
+
+	std::cout << "lowpoints: ";
+	std::vector<int> lows;
+	int sum = 0;
+	for (std::size_t i=0; i<grid.size(); i++) {
+		if (is_min(grid, size, i)) {
+			std::cout << "(" << itox(size, i) << " " << itoy(size, i) << "), ";
+			sum += grid[i]+1;
+			lows.push_back(i);
+		}
+	}
+
+	std::cout << std::endl;
+	std::cout << "sum of risk: " << sum << std::endl;
+
+	std::vector<int> isles;
+	for (auto low : lows) {
+		auto visit = flood(grid, size, low);
+		isles.push_back(visit.size());
+	}
+	std::sort(std::begin(isles), std::end(isles), std::greater<int>());
+
+	int res = isles[0] * isles[1] * isles[2];
+	std::cout << "three largest basins multiplied: " << res << std::endl;
+	return 0;
+}