1 files changed, 154 insertions, 0 deletions
diff --git a/2021/day9/09.cpp b/2021/day9/09.cpp
new file mode 100644
index 0000000..5026a77
--- /dev/null
+++ b/2021/day9/09.cpp
@@ -0,0 +1,154 @@
+#include <iostream>
+#include <fstream>
+#include <string>
+#include <cerrno>
+#include <vector>
+#include <set>
+#include <queue>
+#include <algorithm>
+
+using namespace std;
+
+std::string get_file_contents(const char *filename)
+{
+  std::ifstream in(filename, std::ios::in | std::ios::binary);
+  if (!in)
+    {
+      throw(errno);
+    }
+
+  std::string contents;
+  in.seekg(0, std::ios::end);
+  contents.resize(in.tellg());
+  in.seekg(0, std::ios::beg);
+
+  in.read(&contents[0], contents.size());
+  in.close();
+  return contents;
+}
+
+using heightmap_t = vector<vector<int>>;
+
+heightmap_t parse_height_map(const char* input, int len){
+  vector<vector<int>> output;
+  vector<int> currRow;
+  for (int i = 0; i < len; i++) {
+    if (input[i] == '\n') {
+      output.push_back(currRow);
+      currRow = vector<int>();
+    } else {
+      currRow.push_back(input[i] - '0');
+    }
+  }
+  if (currRow.size() > 0) {
+    output.push_back(currRow);
+  }
+
+  return output;
+}
+
+bool isLowPoint(const heightmap_t &heightmap, int x, int y) {
+  int point = heightmap[y][x];
+  for (int dx = -1; dx < 2; dx++) {
+    for (int dy = -1; dy < 2; dy++) {
+      int checkX = x + dx;
+      int checkY = y + dy;
+      if ((checkX == x && checkY == y) ||
+          checkX < 0 || checkY < 0 ||
+          checkY >= heightmap.size() || checkX >= heightmap[checkY].size())
+        continue;
+      if (heightmap[checkY][checkX] <= point) {
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
+struct coord {
+  int x;
+  int y;
+
+
+  coord(int x, int y) {
+    this->x = x;
+    this->y = y;
+  }
+
+  int hash() const {
+    return this->x << 16 | this->y;
+  }
+
+  bool operator==(const coord &a) const {
+    return a.x == x && a.y == y;
+  }
+
+  bool operator<(const coord &a) const {
+    return this->hash() < a.hash();
+  }
+};
+
+int findBasinSize(const heightmap_t &heightmap, int startX, int startY) {
+  set<coord> visited;
+  queue<coord> queue;
+  queue.emplace(startX, startY);
+
+  int totalSize = 0;
+  while(!queue.empty()) {
+    int x = queue.front().x;
+    int y = queue.front().y;
+    queue.pop();
+    if (x < 0 || y < 0 || y >= heightmap.size() || x >= heightmap[y].size() || heightmap[y][x] == 9 || visited.count({x, y})) {
+      continue;
+    }
+
+    totalSize++;
+    visited.insert({x, y});
+
+    for (int dx = -1; dx < 2; dx++) {
+      for (int dy = -1; dy < 2; dy++) {
+        int checkX = x + dx;
+        int checkY = y + dy;
+        if (dx != 0 && dy != 0) // dont allow diagonals / {x, y}
+          continue;
+        queue.emplace(checkX, checkY);
+      }
+    }
+  }
+
+  return totalSize;
+}
+
+int main() {
+  std::string input = get_file_contents("input");
+  auto heightmap = parse_height_map(input.c_str(), input.length());
+
+  int lowPointCount = 0;
+  int totalRisk = 0;
+  int topBasins[] = {0, 0, 0};
+  for (int y = 0; y < heightmap.size(); y++) {
+    for (int x = 0; x < heightmap[y].size(); x++) {
+      if (isLowPoint(heightmap, x, y)) {
+        lowPointCount++;
+        totalRisk += heightmap[y][x] + 1;
+
+        int basinSize = findBasinSize(heightmap, x, y);
+        for (int i = 0; i < 3; i++) {
+          if (topBasins[i] < basinSize) {
+            topBasins[i] = basinSize;
+            break;
+          }
+        }
+        sort(topBasins, topBasins + 3);
+      }
+    }
+  }
+
+  int acc = 1;
+  for (int i = 0; i < 3; i++) {
+    acc *= topBasins[i];
+  }
+  printf("Total low points: %d\n", lowPointCount);
+  printf("Total risk: %d\n", totalRisk);
+  printf("Part B answer: %d\n", acc);
+}