do day 4

input/2025-4.sample.txt

@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.

src/day4.cpp

@@ -0,0 +1,97 @@
+// :autocmd BufWritePost *.cpp :silent exec "!./build 4"
+#include <cstdint>
+#include <fstream>
+#include <iostream>
+#include <vector>
+#include <algorithm>
+
+// investigate nob.h and c macros, macroify derive(eq, hash, ...)
+struct Roll {
+  int64_t row;
+  int64_t col;
+};
+
+bool operator==(const Roll &lhs, const Roll &rhs) {
+  return lhs.row == rhs.row && lhs.col == rhs.col;
+}
+
+using Grid = std::vector<std::string>;
+using Rolls = std::vector<Roll>;
+
+int64_t part_1(const Grid &grid, const Rolls &rolls);
+int64_t part_2(Grid, std::vector<Roll> rolls);
+
+int main() {
+  std::ifstream file{"input/2025-4.txt"};
+  std::string line;
+  Roll roll_iter{};
+  std::vector<Roll> rolls;
+  Grid grid;
+  while (std::getline(file, line)) {
+    grid.push_back(line);
+    for (char c : line) {
+      if (c == '@') {
+        Roll local_copy = roll_iter;
+        rolls.push_back(local_copy);
+      }
+      roll_iter.col += 1;
+    }
+    roll_iter.row += 1;
+    roll_iter.col = 0;
+  }
+  std::cout << "Part 1: " << part_1(grid, rolls) << std::endl;
+  std::cout << "Part 2: " << part_2(grid, rolls) << std::endl;
+}
+
+void is_roll_helper(size_t row, size_t col, const Grid &grid, short &ans) {
+  if (row >= 0 && row < grid.size() && col >= 0 && col < grid[row].size() &&
+      grid[row][col] == '@') {
+    ans += 1;
+  }
+}
+
+short num_neighbors(const Grid &grid, const Roll &roll) {
+  short ans{0};
+  is_roll_helper(roll.row - 1, roll.col - 1, grid, ans);
+  is_roll_helper(roll.row - 1, roll.col, grid, ans);
+  is_roll_helper(roll.row - 1, roll.col + 1, grid, ans);
+  is_roll_helper(roll.row, roll.col - 1, grid, ans);
+  is_roll_helper(roll.row, roll.col + 1, grid, ans);
+  is_roll_helper(roll.row + 1, roll.col - 1, grid, ans);
+  is_roll_helper(roll.row + 1, roll.col, grid, ans);
+  is_roll_helper(roll.row + 1, roll.col + 1, grid, ans);
+  return ans;
+}
+
+int64_t part_1(const Grid &grid, const Rolls &rolls) {
+  int64_t ans{0};
+  for (auto roll : rolls) {
+    auto cnt{num_neighbors(grid, roll)};
+    if (cnt < 4) {
+      ans += 1;
+    }
+  }
+  return ans;
+}
+
+int64_t part_2(Grid grid, Rolls rolls) {
+  int64_t last_roll_cnt = INT64_MAX;
+  int64_t ans{0};
+  Rolls to_delete{};
+  do {
+    to_delete.clear();
+    for (auto roll : rolls) {
+      auto cnt{num_neighbors(grid, roll)};
+      if (cnt < 4) {
+        ans += 1;
+        to_delete.emplace_back(roll);
+      }
+    }
+
+    for (auto roll : to_delete) {
+      std::erase(rolls, roll);
+      grid[roll.row][roll.col] = '.';
+    }
+  } while (to_delete.size() != 0);
+  return ans;
+}