2025 day 4: solve part 2

Author: yut23

2025/answer_tests/day04/example1.txt

@@ -1,2 +1,3 @@
 Day 4:
 13
+43

2025/src/day04.cpp

@@ -15,8 +15,16 @@ int main(int argc, char **argv) {
 
     auto grid = aoc::day04::read_input(infile);
 
-    auto part1 = aoc::day04::count_forklift_accessible(grid);
+    int part1 = aoc::day04::count_forklift_accessible<aoc::PART_1>(grid);
     std::cout << part1 << "\n";
 
+    int part2 = 0;
+    int removed;
+    do {
+        removed = aoc::day04::count_forklift_accessible<aoc::PART_2>(grid);
+        part2 += removed;
+    } while (removed > 0);
+    std::cout << part2 << "\n";
+
     return 0;
 }

2025/src/day04.hpp

@@ -17,9 +17,10 @@ namespace aoc::day04 {
 
 using aoc::ds::Grid;
 
-int count_forklift_accessible(const Grid<char> &grid) {
+template <aoc::Part PART>
+int count_forklift_accessible(Grid<char> &grid) {
     int count = 0;
-    grid.for_each([&count, &grid](char c, const Pos &center) {
+    grid.for_each([&count, &grid](char &c, const Pos &center) {
         if (c != '@') {
             return;
         }
@@ -32,6 +33,9 @@ int count_forklift_accessible(const Grid<char> &grid) {
         // the 4 in the question doesn't include the center roll
         if (adj_count < 5) {
             ++count;
+            if constexpr (PART == aoc::PART_2) {
+                c = '.';
+            }
         }
     });
     return count;

aoc_lib/src/ds/grid.hpp

@@ -339,6 +339,26 @@ struct Grid {
         }
     }
 
+    /**
+     * Calls func(value[, pos]) for each position in the grid. Allows in-place
+     * modification of the value.
+     */
+    template <typename Func>
+        requires std::invocable<Func, value_type &, const Pos &> ||
+                 std::invocable<Func, value_type &>
+    constexpr void for_each(Func &&func) {
+        Pos p;
+        for (p.y = 0; p.y < height; ++p.y) {
+            for (p.x = 0; p.x < width; ++p.x) {
+                if constexpr (std::invocable<Func, value_type &, const Pos &>) {
+                    func((*this)[p], p);
+                } else {
+                    func((*this)[p]);
+                }
+            }
+        }
+    }
+
     /**
      * Calls formatter(value[, pos]) for each position in the grid in row-major
      * order, and outputs a newline at the end of each row.