AoC 2024 day 18 part 2

aoc2024/src/day18/python/solution.py

@@ -13,16 +13,35 @@
 # limitations under the License.
 from typing import Sequence
 from common.python3.graph_utils import shortest_distance_bfs
+from common.python3.types import Position
 
 _CORRUPTED = '#'
 _NOT_CORRUPTED = '.'
 
 
+def _corrupt_grid(grid: list[list[str]], corrupted_positions: tuple[tuple[int, ...], ...]) -> None:
+  for i in range(len(corrupted_positions)):
+    pos = corrupted_positions[i]
+    grid[pos[1]][pos[0]] = _CORRUPTED
+
+
 def min_steps_to_exit(input: Sequence[str], num_bytes: int = 1024,
                       width: int = 71, length: int = 71) -> int:
   corrupted_positions = tuple(map(lambda entry: tuple(map(int, entry.split(','))), input))
   grid = [[_NOT_CORRUPTED for _ in range(width)] for _ in range(length)]
-  for i in range(num_bytes):
-    pos = corrupted_positions[i]
-    grid[pos[1]][pos[0]] = _CORRUPTED
+  _corrupt_grid(grid, corrupted_positions[:num_bytes])
   return shortest_distance_bfs(grid=grid, predicate=lambda x, y: grid[y][x] != _CORRUPTED)
+
+
+def find_first_byte_preventing_exit(input: Sequence[str],
+                                    width: int = 71, length: int = 71) -> int:
+  corrupted_positions = tuple(map(lambda entry: tuple(map(int, entry.split(','))), input))
+  for i in range(len(corrupted_positions)):
+    try:
+      grid = [[_NOT_CORRUPTED for _ in range(width)] for _ in range(length)]
+      _corrupt_grid(grid, corrupted_positions[:i + 1])
+      _ = shortest_distance_bfs(grid=grid, predicate=lambda x, y: grid[y][x] != _CORRUPTED)
+    except ValueError:
+      # Found the byte that prevents the exit.
+      return corrupted_positions[i]
+  raise ValueError('There is no byte that prevents the exit.')

aoc2024/test/day18/python/test_solution.py

@@ -14,7 +14,7 @@
 import unittest
 
 from common.python3.AdventOfCodeTestCase import AdventOfCodeTestCase
-from aoc2024.src.day18.python.solution import min_steps_to_exit
+from aoc2024.src.day18.python.solution import min_steps_to_exit, find_first_byte_preventing_exit
 
 
 class TestSolution(AdventOfCodeTestCase):
@@ -28,6 +28,12 @@ def test_part1_withExample_success(self):
   def test_part1_withPuzzleInput_success(self):
     self.assertEqual(234, min_steps_to_exit(self.input, num_bytes=1024))
 
+  def test_part2_withExample_success(self):
+    self.assertEqual((6, 1), find_first_byte_preventing_exit(self.examples[0], width=7, length=7))
+
+  def test_part2_withPuzzleInput_success(self):
+    self.assertEqual((58, 19), find_first_byte_preventing_exit(self.input))
+
 
 if __name__ == '__main__':
   unittest.main()

common/python3/graph_utils.py

@@ -30,3 +30,4 @@ def shortest_distance_bfs(grid: list[list[str]], visited: set[Position] = None,
               and predicate(next_pos[0], next_pos[1])):
         visited.add(next_pos)
         queue.append((next_pos, level + 1))
+  raise ValueError('Could not reach the exit.')