AoC 2024 day 12 part 2

aoc2024/src/day12/python/solution.py

@@ -17,28 +17,51 @@
 type Pos = tuple[int, int]  # (y,x)
 
 
+def _update_perimeter(perimeters: dict[str, list[int]], pos: Pos, direction: tuple[int, int]):
+  """Updates perimeter data."""
+  if direction in ((0, 1), (0, -1)):  # Left/right
+    perimeters[f'{direction}:{pos[1]}'].append(pos[0])
+  else:  # Up/down
+    perimeters[f'{direction}:{pos[0]}'].append(pos[1])
+
+
 def _dfs_helper(pos: Pos, flower: str, garden_map: Sequence[str],
-                visited: set[Pos], area: list[int], perimeter: list[int]):
+                visited: set[Pos], area: list[int], perimeters: dict[str, list[int]],
+                direction: tuple[int, int] | None = None):
+  """Traverses the garden with DSF, tracking area and perimeter data."""
   if (pos[0] < 0 or pos[0] > len(garden_map) - 1
           or pos[1] < 0 or pos[1] > len(garden_map[0]) - 1):
     # Out of bounds.
-    perimeter[0] += 1
+    _update_perimeter(perimeters, pos, direction)
     visited.add(pos)
     return
   if garden_map[pos[0]][pos[1]] != flower:
     # Facing another region.
-    perimeter[0] += 1
+    _update_perimeter(perimeters, pos, direction)
     return
   # Same region.
   area[0] += 1
   visited.add(pos)
   for direction in ((-1, 0), (0, 1), (1, 0), (0, -1)):
     next_pos = (pos[0] + direction[0], pos[1] + direction[1])
     if next_pos not in visited:
-      _dfs_helper(next_pos, flower, garden_map, visited, area, perimeter)
+      _dfs_helper(next_pos, flower, garden_map, visited, area, perimeters, direction)
+
+
+def _calculate_sides(perimeters: dict[str, list[int]]) -> int:
+  """Calculates number of sides given perimeter data."""
+  sides = 0
+  for coords in perimeters.values():
+    sides += 1  # Each group of coordinates represents at least one side.
+    sorted_coords = sorted(coords)
+    for i in range(1, len(sorted_coords)):
+      if sorted_coords[i] > sorted_coords[i - 1] + 1:
+        # Coordinates are disjointed, hence 2 different sides.
+        sides += 1
+  return sides
 
 
-def calculate_fencing_price(garden_map: Sequence[str]) -> int:
+def calculate_fencing_price(garden_map: Sequence[str], price_by_side=False) -> int:
   """Calculates the price to fence all regions.
   A region price is calculated as its area multiplied by its perimeter."""
   visited: dict[str, set[Pos]] = defaultdict(set)  # Tracks region positions.
@@ -48,7 +71,14 @@ def calculate_fencing_price(garden_map: Sequence[str]) -> int:
       flower = garden_map[y][x]
       pos = (y, x)
       if pos not in visited[flower]:
-        area, perimeter = [0], [0]  # Pass by reference
-        _dfs_helper(pos, garden_map[y][x], garden_map, visited[flower], area, perimeter)
-        total_price += area[0] * perimeter[0]
+        area = [0]  # Pass by reference
+        # Track perimeter data. Key by direction at x or y coordinate:
+        # UP/DOWN at x coordinate -> list of y coordinates
+        # LEFT/RIGHT at y coordinate -> list of x coordinates
+        perimeters: dict[str, list[int]] = defaultdict(list)
+        _dfs_helper(pos, garden_map[y][x], garden_map, visited[flower], area, perimeters)
+        if price_by_side:
+          total_price += area[0] * _calculate_sides(perimeters)
+        else:
+          total_price += area[0] * sum([len(values) for values in perimeters.values()])
   return total_price

aoc2024/test/day12/python/example4.txt

@@ -0,0 +1,5 @@
+EEEEE
+EXXXX
+EEEEE
+EXXXX
+EEEEE

aoc2024/test/day12/python/example5.txt

@@ -0,0 +1,6 @@
+AAAAAA
+AAABBA
+AAABBA
+ABBAAA
+ABBAAA
+AAAAAA

aoc2024/test/day12/python/test_solution.py

@@ -34,6 +34,24 @@ def test_part1_withLargerExample_calculates(self):
   def test_part1_withPuzzleInput_calculates(self):
     self.assertEqual(1465112, calculate_fencing_price(self.input))
 
+  def test_part2_with4Areas_calculates(self):
+    self.assertEqual(80, calculate_fencing_price(self.examples[0], price_by_side=True))
+
+  def test_part2_withAreaHasHoles_calculates(self):
+    self.assertEqual(436, calculate_fencing_price(self.examples[1], price_by_side=True))
+
+  def test_part2_withEShape_calculates(self):
+    self.assertEqual(236, calculate_fencing_price(self.examples[3], price_by_side=True))
+
+  def test_part2_withAbba_calculates(self):
+    self.assertEqual(368, calculate_fencing_price(self.examples[4], price_by_side=True))
+
+  def test_part2_withLargerExample_calculates(self):
+    self.assertEqual(1206, calculate_fencing_price(self.examples[2], price_by_side=True))
+
+  def test_part2_withPuzzleInput_calculates(self):
+    self.assertEqual(893790, calculate_fencing_price(self.input, price_by_side=True))
+
 
 if __name__ == '__main__':
   unittest.main()