2024 day 12 part 1

2024/day12/day12.py

@@ -0,0 +1,83 @@
+from dataclasses import dataclass
+from collections import defaultdict
+
+
+@dataclass(frozen=True)
+class Vec2d:
+    x: int
+    y: int
+
+    def __add__(self, other): 
+        return Vec2d(self.x + other.x, self.y + other.y)
+
+
+DIRECTIONS = (
+    Vec2d(0, -1),   # N
+    Vec2d(1, 0),    # E
+    Vec2d(0, 1),    # S
+    Vec2d(-1, 0),   # W
+)
+
+def in_bounds(grid, pos):
+    return 0 <= pos.y < len(grid) and 0 <= pos.x < len(grid[0])
+
+
+def visit_region(grid, root):
+    region = set()
+    # BFS
+    queue = [root]
+    visited = set()
+    region.add(root)
+    visited.add(root)
+
+    while queue != []:
+        node = queue.pop(0)
+        for d in DIRECTIONS:
+            new_pos = node + d
+            if in_bounds(grid, new_pos) and new_pos not in visited:
+                visited.add(new_pos)
+                if grid[root.y][root.x] == grid[new_pos.y][new_pos.x]:
+                    queue.append(new_pos)
+                    region.add(new_pos)
+
+    return region
+ 
+
+def get_perimeter(grid, region):
+    perimeter = 0
+    for node in region:
+        for d in DIRECTIONS:
+            new_pos = node + d
+            if not in_bounds(grid, new_pos) or grid[new_pos.y][new_pos.x] != grid[node.y][node.x]:
+                perimeter += 1
+    return perimeter
+
+
+def main(grid):
+    # build list of regions using BFS
+    regions = []
+    visited = set()
+    for y, row in enumerate(grid):
+        for x, c in enumerate(row):
+            pos = Vec2d(x, y)
+            if pos not in visited:
+                region = visit_region(grid, pos)
+                visited.update(region)
+                regions.append((c, region))
+
+    total = 0
+    for region in regions:
+        c, nodes = region
+        area = len(nodes)
+        perimeter = get_perimeter(grid, nodes)
+        #print(f"Region {c} area {area} perimeter {perimeter}")
+        total += area * perimeter
+    print("Part 1: ", total)
+
+if __name__ == "__main__":
+    import sys
+    infile = sys.argv[1] if 1 < len(sys.argv) else "example.txt"
+    with open(infile) as f:
+        grid = [l.rstrip() for l in f.readlines()]
+        main(grid)
+