2025 day 4

adventofcode/2025/day4/day4.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+import fileinput
+from collections import deque
+
+DIRECTIONS = [
+    0 - 1j,  # NORTH
+    1 - 1j,  # NORTHEAST
+    1 + 0j,  # EAST
+    1 + 1j,  # SOUTHEAST
+    0 + 1j,  # SOUTH
+    -1 + 1j,  # SOUTHWEST
+    -1 + 0j,  # WEST
+    -1 - 1j,  # NORTHWEST
+]
+
+
+def parse_grid(lines):
+    grid = set()
+    for y, line in enumerate(lines):
+        for x, c in enumerate(line):
+            if c == "@":
+                grid.add(complex(x, y))
+    return grid
+
+
+def can_be_accessed(roll, grid):
+    """
+    The forklifts can only access a roll of paper if there are fewer than four
+    rolls of paper in the eight adjacent positions.
+    """
+    total = sum(1 for direction in DIRECTIONS if roll + direction in grid)
+    return total < 4
+
+
+def main(inp):
+    # Part 1
+    grid = parse_grid(inp)
+    total = 0
+    total = sum(1 for roll in grid if can_be_accessed(roll, grid))
+    print(f"Part 1: {total}")
+
+    # Part 2
+    total = 0
+    # use a queue to avoid rechecking all rolls each iteration
+    # we only check rolls that are adjacent to a roll that was removed
+    accessible_queue = deque(roll for roll in grid if can_be_accessed(roll, grid))
+    while accessible_queue:
+        roll = accessible_queue.popleft()
+        if roll not in grid:
+            continue  # roll was already removed
+
+        total += 1
+        grid.remove(roll)
+        # check if new rolls are accessible
+        for direction in DIRECTIONS:
+            neighbor = roll + direction
+            if neighbor in grid and can_be_accessed(neighbor, grid):
+                accessible_queue.append(neighbor)
+    print(f"Part 2: {total}")
+
+
+if __name__ == "__main__":
+    lines = [x.rstrip() for x in fileinput.input()]
+    main(lines)