2024 day 18

2024/day18/day18.py

@@ -0,0 +1,81 @@
+from dataclasses import dataclass
+from heapq import heappush, heappop
+
+
+DIRECTIONS = (
+    1 + 0j,  # EAST
+    0 + 1j,  # SOUTH
+    -1 + 0j,  # WEST
+    0 - 1j,  # NORTH
+)
+
+
+@dataclass(frozen=True)
+class Node:
+    pos: complex
+    cost: int = 0
+
+    def __lt__(self, other):
+        return self.cost < other.cost
+
+
+def can_reach(pos, obstacles, grid_size):
+    height, width = grid_size
+    x, y = int(pos.real), int(pos.imag)
+    if 0 <= x < width and 0 <= y < height:
+        if (x, y) not in obstacles:
+            return True
+    return False
+
+
+def search(obstacles, start, goal, grid_size):  # could just use bfs?
+    queue = [Node(start)]
+    visited = set()
+    i = 0
+    while queue != []: 
+        node = heappop(queue)
+        visited.add(node.pos)
+        if node.pos == goal:
+            return node.cost
+        for direction in DIRECTIONS:
+            new_pos = node.pos + direction
+            n = Node(new_pos, node.cost + 1)
+            if n not in queue and n.pos not in visited and can_reach(n.pos, obstacles, grid_size):
+                heappush(queue, n)
+    return -1
+
+
+def find_path(coords, limit, grid_size):
+    obstacles = coords[:limit]
+    start = 0+0j
+    goal = complex(grid_size[0] - 1, grid_size[1] - 1)
+    cost = search(obstacles, start, goal, grid_size)
+    return cost
+
+
+def main(coords, limit, grid_size):
+    part1 = find_path(coords, limit, grid_size)
+    print("Part 1: ", part1)
+
+    # binary search for part 2
+    low, high = limit, len(coords)
+    while high - low > 1:
+        i = (low + high) // 2
+        if find_path(coords, i, grid_size) != -1: 
+            low = i
+        else:
+            high = i
+    print("Part 2: ", coords[low])
+
+
+if __name__ == "__main__":
+    import sys
+    infile = sys.argv[1] if len(sys.argv) > 1 else "example.txt"
+    with open(infile) as f:
+        lines = f.readlines()
+        lines = [tuple(map(int, l.rstrip().split(","))) for l in lines]
+        if infile == "example.txt":
+            main(lines, 12, (7, 7))
+        else:
+            main(lines, 1024, (71, 71))
+