Day 16

day16.py

@@ -0,0 +1,154 @@
+#!/usr/bin/env python3
+import math
+from enum import Enum, auto
+from typing import Tuple, Dict, Set
+from copy import copy
+
+
+class Tile(Enum):
+    WALL = auto()
+    PATH = auto()
+
+class Direction(Enum):
+    NORTH = auto()
+    EAST = auto()
+    SOUTH = auto()
+    WEST = auto()
+
+type Pos = Tuple[int,int]
+
+maze = []
+end: Pos = (0,0)
+player: Pos = (0,0)
+
+with open("day16") as f:
+    for y,l in enumerate(f):
+        temp = []
+        for x,c in enumerate(l):
+            if c == '#':
+                temp.append(Tile.WALL)
+            else:
+                temp.append(Tile.PATH)
+
+            if c == 'E':
+                end = (y,x)
+
+            if c == 'S':
+                player = (y,x)
+
+        maze.append(temp)
+
+
+def next_moves(pos, direction, cost):
+    moves = []
+
+    for d in Direction:
+        if((d == Direction.NORTH and direction == Direction.SOUTH)
+           or (d == Direction.SOUTH and direction == Direction.NORTH)
+           or (d == Direction.WEST and direction == Direction.EAST)
+           or (d == Direction.EAST and direction == Direction.WEST)):
+            continue # only 90°
+
+        p = pos
+        if d == Direction.NORTH:
+            p = (p[0]-1, p[1])
+        if d == Direction.EAST:
+            p = (p[0], p[1]+1)
+        if d == Direction.SOUTH:
+            p = (p[0]+1, p[1])
+        if d == Direction.WEST:
+            p = (p[0], p[1]-1)
+
+        # can't move into wall
+        if maze[p[0]][p[1]] == Tile.WALL: continue
+
+        # because I'm a good, defensive programmer lol
+        if p[0] < 0 or p[1] < 0: raise RuntimeError("Crossed a wall")
+        if p[0] >= len(maze) or p[1] >= len(maze[0]): raise RuntimeError("Crossed a wall")
+
+
+        c = cost
+        if d == direction: c += 1
+        else: c += 1000+1
+
+        moves.append((p,d,c))
+
+    return moves
+
+visited: Dict[Pos, Set[Tuple[int, Direction]]] = {}
+queue = [(player, Direction.EAST, 0)]
+hits = []
+cheap_map = [[math.inf for _ in range(len(maze[0]))] for _ in range(len(maze))]
+
+while queue:
+    w = queue.pop()
+    if w[0] in visited:
+        v = visited[w[0]]
+        v0 = next(iter(visited[w[0]])) # just get any element, for cost doesn't matter
+        # this is not the cheapest path, prune search here
+        if v0[0] < w[2]: continue
+        # this is the cheapest path so far, replace all others found so far
+        if v0[0] > w[2]: visited[w[0]] = {(w[2], w[1])}
+        # another possibility, equally good as the others, but potentially different direction
+        else: visited[w[0]].add((w[2], w[1]))
+    else:
+        visited[w[0]] = {(w[2], w[1])}
+
+    if w[2] < cheap_map[w[0][0]][w[0][1]]:
+        cheap_map[w[0][0]][w[0][1]] = w[2]
+
+    if w[0] == end:
+        hits.append(w[2])
+
+    queue.extend(next_moves(w[0], w[1], w[2]))
+
+print(min(hits))
+
+tiles = set()
+queue = [(player, Direction.EAST, 0, [player])]
+while True:
+    if not queue: break
+
+    w = queue.pop()
+
+    if w[0] == end and w[2] == min(hits):
+        tiles.update(w[3])
+        continue
+
+    tainted = False
+    # we might cross a path that is cheaper at this point
+    # but which has to take a turn while we don't
+    if cheap_map[w[0][0]][w[0][1]] < w[2]:
+        tainted = True
+
+    nms = next_moves(w[0], w[1], w[2])
+
+    for nm in nms:
+        # if we're tainted we are either back
+        # to cheapest path here or we won't come back
+        if tainted and cheap_map[nm[0][0]][nm[0][1]] < nm[2]:
+            continue
+        l = copy(w[3]); l.append(nm[0]);
+        queue.append((nm[0], nm[1], nm[2], l))
+
+print(len(tiles))
+
+def pprint():
+    for i,y in enumerate(maze):
+        for j,x in enumerate(y):
+            if (i,j) in tiles:
+                print("O", sep=" ", end="")
+            elif x == Tile.WALL:
+                print("#", sep=" ", end = "")
+            elif x == Tile.PATH:
+                print(".", sep=" ", end = "")
+        print()
+
+def pprint_cheap():
+    for y in cheap_map:
+        for x in y:
+            if x == math.inf:
+                print("#"*6, sep=" ", end = "")
+            else:
+                print(f" {x:<5}", sep=" ", end="")
+        print()