Solve 2023 day 21

2023/21_steps.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+import sys
+from collections import deque
+
+def bfs(grid):
+    h = len(grid)
+    w = len(grid[0])
+    start = next((x, y) for y in range(h) for x in range(w) if grid[y][x] == 'S')
+    visited = {start: 0}
+    work = deque([(start, 0)])
+    while work:
+        (x, y), steps = work.popleft()
+        for nb in ((x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)):
+            nx, ny = nb
+            if 0 <= nx < w and 0 <= ny < h:
+                if grid[ny][nx] != '#' and nb not in visited:
+                    visited[nb] = steps + 1
+                    work.append((nb, steps + 1))
+    return visited
+
+def reachable_in_26501365_steps(min_dists):
+    # https://github.com/villuna/aoc23/wiki/A-Geometric-solution-to-advent-of-code-2023,-day-21
+    n = (26501365 - 131 // 2) // 131  # w = h = 131
+    even = odd = even_corners = odd_corners = 0
+    for dist in min_dists.values():
+        is_odd = dist % 2
+        even += 1 - is_odd
+        odd += is_odd
+        if dist > 65:
+            even_corners += 1 - is_odd
+            odd_corners += is_odd
+    whole_squares = (n + 1) ** 2 * odd + n * n * even
+    corners = (n + 1) * odd_corners + n * even_corners
+    return whole_squares - corners
+
+min_dists = bfs(sys.stdin.read().split())
+print(sum(dist % 2 == 0 and dist <= 64 for xy, dist in min_dists.items()))
+print(reachable_in_26501365_steps(min_dists))

2023/input/21

@@ -0,0 +1,131 @@
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