Add solution to 2024-12-21

Author: fuglede

2024/day21/solutions.py

@@ -0,0 +1,69 @@
+from itertools import pairwise
+import networkx as nx
+from functools import cache
+
+with open("input") as f:
+    ls = f.read().strip().split("\n")
+
+
+numpad = {
+    0: "7",
+    1j: "8",
+    2j: "9",
+    1: "4",
+    1 + 1j: "5",
+    1 + 2j: "6",
+    2: "1",
+    2 + 1j: "2",
+    2 + 2j: "3",
+    3: None,
+    3 + 1j: "0",
+    3 + 2j: "A",
+}
+
+keypad = {
+    0: None,
+    1j: "^",
+    2j: "A",
+    1: "<",
+    1 + 1j: "v",
+    1 + 2j: ">",
+}
+
+fourdir = {1: "v", -1: "^", 1j: ">", -1j: "<"}
+
+
+def shortest_paths(grid):
+    G = nx.DiGraph((z, z + dz) for z in grid for dz in fourdir if grid.get(z + dz))
+
+    return {
+        (grid[start], grid[end]): [
+            "".join(fourdir[v - u] for u, v in pairwise(path)) for path in paths
+        ]
+        for start, ends in nx.all_pairs_all_shortest_paths(G)
+        for end, paths in ends.items()
+    }
+
+
+shortest_numpad = shortest_paths(numpad)
+shortest_keypad = shortest_paths(keypad)
+
+
+@cache
+def shortest_path_length(bot, keys, target_bot):
+    if bot == target_bot + 1:
+        return len(keys)
+    all_shortest = shortest_keypad if bot else shortest_numpad
+    return sum(
+        min(
+            shortest_path_length(bot + 1, path + "A", target_bot)
+            for path in all_shortest[start, end]
+        )
+        for start, end in pairwise("A" + keys)
+    )
+
+
+for target_bot in 2, 25:
+    print(
+        sum(int(code[:-1]) * shortest_path_length(0, code, target_bot) for code in ls)
+    )