From f4edfaa6802cdec39bc48f3b3e2c01b7564196be Mon Sep 17 00:00:00 2001
From: felixm <mail@felixm.de>
Date: Fri, 20 Dec 2024 01:54:25 -0500
Subject: [PATCH] Solve 2024 day 20

---
 2024/d20.py | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 README.md   |  2 +-
 2 files changed, 91 insertions(+), 1 deletion(-)
 create mode 100644 2024/d20.py

diff --git a/2024/d20.py b/2024/d20.py
new file mode 100644
index 0000000..f5f746a
--- /dev/null
+++ b/2024/d20.py
@@ -0,0 +1,90 @@
+from collections import deque
+from lib import get_data
+from lib import Grid2D
+
+
+data = """###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############
+"""
+data = get_data(__file__)
+
+g = Grid2D(data)
+
+(start,) = g.find("S")
+(end,) = g.find("E")
+
+cost = 0
+sd = dict()
+de = dict()
+
+queue = deque([(start[0], start[1], 0)])
+seen = set()
+while queue:
+    r, c, cost = queue.popleft()
+
+    if (r, c) in sd:
+        continue
+    sd[(r, c)] = cost
+
+    if (r, c) == end:
+        break
+
+    for r, c in g.neighbors_ort((r, c)):
+        if g[(r, c)] != "#":
+            queue.append((r, c, cost + 1))
+
+seen = set()
+queue = deque([(end[0], end[1], 0)])
+while queue:
+    r, c, cost = queue.popleft()
+
+    if (r, c) in de:
+        continue
+    de[(r, c)] = cost
+
+    if (r, c) == start:
+        break
+
+    for r, c in g.neighbors_ort((r, c)):
+        if g[(r, c)] != "#":
+            queue.append((r, c, cost + 1))
+
+for max_steps in [2, 20]:
+    cheats = set()
+    for cr, cc in g.find("S."):
+        start = cr, cc, 0
+        seen = set()
+        queue = deque([start])
+        while queue:
+            r, c, steps = queue.popleft()
+            if steps > max_steps:
+                continue
+
+            if (r, c) in seen:
+                continue
+            seen.add((r, c))
+
+            if g[(r, c)] != "#":
+                ncost = sd[(cr, cc)] + steps + de[(r, c)]
+                if cost - ncost >= 100:
+                    cheats.add((cr, cc, r, c))
+
+            for dr, dc in [(-1, 0), (0, 1), (1, 0), (0, -1)]:
+                nr, nc = r + dr, c + dc
+                if g.contains((nr, nc)):
+                    queue.append((nr, nc, steps + 1))
+
+    print(len(cheats))
diff --git a/README.md b/README.md
index e9cacdc..0a9ca82 100644
--- a/README.md
+++ b/README.md
@@ -310,5 +310,5 @@ and focus. Of course, learning more algorithms and techniques helps.
 - Day 17: `17:34:16 23722      0       >24h  17778      0`
 - Day 18: `14:35:01 20398      0   14:37:18  19550      0`
 - Day 19: `00:14:37  2001      0   00:19:43   1584      0`
-- Day 20:
+- Day 20: `01:08:53   3637     0   01:53:01   2837      0`