From 2d196f9c5234560889d8972152f1ebdb60ce6dad Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Fri, 20 Dec 2024 01:39:40 -0500
Subject: [PATCH] 2024: d20: ex2: add solution

---
 2024/d20/ex2/ex2.py | 107 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100755 2024/d20/ex2/ex2.py

diff --git a/2024/d20/ex2/ex2.py b/2024/d20/ex2/ex2.py
new file mode 100755
index 0000000..614580a
--- /dev/null
+++ b/2024/d20/ex2/ex2.py
@@ -0,0 +1,107 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from collections.abc import Iterator
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+    def neighbours(self) -> Iterator["Point"]:
+        for dx, dy in (
+            (-1, 0),
+            (1, 0),
+            (0, -1),
+            (0, 1),
+        ):
+            yield Point(self.x + dx, self.y + dy)
+
+
+class ParsedMap(NamedTuple):
+    start: Point
+    end: Point
+    tracks: set[Point]
+
+
+MIN_SAVE = 100
+
+
+def solve(input: str) -> int:
+    def parse(input: list[str]) -> ParsedMap:
+        start: Point | None = None
+        end: Point | None = None
+        tracks: set[Point] = set()
+
+        for x, line in enumerate(input):
+            for y, c in enumerate(line):
+                if c == "#":
+                    continue
+                p = Point(x, y)
+                if c == "S":
+                    start = p
+                elif c == "E":
+                    end = p
+                tracks.add(p)
+
+        assert start is not None and end is not None  # Sanity check
+        return ParsedMap(start, end, tracks)
+
+    def flood_distance(start: Point, points: set[Point]) -> dict[Point, int]:
+        res = {start: 0}
+        queue = {start}
+
+        while queue:
+            p = queue.pop()
+            dist = res[p]
+            for n in p.neighbours():
+                if n in res:
+                    continue
+                if n not in points:
+                    continue
+                res[n] = dist + 1
+                queue.add(n)
+
+        return res
+
+    def dist(a: Point, b: Point) -> int:
+        return abs(a.x - b.x) + abs(a.y - b.y)
+
+    def disk(p: Point, radius: int) -> Iterator[Point]:
+        for dx, dy in itertools.product(range(-radius, radius + 1), repeat=2):
+            n = Point(p.x + dx, p.y + dy)
+            if dist(p, n) > radius:
+                continue
+            yield n
+
+    def find_cheats(start: Point, end: Point, tracks: set[Point]) -> int:
+        start_dist = flood_distance(start, tracks)
+        end_dist = flood_distance(end, tracks)
+
+        assert start_dist[end] == end_dist[start]
+        fastest = start_dist[end]
+
+        res = 0
+        for a in tracks:
+            for b in disk(a, 20):
+                if b not in tracks:
+                    continue
+                time = start_dist[a] + dist(a, b) + end_dist[b]
+                if (fastest - time) < MIN_SAVE:
+                    continue
+                res += 1
+        return res
+
+    start, end, tracks = parse(input.splitlines())
+    return find_cheats(start, end, tracks)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()