From bd30ae97ed1d00618c5da74e15463d38043c1777 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Thu, 21 Dec 2023 10:53:48 +0100
Subject: [PATCH] 2023: d21: ex1: add solution

---
 2023/d21/ex1/ex1.py | 73 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100755 2023/d21/ex1/ex1.py

diff --git a/2023/d21/ex1/ex1.py b/2023/d21/ex1/ex1.py
new file mode 100755
index 0000000..d5bc5ed
--- /dev/null
+++ b/2023/d21/ex1/ex1.py
@@ -0,0 +1,73 @@
+#!/usr/bin/env python
+
+import sys
+from collections import defaultdict, deque
+from typing import NamedTuple, Optional
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+GardenPoints = set[Point]
+
+
+def solve(input: list[str]) -> int:
+    def parse(input: list[str]) -> tuple[GardenPoints, Point]:
+        start: Optional[Point] = None
+        points: GardenPoints = set()
+
+        for x, line in enumerate(input):
+            for y, c in enumerate(line):
+                if c == "#":
+                    continue
+                if c == "S":
+                    start = Point(x, y)
+                points.add(Point(x, y))
+
+        assert start is not None  # Sanity check
+        return points, start
+
+    def explore(points: GardenPoints, start: Point) -> dict[Point, int]:
+        res: dict[Point, int] = {}
+        queue: deque[tuple[Point, int]] = deque([(start, 0)])
+
+        while queue:
+            point, dist = queue.popleft()
+            # If we already saw the point, then we saw it at a smaller distance
+            if point in res:
+                continue
+            # If it's not an actual garden point (rocks, out-of-bounds), don't log it
+            if point not in points:
+                continue
+            res[point] = dist
+            for dx, dy in (
+                (-1, 0),
+                (1, 0),
+                (0, -1),
+                (0, 1),
+            ):
+                queue.append((Point(point.x + dx, point.y + dy), dist + 1))
+
+        return res
+
+    def reachable_in(distances: dict[Point, int], steps: int) -> set[Point]:
+        inverse_dist: dict[int, list[Point]] = defaultdict(list)
+        for p, dist in distances.items():
+            inverse_dist[dist].append(p)
+
+        return set(p for i in range(steps % 2, steps + 1, 2) for p in inverse_dist[i])
+
+    points, start = parse(input)
+    distances = explore(points, start)
+    return len(reachable_in(distances, 64))
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()