From 5ebf620992629982ea97f3351b5ae1a40170f160 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Sun, 24 Dec 2023 13:35:15 +0100
Subject: [PATCH] 2023: d24: ex1: add solution

---
 2023/d24/ex1/ex1.py | 78 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100755 2023/d24/ex1/ex1.py

diff --git a/2023/d24/ex1/ex1.py b/2023/d24/ex1/ex1.py
new file mode 100755
index 0000000..0145669
--- /dev/null
+++ b/2023/d24/ex1/ex1.py
@@ -0,0 +1,78 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from decimal import Decimal
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+    z: int
+
+
+class HailStone(NamedTuple):
+    pos: Point
+    vel: Point
+
+
+def solve(input: list[str]) -> int:
+    def parse_line(line: str) -> HailStone:
+        pos, vel = line.split(" @ ")
+        return HailStone(
+            Point(*map(int, pos.split(", "))), Point(*map(int, vel.split(", ")))
+        )
+
+    def parse(input: list[str]) -> list[HailStone]:
+        return [parse_line(line) for line in input]
+
+    def intersections(hailstones: list[HailStone], boundaries: tuple[int, int]) -> int:
+        def intersects(a: HailStone, b: HailStone) -> bool:
+            # According to wikipedia, if:
+            # x = a_px + t * a_vx = b_px + u * b_vx
+            # y = a_py + t * a_vy = b_py + u * b_vy
+            # then:
+            # t = ((a_px-b_px)*(-b_vy) - (a_py-b_py)*(-b_vx)) / ((-a_vx)*(-b_vy) - (-a_vy)*(-b_vx))
+            # u = ((a_px-b_px)*(-a_vy) - (a_py-b_py)*(-a_vx)) / ((-a_vx)*(-b_vy) - (-a_vy)*(-b_vx))
+            (a_px, a_py, _), (a_vx, a_vy, _) = a
+            (b_px, b_py, _), (b_vx, b_vy, _) = b
+
+            # Use rationals for extra precision, just in case
+            denom = Decimal(a_vx * b_vy - a_vy * b_vx)
+
+            # Parallel lines
+            if denom == 0:
+                return False
+
+            t = ((a_px - b_px) * (-b_vy) - (a_py - b_py) * (-b_vx)) / denom
+            u = ((a_px - b_px) * (-a_vy) - (a_py - b_py) * (-a_vx)) / denom
+
+            # Intersects in the past
+            if t < 0 or u < 0:
+                return False
+
+            x = a_px + t * a_vx
+            y = a_py + t * a_vy
+
+            # Outside our observation area
+            if not boundaries[0] <= x <= boundaries[1]:
+                return False
+            if not boundaries[0] <= y <= boundaries[1]:
+                return False
+
+            return True
+
+        return sum(intersects(a, b) for a, b in itertools.combinations(hailstones, 2))
+
+    hailstones = parse(input)
+    return intersections(hailstones, (200000000000000, 400000000000000))
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()