From ad739b7fec6bb92127b43b69799019db6759b365 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Sun, 29 Dec 2024 23:30:47 -0500
Subject: [PATCH] 2018: d13: ex2: add solution

---
 2018/d13/ex2/ex2.py | 178 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 178 insertions(+)
 create mode 100755 2018/d13/ex2/ex2.py

diff --git a/2018/d13/ex2/ex2.py b/2018/d13/ex2/ex2.py
new file mode 100755
index 0000000..84fbf5e
--- /dev/null
+++ b/2018/d13/ex2/ex2.py
@@ -0,0 +1,178 @@
+#!/usr/bin/env python
+
+import enum
+import sys
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+class Track(enum.StrEnum):
+    HORIZONTAL = "-"
+    VERTICAL = "|"
+    TURN = "/"
+    ANTI_TURN = "\\"
+    INTERSECTION = "+"
+
+
+class Direction(enum.StrEnum):
+    UP = "^"
+    DOWN = "v"
+    LEFT = "<"
+    RIGHT = ">"
+
+    def step(self, p: Point) -> Point:
+        dx, dy = {
+            Direction.UP: (-1, 0),
+            Direction.DOWN: (1, 0),
+            Direction.LEFT: (0, -1),
+            Direction.RIGHT: (0, 1),
+        }[self]
+        return Point(p.x + dx, p.y + dy)
+
+    def to_track(self) -> Track:
+        if self == Direction.UP or self == Direction.DOWN:
+            return Track.VERTICAL
+        if self == Direction.LEFT or self == Direction.RIGHT:
+            return Track.HORIZONTAL
+        assert False  # Sanity check
+
+
+class IntersectionBehaviour(enum.IntEnum):
+    LEFT = 0
+    STRAIGHT = 1
+    RIGHT = 2
+
+    def apply(self, dir: Direction) -> Direction:
+        if self == IntersectionBehaviour.STRAIGHT:
+            return dir
+        if self == IntersectionBehaviour.LEFT:
+            return {
+                Direction.UP: Direction.LEFT,
+                Direction.LEFT: Direction.DOWN,
+                Direction.DOWN: Direction.RIGHT,
+                Direction.RIGHT: Direction.UP,
+            }[dir]
+        if self == IntersectionBehaviour.RIGHT:
+            return {
+                Direction.UP: Direction.RIGHT,
+                Direction.RIGHT: Direction.DOWN,
+                Direction.DOWN: Direction.LEFT,
+                Direction.LEFT: Direction.UP,
+            }[dir]
+        assert False  # Sanity check
+
+    def next(self) -> "IntersectionBehaviour":
+        return IntersectionBehaviour((self + 1) % 3)
+
+
+class Cart(NamedTuple):
+    pos: Point
+    dir: Direction
+    intersection: IntersectionBehaviour
+
+    def step(self, tracks: dict[Point, Track]) -> "Cart":
+        assert self.pos in tracks  # Sanity check
+        new_pos = self.dir.step(self.pos)
+        track = tracks[new_pos]
+
+        if track in (Track.HORIZONTAL, Track.VERTICAL):
+            assert track == self.dir.to_track()  # Sanity check
+            return Cart(new_pos, self.dir, self.intersection)
+        if track == Track.TURN:
+            new_dir = {
+                Direction.UP: Direction.RIGHT,
+                Direction.DOWN: Direction.LEFT,
+                Direction.LEFT: Direction.DOWN,
+                Direction.RIGHT: Direction.UP,
+            }[self.dir]
+            return Cart(new_pos, new_dir, self.intersection)
+        if track == Track.ANTI_TURN:
+            new_dir = {
+                Direction.UP: Direction.LEFT,
+                Direction.DOWN: Direction.RIGHT,
+                Direction.LEFT: Direction.UP,
+                Direction.RIGHT: Direction.DOWN,
+            }[self.dir]
+            return Cart(new_pos, new_dir, self.intersection)
+        if track == Track.INTERSECTION:
+            new_dir = self.intersection.apply(self.dir)
+            return Cart(new_pos, new_dir, self.intersection.next())
+        assert False  # Sanity check
+
+
+def solve(input: str) -> str:
+    def parse(input: list[str]) -> tuple[dict[Point, Track], list[Cart]]:
+        tracks: dict[Point, Track] = {}
+        carts: list[Cart] = []
+
+        for x, line in enumerate(input):
+            for y, c in enumerate(line):
+                p = Point(x, y)
+                if c in Track:
+                    tracks[p] = Track(c)
+                elif c in Direction:
+                    carts.append(Cart(p, Direction(c), IntersectionBehaviour.LEFT))
+
+        # Don't forget the tracks under the carts
+        for cart in carts:
+            tracks[cart.pos] = cart.dir.to_track()
+
+        return tracks, carts
+
+    def print_state(tracks: dict[Point, Track], carts: list[Cart]) -> None:
+        cart_points = {cart.pos: cart for cart in carts}
+        max_x, max_y = max(p.x for p in tracks), max(p.y for p in tracks)
+
+        print()
+        for x in range(0, max_x + 1):
+            for y in range(0, max_y + 1):
+                p = Point(x, y)
+                if p in cart_points:
+                    print(str(cart_points[p].dir), end="")
+                elif p in tracks:
+                    print(str(tracks[p]), end="")
+                else:
+                    print(" ", end="")
+            print()
+
+    tracks, carts = parse(input.splitlines())
+    assert len(carts) % 2 == 1  # Sanity check
+
+    while True:
+        cart_positions = {cart.pos for cart in carts}
+        crashed: set[Point] = set()
+        new_carts: list[Cart] = []
+        for cart in carts:
+            # Already crashed, nothing to do
+            if cart.pos in crashed:
+                continue
+            new_cart = cart.step(tracks)
+            # Is there a crash
+            if new_cart.pos in cart_positions:
+                cart_positions.remove(new_cart.pos)
+                crashed.add(new_cart.pos)
+                continue
+            cart_positions.remove(cart.pos)
+            cart_positions.add(new_cart.pos)
+            new_carts.append(new_cart)
+        new_carts = [cart for cart in new_carts if cart.pos not in crashed]
+        new_carts.sort()
+        carts = new_carts
+        assert carts  # Sanity check
+        if len(carts) == 1:
+            return f"{carts[0].pos.y},{carts[0].pos.x}"
+
+    assert False  # Sanity check
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()