From bbafa72fd365769a08cbbfedfeb2dbc1b0b8738f Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Sun, 11 May 2025 01:12:58 +0100
Subject: [PATCH] 2017: d19: ex1: add solution

---
 2017/d19/ex1/ex1.py | 73 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100755 2017/d19/ex1/ex1.py

diff --git a/2017/d19/ex1/ex1.py b/2017/d19/ex1/ex1.py
new file mode 100755
index 0000000..6cc724c
--- /dev/null
+++ b/2017/d19/ex1/ex1.py
@@ -0,0 +1,73 @@
+#!/usr/bin/env python
+
+import enum
+import sys
+from collections.abc import Iterator
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+class Direction(enum.Enum):
+    UP = Point(-1, 0)
+    DOWN = Point(1, 0)
+    LEFT = Point(0, -1)
+    RIGHT = Point(0, 1)
+
+    def apply(self, p: Point) -> Point:
+        dx, dy = self.value
+        return Point(p.x + dx, p.y + dy)
+
+    def turns(self) -> Iterator["Direction"]:
+        match self:
+            case Direction.UP | Direction.DOWN:
+                turns = (Direction.LEFT, Direction.RIGHT)
+            case Direction.LEFT | Direction.RIGHT:
+                turns = (Direction.UP, Direction.DOWN)
+        yield from turns
+
+
+def solve(input: str) -> str:
+    def parse(input: str) -> dict[Point, str]:
+        return {
+            Point(x, y): c
+            for x, line in enumerate(input.splitlines())
+            for y, c in enumerate(line)
+            if c != " "
+        }
+
+    def packet_spell(map: dict[Point, str]) -> str:
+        letters: list[str] = []
+        # Travel starts downward, from outside the map before the first line
+        pos = min(map.keys(), key=lambda p: p.x)
+        dir = Direction.DOWN
+
+        while True:
+            if (letter := map[pos]) not in ("+", "|", "-"):
+                letters.append(letter)
+            new_dir = dir
+            new_pos = dir.apply(pos)
+            if new_pos not in map:
+                for new_dir in dir.turns():
+                    if (new_pos := new_dir.apply(pos)) in map:
+                        break
+                else:
+                    # Must be the end of the line, stop looping
+                    return "".join(letters)
+            dir = new_dir
+            pos = new_pos
+
+    map = parse(input)
+    return packet_spell(map)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()