2017: d19: ex2: add solution

2017/d19/ex2/ex2.py

@@ -0,0 +1,75 @@
+#!/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) -> int:
+    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_walk(map: dict[Point, str]) -> int:
+        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
+
+        steps = 0
+        while True:
+            if (letter := map[pos]) not in ("+", "|", "-"):
+                letters.append(letter)
+            new_dir = dir
+            new_pos = dir.apply(pos)
+            steps += 1
+            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 steps
+            dir = new_dir
+            pos = new_pos
+
+    map = parse(input)
+    return packet_walk(map)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()