From f9c1e152c33b0f745edb55555156e79cca1008ab Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Mon, 18 Dec 2023 08:02:58 +0000
Subject: [PATCH] 2023: d18: ex1: add solution

---
 2023/d18/ex1/ex1.py | 89 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100755 2023/d18/ex1/ex1.py

diff --git a/2023/d18/ex1/ex1.py b/2023/d18/ex1/ex1.py
new file mode 100755
index 0000000..be11cb2
--- /dev/null
+++ b/2023/d18/ex1/ex1.py
@@ -0,0 +1,89 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from enum import StrEnum
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+class Direction(StrEnum):
+    UP = "U"
+    DOWN = "D"
+    LEFT = "L"
+    RIGHT = "R"
+
+    def apply(self, pos: Point, n: int = 1) -> Point:
+        DIRECTIONS = {
+            "U": Point(-1, 0),
+            "D": Point(1, 0),
+            "L": Point(0, -1),
+            "R": Point(0, 1),
+        }
+        dx, dy = DIRECTIONS[self.value]
+        return Point(pos.x + dx * n, pos.y + dy * n)
+
+
+DigPlanStep = tuple[Direction, int]
+DigPlan = list[DigPlanStep]
+
+
+def solve(input: list[str]) -> int:
+    def parse_line(line: str) -> DigPlanStep:
+        dir, n, _ = line.split()
+        return Direction(dir), int(n)
+
+    def parse(input: list[str]) -> DigPlan:
+        return list(map(parse_line, input))
+
+    def dig_trench(plan: DigPlan) -> list[Point]:
+        points = [Point(0, 0)]
+
+        for direction, n in plan:
+            points.append(direction.apply(points[-1], n))
+        # The trench should loop back to the start, make sure we don't count it twice
+        if points[-1] == Point(0, 0):
+            del points[-1]
+
+        return points
+
+    def lagoon_volume(trench: list[Point]) -> int:
+        def shoelace_area(points: list[Point]) -> int:
+            # Must be integer because pipes follow the grid, and can't cut squares in half
+            return abs(
+                sum(
+                    (points[i - 1].x * points[i].y) - (points[i].x * points[i - 1].y)
+                    for i in range(len(points))
+                )
+                // 2
+            )
+
+        def perimeter(points: list[Point]) -> int:
+            res = 0
+
+            for p, n in itertools.pairwise(itertools.chain(points, [points[0]])):
+                res += abs(n.x - p.x) + abs(n.y - p.y)
+
+            return res
+
+        area = shoelace_area(trench)
+        trench_points = perimeter(trench)
+        interior_points = area - trench_points // 2 + 1
+        return interior_points + trench_points
+
+    plan = parse(input)
+    trench = dig_trench(plan)
+    return lagoon_volume(list(trench))
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()