Implement 2024 day 18

Author: bertptrs

2024/src/aoc/days/day18.py

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+from collections import deque
+
+from . import SeparateRunner
+
+
+def parse_input(data: str) -> list[tuple[int, int]]:
+    return [tuple(map(int, line.split(","))) for line in data.strip().split("\n")]
+
+
+def find_exit(fallen: set[tuple[int, int]], width: int, height: int) -> int | None:
+    todo = deque([(0, 0, 0)])
+
+    best = {(0, 0): 0}
+
+    def enqueue(dist: int, x: int, y: int):
+        # print(f"trying {x},{y}")
+        if (x, y) in fallen:
+            return
+
+        if (x, y) not in best or best[x, y] > dist:
+            best[x, y] = dist
+            todo.append((dist, x, y))
+
+    while todo:
+        dist, x, y = todo.popleft()
+        # print(x, y)
+
+        if x == width - 1 and y == height - 1:
+            return dist
+
+        if x > 0:
+            enqueue(dist + 1, x - 1, y)
+
+        if x + 1 < width:
+            enqueue(dist + 1, x + 1, y)
+
+        if y > 0:
+            enqueue(dist + 1, x, y - 1)
+
+        if y + 1 < height:
+            enqueue(dist + 1, x, y + 1)
+
+
+class DayRunner(SeparateRunner):
+    @classmethod
+    def part1(
+        cls, input: str, width: int = 71, height: int = 71, limit: int = 1024
+    ) -> int:
+        falling = parse_input(input)
+
+        return find_exit(set(falling[:limit]), width, height)
+
+    @classmethod
+    def part2(cls, input: str, width: int = 71, height: int = 71) -> str:
+        falling = parse_input(input)
+
+        lower = 0
+        upper = len(falling)
+
+        while lower < upper:
+            mid = lower + (upper - lower) // 2
+
+            if find_exit(set(falling[:mid]), width, height) is not None:
+                lower = mid + 1
+            else:
+                upper = mid
+
+        first_blocker = falling[lower - 1]
+
+        return f"{first_blocker[0]},{first_blocker[1]}"

2024/tests/samples/18.txt

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+5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0

2024/tests/test_day18.py

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+from aoc.days.day18 import DayRunner
+
+from . import get_data
+
+
+def test_sample_part1() -> None:
+    assert DayRunner.part1(get_data(18), width=7, height=7, limit=12) == 22
+
+
+def test_sample_part2() -> None:
+    assert DayRunner.part2(get_data(18), width=7, height=7) == "6,1"