2022: d15: ex2: add solution

2022/d15/ex2/ex2.py

@@ -0,0 +1,112 @@
+#!/usr/bin/env python
+
+import dataclasses
+import functools
+import sys
+from collections.abc import Iterable
+from typing import NamedTuple, Optional
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+    @classmethod
+    def from_input(cls, input: str) -> "Point":
+        assert input.startswith("x=")  # Sanity check
+        x, y = input.split(", ")
+        return cls(int(x.split("=")[-1]), int(y.split("=")[-1]))
+
+
+class Interval(NamedTuple):
+    start: int
+    end: int
+
+    def as_set(self) -> set[int]:
+        return set(range(self.start, self.end + 1))
+
+
+def merge_intervals(intervals: Iterable[Interval]) -> list[Interval]:
+    intervals = sorted(intervals)
+
+    res = [intervals[0]]
+    for candidate in intervals[1:]:
+        # Range is inclusive in both end, so add 1 to end in case of near miss
+        if (res[-1].end + 1) >= candidate.start:
+            new_end = max(res[-1].end, candidate.end)
+            res[-1] = Interval(res[-1].start, new_end)
+        else:
+            res.append(candidate)
+
+    return res
+
+
+def distance(a: Point, b: Point) -> int:
+    return abs(a.x - b.x) + abs(a.y - b.y)
+
+
+@dataclasses.dataclass
+class SensorData:
+    pos: Point
+    beacon: Point
+
+    @classmethod
+    def from_input(cls, input: str) -> "SensorData":
+        assert input.startswith("Sensor at x=")  # Sanity check
+
+        mid = input.index(":")
+        sensor, beacon = input[:mid], input[mid:]
+        return cls(
+            Point.from_input(sensor[sensor.index("x") :]),
+            Point.from_input(beacon[beacon.index("x") :]),
+        )
+
+    @functools.cached_property
+    def safe_range(self) -> int:
+        return distance(self.pos, self.beacon)
+
+    def scan_row(self, row: int) -> Optional[Interval]:
+        distance = abs(row - self.pos.y)
+        dx = self.safe_range - distance
+        if dx < 0:
+            return None
+        return Interval(self.pos.x - dx, self.pos.x + dx)
+
+
+def solve(input: list[str]) -> int:
+    def points_without_sos(data: list[SensorData], row: int) -> list[Interval]:
+        intervals = (d.scan_row(row) for d in data)
+        return merge_intervals(i for i in intervals if i is not None)
+
+    def find_hole(intervals: list[Interval], max_coord: int) -> Optional[int]:
+        for i in intervals:
+            if i.start > 0:
+                return i.start - 1
+            if i.end < max_coord:
+                return i.end + 1
+
+        return None
+
+    def find_sos(data: list[SensorData], max_coord: int) -> Point:
+        for row in range(0, max_coord + 1):
+            intervals = points_without_sos(data, row)
+            if (hole := find_hole(intervals, max_coord)) is not None:
+                return Point(hole, row)
+        assert False  # Sanity check
+
+    def tuning_frequency(p: Point) -> int:
+        return p.x * 4000000 + p.y
+
+    data = [SensorData.from_input(line) for line in input]
+    MAX_COORD = 4_000_000
+    sos_beacon = find_sos(data, MAX_COORD)
+    return tuning_frequency(sos_beacon)
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()