day 15

Author: startrekdude

Diff for: 15/sample-input.txt

@@ -0,0 +1,14 @@
+Sensor at x=2, y=18: closest beacon is at x=-2, y=15
+Sensor at x=9, y=16: closest beacon is at x=10, y=16
+Sensor at x=13, y=2: closest beacon is at x=15, y=3
+Sensor at x=12, y=14: closest beacon is at x=10, y=16
+Sensor at x=10, y=20: closest beacon is at x=10, y=16
+Sensor at x=14, y=17: closest beacon is at x=10, y=16
+Sensor at x=8, y=7: closest beacon is at x=2, y=10
+Sensor at x=2, y=0: closest beacon is at x=2, y=10
+Sensor at x=0, y=11: closest beacon is at x=2, y=10
+Sensor at x=20, y=14: closest beacon is at x=25, y=17
+Sensor at x=17, y=20: closest beacon is at x=21, y=22
+Sensor at x=16, y=7: closest beacon is at x=15, y=3
+Sensor at x=14, y=3: closest beacon is at x=15, y=3
+Sensor at x=20, y=1: closest beacon is at x=15, y=3

Diff for: 15/solve.py

@@ -0,0 +1,94 @@
+import re
+
+from dataclasses import dataclass
+from itertools import chain
+from operator import attrgetter
+from sys import argv as args
+
+path = args[1] if len(args) >= 2 else "sample-input.txt"
+with open(path) as f:
+	lines = f.read().splitlines()
+
+@dataclass(frozen=True)
+class Point:
+	x : int
+	y : int
+	
+	def distance(self, other):
+		return abs(self.x - other.x) + abs(self.y - other.y)
+
+@dataclass(frozen=True)
+class Sensor:
+	pos        : Point
+	beacon_pos : Point
+	
+	def in_range(self, p):
+		return self.pos.distance(p) <= self.pos.distance(self.beacon_pos)
+
+sensors = []
+beacons = set()
+
+for line in lines:
+	nums = [int(x) for x in re.findall("[-\d]+", line)]
+	beacon = Point(nums[2], nums[3])
+	beacons.add(beacon)
+	sensors.append(Sensor(
+		pos       =Point(nums[0], nums[1]),
+		beacon_pos=beacon
+	))
+
+Y = 2000000
+
+xlo = min(chain((s.pos.x for s in sensors), (s.beacon_pos.x for s in sensors))) * 2
+xhi = max(chain((s.pos.x for s in sensors), (s.beacon_pos.x for s in sensors))) * 2
+
+count = 0
+for x in range(xlo, xhi + 1):
+	p = Point(x, Y)
+	
+	if p in beacons:
+		continue
+	
+	if any(s.in_range(p) for s in sensors):
+		count += 1
+
+print(count)
+
+@dataclass(frozen=True)
+class Range:
+	lo : int
+	hi : int
+
+def analyze_column(x):
+	ranges = []
+	for sensor in sensors:
+		xDelta = abs(sensor.pos.x - x)
+		distance = sensor.pos.distance(sensor.beacon_pos)
+		yPos = sensor.pos.y
+		yDelta = distance - xDelta
+		
+		yLo = yPos - yDelta
+		yHi = yPos + yDelta
+		if yLo <= yHi:
+			ranges.append(Range(yLo, yHi))
+	
+	ranges.sort(key=attrgetter("lo"))
+	
+	for i in range(len(ranges) - 1):
+		r1 = ranges[i]
+		r2 = ranges[i + 1]
+		maybe_gap = r1.hi < r2.lo - 1
+		if maybe_gap:
+			gap_y = r1.hi + 1
+			if all(gap_y < r.lo or gap_y > r.hi for r in ranges):
+				return Point(x, gap_y)
+	return None
+
+def tuning_frequency(p):
+	return p.x * 4000000 + p.y
+
+for x in range(4000001):
+	p = analyze_column(x)
+	if p:
+		print(tuning_frequency(p))
+		break

Diff for: README.md

@@ -20,4 +20,5 @@ Organized in a fairly self-explanatory way; `01` has the first day and so on. My
 | Day 12 | `00:18:51` | `00:22:30` |
 | Day 13 | `00:09:30` | `00:15:29` |
 | Day 14 | `00:18:18` | `00:21:25` |
+| Day 15 | `00:15:43` | `00:40:24` |