day 15

src/main/kotlin/me/gergo/Aoc12.kt

@@ -112,4 +112,4 @@ fun neighbors(width: Int, height: Int, c: Coordinate) = sequence {
     if (y > 0) yield(Coordinate(x, y - 1))
 }
 
-fun manhattanDistance(a: Coordinate, b: Coordinate): Double = (abs(b.x - a.x) + abs(b.y - a.y)).toDouble()
+private fun manhattanDistance(a: Coordinate, b: Coordinate): Double = (abs(b.x - a.x) + abs(b.y - a.y)).toDouble()

src/main/kotlin/me/gergo/Aoc15.kt

@@ -0,0 +1,62 @@
+package me.gergo
+
+import java.io.File
+import kotlin.math.abs
+
+fun main() {
+    val sensors = File("src/main/resources/input15.txt").readLines()
+        .map(::parseSensors)
+
+    val maxRange = sensors.maxOf(Sensor::range)
+    val minX = sensors.minOf { it.closestBeacon.x } - maxRange
+    val maxX = sensors.maxOf { it.closestBeacon.x } + maxRange
+    val beacons = sensors.map(Sensor::closestBeacon).toSet()
+
+    // Part One
+    val atY = 2000000
+    val result1 = (minX..maxX).count { x ->
+        sensors.any { it.inRange(Coordinate(x, atY)) } // Number of coordinates covered by the sensor in our row,
+    } - beacons.count { it.y == atY } // ...minus the number of known beacons in that row
+
+    println("Positions that cannot contain a beacon: $result1")
+
+    // Part Two
+    val max = 4000000
+    val tuningFrequency =
+        sensors.flatMap(Sensor::outerBorder) // The search space can be greatly limited by considering the outer border of the sensor range
+            .filter { it.x in 0..max && it.y in 0..max } // ...except for all the coordinates that fall outside the 0..4000000 range
+            .distinct() // ...removing duplicates
+            .filter { c -> sensors.none { s -> s.inRange(c) } } // ...finally selecting the undetected coordinate
+            .map { p -> p.x * 4000000L + p.y }
+    println("Distress beacon tuning frequency: $tuningFrequency")
+}
+
+private data class Sensor(val position: Coordinate, val closestBeacon: Coordinate) {
+    val range = manhattanDistance(position, closestBeacon.x, closestBeacon.y)
+    fun inRange(c: Coordinate) = manhattanDistance(position, c.x, c.y) <= range
+
+    fun outerBorder() = sequence<Coordinate> {
+        val radius = range
+
+        for (i in 0..radius) { // Left to top
+            yield(Coordinate(position.x - radius - 1 + i, position.y - i))
+        }
+        for (i in 0..radius) { // Top to right
+            yield(Coordinate(position.x + i, position.y - radius - 1 + i))
+        }
+        for (i in 0..radius) { // Right to bottom
+            yield(Coordinate(position.x + radius + 1 - i, position.y + i))
+        }
+        for (i in 0..radius) { // Bottom to left
+            yield(Coordinate(position.x - i, position.y + radius + 1 - i))
+        }
+    }
+}
+
+private fun manhattanDistance(a: Coordinate, x: Int, y: Int) = abs(a.x - x) + abs(a.y - y)
+
+private val SensorBeaconFormat = Regex("Sensor at x=(-?\\d+), y=(-?\\d+): closest beacon is at x=(-?\\d+), y=(-?\\d+)")
+private fun parseSensors(line: String): Sensor {
+    val (_, sx, sy, bx, by) = SensorBeaconFormat.matchEntire(line)!!.groupValues
+    return Sensor(Coordinate(sx.toInt(), sy.toInt()), Coordinate(bx.toInt(), by.toInt()))
+}

src/main/resources/input15.txt

@@ -0,0 +1,26 @@
+Sensor at x=3772068, y=2853720: closest beacon is at x=4068389, y=2345925
+Sensor at x=78607, y=2544104: closest beacon is at x=-152196, y=4183739
+Sensor at x=3239531, y=3939220: closest beacon is at x=3568548, y=4206192
+Sensor at x=339124, y=989831: closest beacon is at x=570292, y=1048239
+Sensor at x=3957534, y=2132743: closest beacon is at x=3897332, y=2000000
+Sensor at x=1882965, y=3426126: closest beacon is at x=2580484, y=3654136
+Sensor at x=1159443, y=3861139: closest beacon is at x=2580484, y=3654136
+Sensor at x=2433461, y=287013: closest beacon is at x=2088099, y=-190228
+Sensor at x=3004122, y=3483833: closest beacon is at x=2580484, y=3654136
+Sensor at x=3571821, y=799602: closest beacon is at x=3897332, y=2000000
+Sensor at x=2376562, y=1539540: closest beacon is at x=2700909, y=2519581
+Sensor at x=785113, y=1273008: closest beacon is at x=570292, y=1048239
+Sensor at x=1990787, y=38164: closest beacon is at x=2088099, y=-190228
+Sensor at x=3993778, y=3482849: closest beacon is at x=4247709, y=3561264
+Sensor at x=3821391, y=3986080: closest beacon is at x=3568548, y=4206192
+Sensor at x=2703294, y=3999015: closest beacon is at x=2580484, y=3654136
+Sensor at x=1448314, y=2210094: closest beacon is at x=2700909, y=2519581
+Sensor at x=3351224, y=2364892: closest beacon is at x=4068389, y=2345925
+Sensor at x=196419, y=3491556: closest beacon is at x=-152196, y=4183739
+Sensor at x=175004, y=138614: closest beacon is at x=570292, y=1048239
+Sensor at x=1618460, y=806488: closest beacon is at x=570292, y=1048239
+Sensor at x=3974730, y=1940193: closest beacon is at x=3897332, y=2000000
+Sensor at x=2995314, y=2961775: closest beacon is at x=2700909, y=2519581
+Sensor at x=105378, y=1513086: closest beacon is at x=570292, y=1048239
+Sensor at x=3576958, y=3665667: closest beacon is at x=3568548, y=4206192
+Sensor at x=2712265, y=2155055: closest beacon is at x=2700909, y=2519581