Day 15 - puzzle 1.

Author: lakiboy

src/day_15/puzzle.kt

@@ -0,0 +1,53 @@
+package day_15
+
+import println
+import readInput
+import kotlin.math.absoluteValue
+
+data class Point(val x: Int, val y: Int) {
+ override fun toString() = "($x; $y)"
+}
+
+private operator fun Point.minus(other: Point) = (x - other.x).absoluteValue + (y - other.y).absoluteValue
+
+private fun MatchResult.toPoint() = Point(
+ groupValues[1].toInt(),
+ groupValues[2].toInt(),
+)
+
+fun puzzle1(input: List<String>, row: Int): Int {
+ val ignoreX = hashSetOf<Int>()
+ val xRanges = mutableListOf<IntRange>()
+
+ input.forEach { line ->
+ val (match1, match2) = """x=(-?\d+), y=(-?\d+)""".toRegex().findAll(line).toList()
+
+ val sensor = match1.toPoint()
+ val beacon = match2.toPoint()
+ val distance = sensor - beacon
+ val radius = if (sensor.y < row) { sensor.y + distance - row } else { row - (sensor.y - distance) }
+
+ if (radius > 0) {
+ xRanges += (sensor.x - radius)..(sensor.x + radius)
+ }
+
+ if (beacon.y == row) {
+ ignoreX += beacon.x
+ }
+ }
+
+ return xRanges
+ .zipWithNext { a, b -> a.toSet() union b.toSet() }
+ .flatten()
+ .distinct()
+ .subtract(ignoreX)
+ .size
+}
+
+fun main() {
+ val testInput = readInput("day_15/input_test")
+ check(puzzle1(testInput, 10) == 26)
+
+ val input = readInput("day_15/input")
+ puzzle1(input, 2_000_000).println() // 4873353
+}