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diff --git a/‎src/main/kotlin/advent2021/Day05.kt
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Lines changed: 36 additions & 0 deletions
diff --git a/‎src/main/kotlin/advent2021/shared/Geometry.kt
Lines changed: 19 additions & 0 deletions b/‎src/main/kotlin/advent2021/shared/Geometry.kt
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+package advent2021
+
+import Resources
+import advent2021.shared.Point2d
+
+class Day05(input: List<String>) {
+
+    private val instructions = input.map { parseRow(it) }
+
+    fun solvePart1(): Int =
+        solve { it.first sharesAxisWith it.second }
+
+    fun solvePart2(): Int =
+        solve { true }
+
+    private fun solve(lineFilter: (Pair<Point2d, Point2d>) -> Boolean) =
+        instructions
+            .filter { lineFilter(it) }
+            .flatMap { it.first lineTo it.second }
+            .groupingBy { it }
+            .eachCount()
+            .count { it.value > 1 }
+
+    private fun parseRow(input: String): Pair<Point2d, Point2d> =
+        Pair(
+            input.substringBefore(" ").split(",").map { it.toInt() }.let { Point2d(it.first(), it.last()) },
+            input.substringAfterLast(" ").split(",").map { it.toInt() }.let { Point2d(it.first(), it.last()) }
+        )
+}
+
+fun main() {
+    val input = Resources.resourceAsListOfString("advent2021/day05.txt")
+    val day = Day05(input = input)
+    println("Part 1: ${day.solvePart1()}")
+    println("Part 2: ${day.solvePart2()}")
+}
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+package advent2021.shared
+
+import kotlin.math.absoluteValue
+import kotlin.math.sign
+
+data class Point2d(val x: Int, val y: Int) {
+
+    infix fun sharesAxisWith(that: Point2d): Boolean =
+        x == that.x || y == that.y
+
+    infix fun lineTo(that: Point2d): List<Point2d> {
+        val xDelta = (that.x - x).sign
+        val yDelta = (that.y - y).sign
+
+        val steps = maxOf((x - that.x).absoluteValue, (y - that.y).absoluteValue)
+
+        return (1 .. steps).scan(this) { last, _ -> Point2d(last.x + xDelta, last.y + yDelta) }
+    }
+}