day 9

src/main/kotlin/me/gergo/Aoc09.kt

@@ -0,0 +1,87 @@
+package me.gergo
+
+import java.io.File
+import java.lang.Integer.max
+import kotlin.math.abs
+import kotlin.math.sign
+
+fun main() {
+    val steps = File("src/main/resources/input09.txt").readLines()
+        .map(::parseRopeStep)
+
+    // val ropeField = RopeField(6, 5, 2) // For Part One
+    val ropeField = RopeField(6, 5, 10) // For Part Two
+    for (step in steps) {
+        println(step)
+        ropeField.moveBy(step)
+    }
+    println("Positions visited by tail: ${ropeField.positionsVisitedByTail().count()}")
+}
+
+enum class Direction { L, R, U, D }
+data class RopeStep(val dir: Direction, val steps: Int)
+data class Position(val x: Int, val y: Int) {
+    fun distanceFrom(o: Position) = max(abs(x - o.x), abs(y - o.y))
+}
+
+class Rope(internal val knots: MutableList<Position>) {
+
+    fun head() = knots[0]
+    fun tail() = knots[knots.size - 1]
+
+    internal fun moveHead(dir: Direction) {
+        val head = head()
+        knots[0] = when (dir) {
+            Direction.L -> Position(head.x - 1, head.y)
+            Direction.R -> Position(head.x + 1, head.y)
+            Direction.U -> Position(head.x, head.y + 1)
+            Direction.D -> Position(head.x, head.y - 1)
+        }
+        for (i in 1 until knots.size) {
+            val prev = knots[i - 1]
+            var curr = knots[i]
+            while (prev.distanceFrom(curr) > 1) {
+                val fx = (prev.x - curr.x).sign
+                val fy = (prev.y - curr.y).sign
+                curr = if (fx == 0) {
+                    Position(curr.x, curr.y + fy)
+                } else if (fy == 0) {
+                    Position(curr.x + fx, curr.y)
+                } else {
+                    Position(curr.x + fx, curr.y + fy)
+                }
+            }
+            knots[i] = curr
+        }
+    }
+}
+
+class RopeField(private val width: Int, private val height: Int, ropeLength: Int) {
+    private val rope = Rope(MutableList(ropeLength) { Position(0, 0) })
+    private val tailPositions = mutableListOf<Position>()
+
+    fun moveBy(step: RopeStep) {
+        for (s in 1..step.steps) {
+            rope.moveHead(step.dir)
+            tailPositions.add(rope.tail())
+            // println(this) // For simple debugging, does not work for negative coordinates!
+        }
+    }
+
+    override fun toString(): String {
+        val buf = Array(height) { CharArray(width) { '.' } }
+        for (i in 1 until rope.knots.size) {
+            val knot = rope.knots[i]
+            buf[height - 1 - knot.y][knot.x] = i.digitToChar()
+        }
+        buf[height - 1 - rope.head().y][rope.head().x] = 'H'
+        return buf.joinToString("\n") { it.joinToString("") } + '\n'
+    }
+
+    fun positionsVisitedByTail() = tailPositions.toSet()
+}
+
+fun parseRopeStep(line: String): RopeStep {
+    val (dir, steps) = line.split(" ")
+    return RopeStep(Direction.valueOf(dir), steps.toInt())
+}