Solution 2018-13, part 2 (Mine Cart Madness)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2018/day13/MineCartMadness.kt

@@ -9,7 +9,7 @@ import de.ronny_h.aoc.extensions.grids.SimpleCharGrid
 import de.ronny_h.aoc.extensions.grids.Turn
 import de.ronny_h.aoc.extensions.grids.Turn.*
 
-fun main() = MineCartMadness().run("33,69", "")
+fun main() = MineCartMadness().run("33,69", "135,9")
 
 class MineCartMadness : AdventOfCode<String>(2018, 13) {
     override fun part1(input: List<String>): String {
@@ -18,12 +18,13 @@ class MineCartMadness : AdventOfCode<String>(2018, 13) {
     }
 
     override fun part2(input: List<String>): String {
-        return ""
+        val remaining = Track(input).moveCartsUntilOnlyOneRemains()
+        return "${remaining.col},${remaining.row}"
     }
 }
 
 class Track(input: List<String>) : SimpleCharGrid(input, ' ') {
-    private val carts: List<Cart> = buildList {
+    private val carts: MutableList<Cart> = buildList {
         forEachCoordinates { position, element ->
             when (element) {
                 '<' -> {
@@ -47,24 +48,40 @@ class Track(input: List<String>) : SimpleCharGrid(input, ' ') {
                 }
             }
         }.last()
-    }
+    }.toMutableList()
 
     fun moveCartsUntilFirstCrash(): Coordinates {
         var collision: Coordinates? = null
         while (collision == null) {
-            collision = tick()
+            collision = tickUntilFirstCollision()
         }
         return collision
     }
 
-    private fun tick(): Coordinates? {
+    fun moveCartsUntilOnlyOneRemains(): Coordinates {
+        while (carts.size != 1) {
+            tickRemovingCollidingCarts()
+        }
+        return carts.single().position
+    }
+
+    private fun tickUntilFirstCollision(): Coordinates? {
         for (cart in carts.sortedBy { it.position }) {
             cart.move()
             carts.firstDuplicate(Cart::position)?.also { return it }
         }
         return null
     }
 
+    private fun tickRemovingCollidingCarts() {
+        for (cart in carts.sortedBy { it.position }) {
+            cart.move()
+            carts.firstDuplicate(Cart::position)?.also { crashed ->
+                carts.removeAll { it.position == crashed }
+            }
+        }
+    }
+
     private fun Cart.move() {
         position += direction
         direction = when (getAt(position)) {

src/test/kotlin/de/ronny_h/aoc/year2018/day13/MineCartMadnessTest.kt

@@ -6,7 +6,8 @@ import io.kotest.matchers.shouldBe
 
 class MineCartMadnessTest : StringSpec({
 
-    val input = """
+    "part 1: the location of the first crash" {
+        val input = """
         /->-\        
         |   |  /----\
         | /-+--+-\  |
@@ -15,12 +16,19 @@ class MineCartMadnessTest : StringSpec({
           \------/   
     """.asList()
 
-    "part 1: the location of the first crash" {
         MineCartMadness().part1(input) shouldBe "7,3"
     }
 
-    "part 2" {
-        val input = listOf("")
-        MineCartMadness().part2(input) shouldBe ""
+    "part 2: the location of the last cart" {
+        val input = """
+            />-<\  
+            |   |  
+            | /<+-\
+            | | | v
+            \>+</ |
+              |   ^
+              \<->/
+          """.asList()
+        MineCartMadness().part2(input) shouldBe "6,4"
     }
 })