1 files changed, 62 insertions, 0 deletions
diff --git a/aoc-2021-kotlin/src/Day17.kt b/aoc-2021-kotlin/src/Day17.kt
new file mode 100644
index 0000000..5e0170b
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day17.kt
@@ -0,0 +1,62 @@
+import kotlin.math.max
+import kotlin.math.sign
+
+object Day17 {
+    data class Target(val left: Int, val right: Int, val bottom: Int, val top: Int)
+
+    private class Probe(private var vel: Pos2D, private val target: Target) {
+        companion object {
+            fun search(target: Target): Pair<Int, Int> {
+                var highest = 0
+                var count = 0
+
+                for (vx in 0..1000) {
+                    for (vy in -1000..1000) {
+                        val probe = Probe(Pos2D(vx, vy), target)
+                        var currentHighest = 0
+
+                        while (probe.canHitTarget) {
+                            probe.step()
+                            currentHighest = max(currentHighest, probe.pos.y)
+
+                            if (probe.isInTarget) {
+                                count++
+                                highest = max(highest, currentHighest)
+                                break
+                            }
+                        }
+                    }
+                }
+
+                return highest to count
+            }
+        }
+
+        private var pos = Pos2D(0, 0)
+
+        private fun step() {
+            pos += vel
+            vel = vel.copy(x = vel.x - vel.x.sign, y = vel.y - 1)
+        }
+
+        private val canHitTarget
+            get() = pos.y > target.bottom
+
+        private val isInTarget
+            get() = pos.x in target.left..target.right && pos.y in target.bottom..target.top
+    }
+
+    fun bothParts(input: Target) = Probe.search(input)
+}
+
+fun main() {
+    val testInput = Day17.Target(20, 30, -10, -5)
+    val testOutput = Day17.bothParts(testInput)
+    check(testOutput.first == 45)
+    check(testOutput.second == 112)
+
+    val input = Day17.Target(192, 251, -89, -59)
+    val output = Day17.bothParts(input)
+    println(output.first)
+    println(output.second)
+}