Optimize KeypadConundrum for performance

But it still consumes too much memory for part two of day 21.
Since we don't need the actual input sequence at all, we might
just calculate its length.

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year24/day21/KeypadConundrum.kt

@@ -5,28 +5,40 @@ import de.ronny_h.aoc.extensions.Direction
 import de.ronny_h.aoc.extensions.Direction.*
 import de.ronny_h.aoc.extensions.SimpleCharGrid
 import de.ronny_h.aoc.extensions.asList
+import de.ronny_h.aoc.extensions.memoize
 
 fun main() = KeypadConundrum().run(94426, 0)
 
 class KeypadConundrum : AdventOfCode<Int>(2024, 21) {
     fun input(code: String, keypad: Keypad, depth: Int): String {
-        if (depth == 0) {
-            return code
-        }
+        data class Parameters(val code: String, val depth: Int)
+
+        val keypads = buildList(depth) {
+            repeat(depth) { add(Keypad(directionalKeypadLayout)) }
+        } + keypad
+        lateinit var inputRec: (Parameters) -> String
+
+        inputRec = { p: Parameters ->
+            if (p.depth == 0) {
+                p.code
+            } else {
+                p.code.map { char ->
+                    keypads[p.depth].moveTo(char).map {
+                        inputRec(Parameters(it, p.depth - 1))
+                    }.minBy { it.length }
+                }.joinToString("")
+            }
+        }.memoize()
 
-        return code.map { char ->
-            keypad.moveTo(char).map {
-                input(it, Keypad(directionalKeypadLayout), depth - 1)
-            }.minBy { it.length }
-        }.joinToString("")
+        return inputRec(Parameters(code, depth))
     }
 
     override fun part1(input: List<String>): Int = input.sumOf {
         input(it, Keypad(numericKeypadLayout), 3).length * it.dropLast(1).toInt()
     }
 
-    override fun part2(input: List<String>): Int {
-        TODO("Not yet implemented")
+    override fun part2(input: List<String>): Int = input.sumOf {
+        input(it, Keypad(numericKeypadLayout), 26).length * it.dropLast(1).toInt()
     }
 }