Day 21, part 1: One shortest sequence for each keypad

But not the shortest sequence over all stages. To get there, we have to
evaluate all shortest paths for each code on each keypad and take the
shortest among all.

Author: rhaendel

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

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+package de.ronny_h.aoc.year24.day21
+
+import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.extensions.Coordinates
+import de.ronny_h.aoc.extensions.Direction
+import de.ronny_h.aoc.extensions.Grid
+import de.ronny_h.aoc.extensions.ShortestPath
+import de.ronny_h.aoc.extensions.aStar
+import de.ronny_h.aoc.extensions.asList
+
+fun main() = KeypadConundrum().run(0, 0)
+
+class KeypadConundrum : AdventOfCode<Int>(2024, 21) {
+    override fun part1(input: List<String>): Int {
+        println("directionalKeypad: $directionalKeypadLayout")
+
+        val numericKeypad = Keypad(numericKeypadLayout)
+        val sequence = input.map { code ->
+            val sequence1 = code.map { position ->
+                numericKeypad.moveTo(position)
+            }.joinToString("")
+            println("robot1: $sequence1")
+
+            val directionalKeypad1 = Keypad(directionalKeypadLayout)
+            val sequence2 = sequence1.map { direction ->
+                directionalKeypad1.moveTo(direction)
+            }.joinToString("")
+            println("robot2: $sequence2")
+
+            val directionalKeypad2 = Keypad(directionalKeypadLayout)
+            val sequence3 = sequence2.map { direction ->
+                directionalKeypad2.moveTo(direction)
+            }.joinToString("")
+            println("$code: $sequence3")
+        }
+
+        return 0
+    }
+
+    override fun part2(input: List<String>): Int {
+        TODO("Not yet implemented")
+    }
+}
+
+/*
++---+---+---+
+| 7 | 8 | 9 |
++---+---+---+
+| 4 | 5 | 6 |
++---+---+---+
+| 1 | 2 | 3 |
++---+---+---+
+    | 0 | A |
+    +---+---+
+*/
+val numericKeypadLayout = """
+    789
+    456
+    123
+     0A
+""".asList()
+
+/*
+    +---+---+
+    | ^ | A |
++---+---+---+
+| < | v | > |
++---+---+---+
+*/
+val directionalKeypadLayout = """
+     ^A
+    <v>
+""".asList()
+
+class Keypad(layout: List<String>) : Grid<Char>(layout, ' ') {
+    override fun Char.toElementType(): Char = this
+
+    data class Node(val direction: Direction, val position: Coordinates) {
+        override fun toString() = "$position$direction"
+    }
+
+    private var currentPosition = find('A')
+
+    fun moveTo(target: Char): String {
+        val targetPosition = find(target)
+        val path = shortestPath(currentPosition, targetPosition)
+        currentPosition = targetPosition
+        val directions = path.path.drop(1).joinToString("") {
+            when (it.direction) {
+                Direction.NORTH -> "^"
+                Direction.EAST -> ">"
+                Direction.SOUTH -> "v"
+                Direction.WEST -> "<"
+            }
+        }
+        return "${directions}A"
+    }
+
+    fun shortestPath(start: Coordinates, goal: Coordinates): ShortestPath<Node> {
+
+        val neighbours: (Node) -> List<Node> = { node ->
+            Direction
+                .entries
+                .map { Node(it, node.position + it) }
+                .filter { getAt(it.position) != nullElement }
+        }
+
+        val d: (Node, Node) -> Int = { a, b ->
+            // pre-condition: a and b are neighbours
+            1
+        }
+
+        val h: (Node) -> Int = { it.position taxiDistanceTo goal }
+
+        return aStar(Node(Direction.EAST, start), { this.position == goal }, neighbours, d, h)
+    }
+}

src/test/kotlin/de/ronny_h/aoc/year24/day21/KeypadConundrumTest.kt

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+package de.ronny_h.aoc.year24.day21
+
+import de.ronny_h.aoc.extensions.asList
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.shouldBe
+
+class KeypadConundrumTest : StringSpec({
+    val input = """
+        029A
+        980A
+        179A
+        456A
+        379A
+    """.asList()
+
+    "make the 1st robot type 029A" {
+        val numericKeypad = Keypad(numericKeypadLayout)
+        numericKeypad.moveTo('0') shouldBe "<A"
+        numericKeypad.moveTo('2') shouldBe "^A"
+        numericKeypad.moveTo('9') shouldBe "^^>A"
+        numericKeypad.moveTo('A') shouldBe "vvvA"
+    }
+
+    "make the 2nd robot type ^^>A" {
+        val directionalKeypad = Keypad(directionalKeypadLayout)
+        directionalKeypad.moveTo('^') shouldBe "<A"
+        directionalKeypad.moveTo('^') shouldBe "A"
+        directionalKeypad.moveTo('>') shouldBe ">vA"
+        directionalKeypad.moveTo('A') shouldBe "^A"
+    }
+
+    "part 1: The sum of the complexities of the five codes" {
+        KeypadConundrum().part1(input) shouldBe 126384
+    }
+})