Add the basic structures for day 21.

2024/src/main/kotlin/aoc/year2024/Day21.kt

@@ -0,0 +1,100 @@
+package aoc.year2024
+
+import aoc.library.Direction
+import aoc.library.Point
+import aoc.library.Puzzle
+import aoc.library.move
+import java.util.PriorityQueue
+
+object Day21 : Puzzle<Long, Long>(day = 21) {
+
+  private const val UP = '^'
+  private const val DOWN = 'v'
+  private const val LEFT = '<'
+  private const val RIGHT = '>'
+  private const val ACTIVATE = 'A'
+
+  /**
+   *     +---+---+
+   *     | ^ | A |
+   * +---+---+---+
+   * | < | v | > |
+   * +---+---+---+
+   */
+  private val DIRECTIONAL_KEYPAD = mapOf(
+    Point(1, 0) to UP,
+    Point(2, 0) to ACTIVATE,
+    Point(0, 1) to LEFT,
+    Point(1, 1) to DOWN,
+    Point(2, 1) to RIGHT,
+  )
+
+  /**
+   * +---+---+---+
+   * | 7 | 8 | 9 |
+   * +---+---+---+
+   * | 4 | 5 | 6 |
+   * +---+---+---+
+   * | 1 | 2 | 3 |
+   * +---+---+---+
+   *     | 0 | A |
+   *     +---+---+
+   */
+  private val NUMERICAL_KEYPAD = mapOf(
+    Point(0, 0) to '7',
+    Point(1, 0) to '8',
+    Point(2, 0) to '9',
+    Point(0, 1) to '4',
+    Point(1, 1) to '5',
+    Point(2, 1) to '6',
+    Point(0, 2) to '1',
+    Point(1, 2) to '2',
+    Point(2, 2) to '3',
+    Point(0, 2) to '1',
+    Point(1, 2) to '2',
+    Point(2, 2) to '3',
+    Point(1, 3) to '0',
+    Point(2, 3) to ACTIVATE,
+  )
+
+  override fun solvePart1(input: String): Long {
+    TODO()
+  }
+
+  private fun Map<Point, Char>.findChar(toFind: Char): Point = firstNotNullOf { (point, char) ->
+    if (char == toFind) point else null
+  }
+
+  private fun findPath(
+    pattern: Map<Point, Char>,
+    from: Char,
+    to: Char,
+  ): String {
+    val fromPosition = pattern.findChar(from)
+    val toPosition = pattern.findChar(to)
+    val visited = mutableSetOf<Point>()
+
+    data class State(val position: Point, val steps: String)
+
+    val queue = PriorityQueue<State>(compareBy { it.steps.length })
+    queue.add(State(fromPosition, ""))
+    while (true) {
+      val candidate = queue.remove()
+      if (candidate.position == toPosition) {
+        return candidate.steps
+      }
+      if (!visited.add(candidate.position)) continue
+      Direction.entries
+        .forEach { direction ->
+          val point = candidate.position.move(direction)
+          if (point in pattern) {
+            queue += State(point, candidate.steps + direction.arrow)
+          }
+        }
+    }
+  }
+
+  override fun solvePart2(input: String): Long {
+    TODO()
+  }
+}

2024/src/test/kotlin/aoc/year2024/Day21Test.kt

@@ -0,0 +1,42 @@
+package aoc.year2024
+
+import aoc.library.solvePart1
+import aoc.library.solvePart2
+import io.kotest.matchers.longs.shouldBeExactly
+import org.junit.jupiter.api.Disabled
+import org.junit.jupiter.api.Test
+
+class Day21Test {
+
+  @Test
+  @Disabled
+  fun part1TestInput() {
+    Day21.solvePart1(
+      """
+      029A
+      980A
+      179A
+      456A
+      379A
+      """.trimIndent(),
+    ) shouldBeExactly 42
+  }
+
+  @Test
+  @Disabled
+  fun part1() {
+    Day21.solvePart1() shouldBeExactly 42
+  }
+
+  @Test
+  @Disabled
+  fun part2TestInput() {
+    Day21.solvePart2() shouldBeExactly 42
+  }
+
+  @Test
+  @Disabled
+  fun part2() {
+    Day21.solvePart2() shouldBeExactly 42
+  }
+}