Add solution for Day 2

src/Day02.kt

@@ -0,0 +1,102 @@
+import java.lang.IllegalArgumentException
+
+fun main() {
+    fun part1(input: List<String>): Int {
+        var total = 0
+        input.forEach { line ->
+            val (opponent, me) = line.split(" ").map { it.toMove() }
+            val winLossAdjustment = if (opponent beats me) {
+                0
+            } else if (me == opponent) {
+                3
+            } else {
+                6
+            }
+            total += winLossAdjustment + me.ordinal + 1
+        }
+        return total
+    }
+
+    fun part2(input: List<String>): Int {
+        var total = 0
+        input.forEach { line ->
+            val (first, second) = line.split(" ")
+            val opponent = first.toMove()
+            val strategy = second.toStrategy()
+            val me = when(strategy) {
+                Strategy.LOSE -> opponent.win()
+                Strategy.TIE -> opponent.tie()
+                Strategy.WIN -> opponent.lose()
+            }
+            total += (strategy.ordinal * 3) + me.ordinal + 1
+        }
+        return total
+    }
+
+    val testInput = readInput("Day02_test")
+    check(part1(testInput) == 15)
+    check(part2(testInput) == 12)
+
+    val input = readInput("Day02")
+    println(part1(input))
+    println(part2(input))
+}
+
+enum class Moves {
+    ROCK {
+        override fun beats(other: Moves): Boolean = other == SCISSORS
+        override fun win(): Moves = SCISSORS
+        override fun lose(): Moves = PAPER
+    },
+    PAPER {
+        override fun beats(other: Moves): Boolean = other == ROCK
+        override fun win(): Moves = ROCK
+        override fun lose(): Moves = SCISSORS
+    },
+    SCISSORS {
+        override fun beats(other: Moves): Boolean = other == PAPER
+        override fun win(): Moves = PAPER
+        override fun lose(): Moves = ROCK
+    };
+
+    /**
+     * Check whether this move would beat the other move
+     */
+    abstract infix fun beats(other: Moves): Boolean
+
+    /**
+     * Return the move that this move would win against
+     */
+    abstract fun win(): Moves
+
+    /**
+     * Return the move this move would tie with
+     */
+    fun tie(): Moves = this
+
+    /**
+     * Return the move this move would lose against
+     */
+    abstract fun lose(): Moves
+}
+
+enum class Strategy {
+    LOSE,
+    TIE,
+    WIN,
+}
+
+fun String.toStrategy() = when (this) {
+    "X" -> Strategy.LOSE
+    "Y" -> Strategy.TIE
+    "Z" -> Strategy.WIN
+    else -> throw IllegalArgumentException("Invalid strategy: $this")
+}
+
+fun String.toMove(): Moves = when (this) {
+    "A", "X" -> Moves.ROCK
+    "B", "Y" -> Moves.PAPER
+    "C", "Z" -> Moves.SCISSORS
+    else -> throw IllegalArgumentException("Invalid move: $this")
+}
+