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src/main/kotlin/rps/Game.kt

@@ -1,18 +1,15 @@
 package rps
 
-import kotlin.random.Random
-
 const val TURNS = 100
-val PLAYER_TWO_CHOICE = Move.ROCK
 
 fun main() {
   println("Outcome from Player 1’s perspective: ${playGame(TURNS)}")
 }
 
 /** Determine the [Outcome] of [first] vs [second] from the perspective of [first]. */
-fun determineOutcome(first: Move, second: Move): Outcome = when {
-  first.strongAgainst() == second -> Outcome.WIN
-  first == second -> Outcome.DRAW
+fun determineOutcome(first: Move, second: Move): Outcome = when(second) {
+  in first.strongAgainst() -> Outcome.WIN
+  first -> Outcome.DRAW
   else -> Outcome.LOSS
 }
 
@@ -24,15 +21,13 @@ fun List<Outcome>.calculateGameSummary() = GameSummary(
   losses = count { it == Outcome.LOSS },
 )
 
-fun playGame(turns: Int): GameSummary = generateSequence { randomMove() }
+fun playGame(turns: Int): GameSummary = generateSequence { randomMove() to randomMove() }
   .take(turns)
-  .map { determineOutcome(it, PLAYER_TWO_CHOICE) }
+  .map { (p1, p2) -> determineOutcome(p1, p2) }
   .toList()
   .calculateGameSummary()
 
-fun randomMove(): Move = Move.values().let { allMoves ->
-  allMoves[Random.nextInt(allMoves.size)]
-}
+fun randomMove(): Move = Move.values().random()
 
 /*
  * Classes and named tuples below this point.
@@ -43,13 +38,13 @@ fun randomMove(): Move = Move.values().let { allMoves ->
  * A possible move in a game of rock-paper-scissors.
  *
  * [strongAgainst] is a function for lazy evaluation because otherwise we can’t access SCISSORS before it’s been defined.
- * If the game needs to be expanded (e.g. the somewhat common “rock, paper, scissors, salamander, spock”),
- * this argument could be a List<Move> instead.
  */
-enum class Move(val strongAgainst: () -> Move) {
-  ROCK({ SCISSORS }),
-  PAPER({ ROCK }),
-  SCISSORS({ PAPER })
+enum class Move(val strongAgainst: () -> Set<Move>) {
+  ROCK({ setOf(SCISSORS, LIZARD) }),
+  PAPER({ setOf(ROCK, SPOCK) }),
+  SCISSORS({ setOf(PAPER, LIZARD) }),
+  LIZARD({ setOf(SPOCK, PAPER) }),
+  SPOCK({ setOf(SCISSORS, ROCK) }),
 }
 
 // purposely not calling this Result to avoid confusion with kotlin.Result and similar

src/test/kotlin/rps/GameTest.kt

@@ -13,6 +13,9 @@ class GameTest {
       Triple(ROCK, PAPER, LOSS),
       Triple(PAPER, SCISSORS, LOSS),
       Triple(ROCK, ROCK, DRAW),
+      Triple(ROCK, LIZARD, WIN),
+      Triple(LIZARD, SPOCK, WIN),
+      Triple(SPOCK, PAPER, LOSS),
     )
     for ((first, second, expected) in games) {
       assertEquals(determineOutcome(first, second), expected)