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BATTLESHIP SIMULATION
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| val random = java.util.Random() | |
| data class TorpedoAttack(val source: Int, val target: Int) | |
| // CommandCenter interface with the default behaviour | |
| interface CommandCenter { | |
| fun fireTorpedo(ships: BooleanArray): TorpedoAttack { | |
| // Flagship is marked as Boolean TRUE | |
| val flagship = ships.indexOfFirst { it } | |
| // Fire torpedo from flagship at random target | |
| return TorpedoAttack(flagship, random.nextInt(5)) | |
| } | |
| fun guideTorpedo(attack: TorpedoAttack): Int { | |
| // Return either -1, 0, 1 to move torpedo a bit | |
| return random.nextInt(3) - 1 | |
| } | |
| fun onTorpedoDetected(attack: TorpedoAttack) { | |
| // Triggered when the opponent fires his torpedo | |
| } | |
| } | |
| fun simulate(): Pair<Boolean, Boolean> { | |
| // Initialize command centers | |
| val yourCC = YourCommandCenter() | |
| val enemyCC = EnemyCommandCenter() | |
| // Initialize battleships | |
| val yourShips = BooleanArray(5) | |
| val enemyShips = BooleanArray(5) | |
| // Randomly place flagships | |
| yourShips[random.nextInt(yourShips.size)] = true | |
| enemyShips[random.nextInt(yourShips.size)] = true | |
| // Your fire first | |
| val yourAttack = yourCC.fireTorpedo(yourShips) | |
| enemyCC.onTorpedoDetected(yourAttack) | |
| // Enemy fires second | |
| val enemyAttack = enemyCC.fireTorpedo(enemyShips) | |
| yourCC.onTorpedoDetected(enemyAttack) | |
| // You guide your torpedo | |
| val yourTarget = yourAttack.target + yourCC.guideTorpedo(yourAttack) % 2 | |
| // Enemy guides his torpedo | |
| val enemyTarget = enemyAttack.target + enemyCC.guideTorpedo(enemyAttack) % 2 | |
| // Check if each side's flagship has been hit | |
| // Note: Both players can be winners at the same time | |
| return Pair( | |
| yourShips.getOrElse(enemyTarget) { false }, | |
| enemyShips.getOrElse(yourTarget) { false } | |
| ) | |
| } | |
| fun main(args: Array<String>) { | |
| var yourWins = 0 | |
| var enemyWins = 0 | |
| for (i in 1..100000) { | |
| val (enemyWon, youWon) = simulate() | |
| if (youWon) yourWins++ | |
| if (enemyWon) enemyWins++ | |
| } | |
| val ratio = yourWins.toDouble() / enemyWins.toDouble() | |
| println("Your wins to enemy's wins ratio is $ratio") | |
| if (ratio < 1.0) println("You lost! Improve your battle logic!") | |
| else if (ratio < 1.25) println("You won by a small margin! Can you do better?") | |
| else println("Congratulations! Your victory is undeniable. Good job!") | |
| } | |
| class EnemyCommandCenter : CommandCenter { | |
| var attackedShip = -1 | |
| override fun fireTorpedo(ships: BooleanArray): TorpedoAttack { | |
| // Enemy has overridden this method to hide his flagship | |
| val flagship = ships.indexOfFirst { it } | |
| var source: Int | |
| do source = random.nextInt(5) while (source == flagship || source == attackedShip) | |
| return TorpedoAttack(source, random.nextInt(5)) | |
| } | |
| override fun guideTorpedo(attack: TorpedoAttack): Int { | |
| // Ensure that torpedo doesn't sway off and always hits at least some ship | |
| return if (attack.target in 1..3) random.nextInt(3) - 1 else 0 | |
| } | |
| override fun onTorpedoDetected(attack: TorpedoAttack) { | |
| attackedShip = attack.target | |
| } | |
| } | |
| class YourCommandCenter : CommandCenter { | |
| /** YourCommandCenter takes control over random = java.util.Random(), | |
| * used to generate the position of flagships | |
| * Therefore, it's possible to predict which values will be generated | |
| * | |
| * weak spot > val random = java.util.Random() | |
| * > enemyShips[random.nextInt(yourShips.size)] = true | |
| */ | |
| /*initializes the seed of random once an instance of the class is created | |
| * even before the first call by the enemy */ | |
| init { | |
| val tmp = java.util.Random().nextLong() | |
| //converting Long to 48-bit seed | |
| val seed = (tmp shl 48) - 1 | |
| //set seed | |
| random.setSeed(seed) | |
| //simulate calls made in simulate() | |
| random.nextInt() | |
| vars.nextRand = random.nextInt(5) | |
| //set the seed again as it has been changed by previous calls | |
| random.setSeed(seed) | |
| } | |
| object vars { | |
| var nextRand: Int? = null | |
| } | |
| override fun fireTorpedo(ships: BooleanArray): TorpedoAttack { | |
| // Flagship is marked as Boolean TRUE | |
| val flagship = ships.indexOfFirst { it } | |
| //Predicted value generated by nextInt() | |
| val target = vars.nextRand!! | |
| return TorpedoAttack(flagship, target) | |
| } | |
| //no need to guide the torpedo - the position of flagship is known | |
| override fun guideTorpedo(attack: TorpedoAttack): Int = 0 | |
| } |
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