GuessGame.java

final class GuessGame {
  public static void main(String[] args) {
    GuessGame game = new GuessGame(new SimpleGame(13, 0, 100));
    System.out.println(game.play());
  }

  private static final class SimpleGame implements GameSetup {
    private final int known;
    private final int lowerBound;
    private final int upperBound;

    SimpleGame(int known, int lowerBound, int upperBound) {
      this.known = known;
      this.lowerBound = lowerBound;
      this.upperBound = upperBound;
    }

    @Override public GameSetup.GuessResult validate(int guess) {
      if (guess == known) {
        return GameSetup.GuessResult.CORRECT;
      }
      if (guess < known) {
        return GameSetup.GuessResult.TOO_LOW;
      }
      return GameSetup.GuessResult.TOO_HIGH;
    }

    @Override public int lowerBound() {
      return lowerBound;
    }

    @Override public int upperBound() {
      return upperBound;
    }
  }

  interface GameSetup {
    enum GuessResult {
      CORRECT, TOO_LOW, TOO_HIGH
    }

    GuessResult validate(int guess);

    int lowerBound();

    int upperBound();
  }

  private final GameSetup setup;

  GuessGame(GameSetup setup) {
    this.setup = setup;
  }

  int play() {
    return guess(0, setup.lowerBound(), setup.upperBound());
  }

  private int guess(int guessCount, int lowerBound, int upperBound) {
    int guess = lowerBound + (upperBound - lowerBound) / 2;
    guessCount++;
    GameSetup.GuessResult result = setup.validate(guess);
    switch (result) {
      case CORRECT:
        return guessCount;
      case TOO_LOW:
        if (guess == upperBound) {
          throw new IllegalStateException("GuessValidator must be wrong. Cannot guess above: "
              + upperBound
              + ". Exhausted "
              + guessCount
              + " guesses.");
        }
        return guess(guessCount, guess + 1, upperBound);
      case TOO_HIGH:
        if (guess == lowerBound) {
          throw new IllegalStateException("GuessValidator must be wrong. Cannot guess below: "
              + lowerBound
              + ". Exhausted "
              + guessCount
              + " guesses.");
        }
        return guess(guessCount, lowerBound, guess - 1);
      default:
        throw new AssertionError("Unimplemented GuessResult: " + result);
    }
  }
}