Introduction

TODO: Introduce game rules and related work.

Notations

Let N be the number of pegs in the code and C be the number of possible peg colors. A code is a vector in \{0, \ldots, C - 1\}^N. The number of possible codes is M = C^N. The game is played as follows:

1. The codemaker chooses a secret code c.
2. The codebreaker tries to guess c in as few as possible iterations. At each iteration t :

• The codebreaker produces a guess code g_t.
• The codemaker provides a feedback f(c, g_t) = \langle b(c,g_t), w(c, g_t) \rangle where b is the number of pegs which are correct in both color and position and w is the number of pegs which are correct in color but not in position.

3. The game ends when the codebreaker guesses the secret code (gets feedback \langle N, 0 \rangle) or when a predefined maximum number of iterations T is reached.

The notion of "correct in color but not in position" is a bit tricky because color repetitions are allowed. To avoid ambiguity, here is a possible feedback implementation:

int[] feedback(int[] code, int[] guess) {
  int[] vCode = new int[C];  // count vector of colors in code
  int[] vGuess = new int[C]; // count vector of colors in guess
  int black = 0;
  for (int i = 0; i < N; i++) {
    if (code[i] == guess[i]) {
      black++;
    } else {
      vCode[code[i]]++;
      vGuess[guess[i]]++;
    }
  }
  int white = 0;
  for (int c = 0; c < C; c++) {
    white += min(vCode[c], vGuess[c]);
  }
  int[] f = {black, white};
  return f;
}

Implementation details

For performance reasons, we represent codes as integers between 0 and M - 1. Writing the code in base C we obtain the vector of peg colors. For example, in the classic settings N = 4 and C = 6, the code 0123 is represented by 0 \times 6^3 + 1 \times 6^2 + 2 \times 6^1 + 3 \times 6^0 = 51.

The feedbacks are also encoded as integers between 0 and F - 1, where F = \frac{(N + 1)(N + 2)}{2}. To reduce the size of arrays indexed by the possible feedbacks, we use the fact that b + w \le N. Here is an example of encoding for N = 3.

Using this encoding, feedback 0 always means that the codebreaker guessed the code. According to the values of N and C, some of the feedbacks above are impossible. For example, the codebreaker can never get a feedback \langle N - 1, 1 \rangle. This encoding tries to find a good trade-off between implementation simplicity and memory economy.

The class CodeToolkit makes these encodings more or less transparent. The main method is int feedback(int code, int guess) For performance reasons, feedbacks are precomputed and stored in a M \times M symmetrical feedbacks matrix. The class also contains some methods for encoding, decoding and displaying codes and feedbacks.

Exhaustive strategies

Evolution

References