negamax_simple_example.cc

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/*
                          Aleph_w
 
  Data structures & Algorithms
  version 2.0.0b
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
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  The above copyright notice and this permission notice shall be included in all
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  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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*/
 
# include <iostream>
 
# include <State_Search.H>
# include <tpl_array.H>
 
using Aleph::Array;
 
namespace
{
 
namespace Search = Aleph::Search;
 
// --- State -----------------------------------------------------------------
 
struct TicTacToeState
{
  Array<int> board;   // 0 = empty, +1 = X, -1 = O
  int player = 1;     // side to move (+1 or -1)
  size_t moves_played = 0;
 
  TicTacToeState() : board(size_t(9), 0) {}
};
 
// --- Domain ----------------------------------------------------------------
 
class TicTacToeDomain
{
public:
  struct Move
  {
    size_t cell = 0;
    bool operator==(const Move &) const noexcept = default;
  };
 
  using State = TicTacToeState;
  using Score = int;
 
  bool is_terminal(const State &state) const
  {
    return winner(state) != 0 or state.moves_played == 9;
  }
 
  Score evaluate(const State &state) const
  {
    const int w = winner(state);
    if (w != 0)
      return w == state.player ? 100 : -100;
    return 0;  // draw
  }
 
  void apply(State &state, const Move &move) const
  {
    state.board[move.cell] = state.player;
    state.player = -state.player;
    ++state.moves_played;
  }
 
  void undo(State &state, const Move &move) const
  {
    --state.moves_played;
    state.player = -state.player;
    state.board[move.cell] = 0;
  }
 
  template <typename Visitor>
  bool for_each_successor(const State &state, Visitor visit) const
  {
    for (size_t i = 0; i < 9; ++i)
      if (state.board[i] == 0 and not visit(Move{i}))
        return false;
    return true;
  }
 
private:
  [[nodiscard]] static int winner(const State &state) noexcept
  {
    static constexpr size_t lines[8][3] = {
      {0, 1, 2}, {3, 4, 5}, {6, 7, 8},
      {0, 3, 6}, {1, 4, 7}, {2, 5, 8},
      {0, 4, 8}, {2, 4, 6}
    };
    for (const auto &line : lines)
      {
        const int a = state.board[line[0]];
        if (a != 0 and a == state.board[line[1]] and a == state.board[line[2]])
          return a;
      }
    return 0;
  }
};
 
// --- Display ---------------------------------------------------------------
 
void print_board(const TicTacToeState &state)
{
  for (size_t row = 0; row < 3; ++row)
    {
      for (size_t col = 0; col < 3; ++col)
        {
          const int cell = state.board[row * 3 + col];
          std::cout << (cell == 1 ? 'X' : cell == -1 ? 'O' : '.');
        }
      std::cout << '\n';
    }
}
 
} // end anonymous namespace
 
int main()
{
  TicTacToeDomain domain;
  TicTacToeState root;
 
  // Run plain Negamax to full depth (no limits needed for a 9-ply game).
  auto result = Search::negamax_search(domain, root);
 
  std::cout << "Tic-Tac-Toe — simple Negamax example\n\n";
 
  print_board(root);
  std::cout << '\n';
 
  std::cout << "Game-theoretic value: " << result.value
            << " (0 = draw)\n";
  if (result.has_principal_variation())
    std::cout << "Best opening move:   cell " << result.first_move().cell << '\n';
  else
    std::cout << "Best opening move:   no principal variation\n";
  std::cout << "Visited states:      " << result.stats.visited_states
            << '\n';
  std::cout << "Heuristic evals:     " << result.stats.heuristic_evaluations
            << '\n';
 
  // Replay the principal variation.
  std::cout << "\nPrincipal variation (" << result.principal_variation.size()
            << " moves):\n";
  TicTacToeState replay = root;
  for (size_t i = 0; i < result.principal_variation.size(); ++i)
    {
      const auto &move = result.principal_variation[i];
      domain.apply(replay, move);
      std::cout << "  Move " << i + 1 << ": cell " << move.cell
                << " by " << (replay.player == -1 ? 'X' : 'O') << '\n';
    }
 
  std::cout << "\nFinal board:\n";
  print_board(replay);
 
  return 0;
}