Android Open Source - android-tic-tac-toe Alpha Beta Search

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License

The source code is released under:

MIT License

Java Source Code

package org.shaon.android.tictactoe.model;
//from w w  w  .  ja  v  a 2s  .c o m
import java.util.ArrayList;
import java.util.List;

import org.shaon.android.tictactoe.model.State.Player;

public class AlphaBetaSearch implements SearchAlgorithm {

  /**
   * Player configuration object.
   */
  private PlayerConfig playerConfig;

  /**
   * Contracts a AlphaBetaSearch object.
   */
  public AlphaBetaSearch() {
    playerConfig = new PlayerConfig();
  }

  /**
   * Set the {@link Player} instance for artificially intelligent player.
   * Delegates {@link PlayerConfig#setAiPlayer(Player)} 
   * 
   * @param aiPlayer Player instance to set. 
   */
  public void setAiPlayer(Player aiPlayer) {
    playerConfig.setAiPlayer(aiPlayer);
  }
  
  /**
   * Get player configuration object.
   * 
   * @return Player configuration object
   */
  public PlayerConfig getPlayerConfig() {
    return playerConfig;
  }
  
  
  /**
   * <pre>alphaBetaSearch
   * function ALPHA-BETA-SEARCH(state) returns an action
   *     inputs: state, current state in game
   * 
   *     v <- MAX-VALUE (state, -INF, +INF)
   *     
   *     return the action in SUCCESSORS(state) with value v
   * </pre>
   * 
   * @param state
   * @return
   */
  public List<ActionState> alphaBetaSearch(State state){
    
    if(state.isFullyEmpty()) {
      return state.successorList(playerConfig.getAiPlayer());
    }
    
    int v = maxValue(state, Integer.MIN_VALUE, Integer.MAX_VALUE);
    
    List<ActionState> maxActionStates = new ArrayList<ActionState>();
    
    for(ActionState as : state.successorList(playerConfig.getAiPlayer())) {
      if(as.getState().getSateValue() == v){
        maxActionStates.add(as);
      }
    }
    
    return maxActionStates;
  }
  
  /**
   * <pre>
   * 
   * function MAX-VALUE (state, a, b) returns a utility value
   * 
   *     input:
   *       state, current state in game
   *       a, the value of the best alternative for MAX along the path to state
   *       b, the value of the best alternative for MIN along the path to state
   * 
   *     if TERMINAL-TEST(state) then return UTILITY(state)
   * 
   *     v <- -INF
   *     for a, s in SUCCESSORS(state) do
   *       v <- MAX(v, MIN-VALUE(s, a, b))
   *       if v >= b then return v
   *       a <- MAX(a, v)
   * 
   *     return v
   * </pre>
   * 
   * @param state
   * @return Maximum value among succor list of state.
   */

  private int maxValue(State state, int a, int b) {
    
    TerminatingCondition tc = state.checkFinished();
    if(tc != null) {
      return tc.utility(playerConfig.getAiPlayer());
    }
    
    int v = Integer.MIN_VALUE;
    
    for(ActionState as : state.successorList(playerConfig.getAiPlayer())) {
      int minValue = minValue(as.getState(), a, b);
      as.getState().setSateValue(minValue);
      v = Math.max(v, minValue);
      if (v >= b) {
        return v;
      }
      a = Math.max(a, v);
    }
    
    return v;
  }
  
  /**
   * <pre>
   * 
   * function MIN-VALUE (state, a, b) returns a utility value
   * 
   *     input:
   *       state, current state in game
   *       a, the value of the best alternative for MAX along the path to state
   *       b, the value of the best alternative for MIN along the path to state
   * 
   *     if TERMINAL-TEST(state) then return UTILITY(state)
   * 
   *     v <- +INF
   * 
   *     for a, s in SUCCESSORS(state) do
   *       v <- MIN(v, MAX-VALUE(s, a, b))
   * 
   *       if v <= a then return v
   *       b <- MIN(b, v)
   * 
   *     return v
   * </pre>
   * 
   * @param state 
   * @return Minimum value among succor list of state.
   */
  
  private int minValue(State state, int a, int b){
    
    TerminatingCondition tc = state.checkFinished();
    if(tc != null) {
      return tc.utility(playerConfig.getAiPlayer());
    }
    
    int v = Integer.MAX_VALUE;
    
    for(ActionState as : state.successorList(playerConfig.getHumanPlayer())) {
      int maxValue = maxValue(as.getState(), a, b);
      as.getState().setSateValue(maxValue);
      v = Math.min(v, maxValue);
      
      if( v <= a) {
        return v;
      }
      b = Math.min(b, v);
    }
    
    return v;
  }

  @Override
  public List<ActionState> search(State state) {
    return alphaBetaSearch(state);
  }
}

Java Source Code List

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