//calculate the heuristic value of a game state with respect to a specific player
int GameHeuristic( const GameState& game, int forwhom, const int* heuristic ) const
{
int result;
result = ChangeHeuristic(game.GetPlayerChange( forwhom ), heuristic );
int best = std::numeric_limits<int>::min();
for(int i=0;i!=game.GetPlayerCount();++i){
if (i!=forwhom){
int score = ChangeHeuristic( game.GetPlayerChange( i ), heuristic );
if (score>best){
best = score;
}
}
}
return result - best;
}
//Get Best Move -
//recursive min/max algorithm explores game states and
//returns a move and its score, for a specific player
MoveScore GetBestMove( const GameState& theGame, const int player_index, int abPrune, const int recursive=1)
{
const Change& player_change = theGame.GetPlayerChange( theGame.GetCurrentPlayer() );
const Change& pool = theGame.GetGameChange();
int bestScore = (player_index == theGame.GetCurrentPlayer())?
std::numeric_limits<int>::min()
:
std::numeric_limits<int>::max();
Coin bestCoin;
Change bestChange;
Change::List* someChange;
if (recycledChangeLists.empty()){
someChange = new Change::List;
}
else{
someChange = recycledChangeLists.back();
recycledChangeLists.pop_back();
}
//for each coin that can be played
for (const Coin *ci=&COINLIST[0];ci!=&COINLIST[COIN_COUNT];++ci){
if (player_change.GetCount(*ci) > 0){
//get all possible sets of change into array
GetAllPossibleChange(
pool,
*ci,
*someChange);
//for each possible set of change
//(reversed iteration improves AB prune by as much as 70%)
const Change::List::reverse_iterator end = someChange->rend();
for(Change::List::reverse_iterator i = someChange->rbegin();i!=end;++i){
//create "move" from coin to give, and change to take
const Move myMove(*ci,*i);
//create gamestate once the move has been played
GameState newGame = theGame.PlayMove( myMove );
//get "score" for this move
int score;
if (recursive>0 && newGame.GetActivePlayers()>1){
//recursivly examine move, get its score
score = GetBestMove( newGame, player_index, bestScore, recursive-1).score;
}
else{
//leaf node - just get heuristic value
score = GameHeuristic( newGame, player_index, myHeuristic );
}
//Min/Max search
if ((player_index == theGame.GetCurrentPlayer() && score > bestScore) ||
(player_index != theGame.GetCurrentPlayer() && score < bestScore))
{
bestScore = score;
bestCoin = *ci;
bestChange = *i;
//Alpha/Beta pruning
if ((player_index == theGame.GetCurrentPlayer() && score > abPrune) ||
(player_index != theGame.GetCurrentPlayer() && score < abPrune)){
goto exit;
}
}
}
}
}
exit:
recycledChangeLists.push_back( someChange );
return MoveScore( Move( bestCoin, bestChange ), bestScore );
}