/**
* Does a depth limited minimax decision search.
*
* @param brd current board state
* @param currDepth current depth in the search
* @param mvStack new ArrayList
* @return move recommendation
*/
private ScoredBreakthroughMove minimax(
EqualBreakthroughState brd, int currDepth, ArrayList<ScoredBreakthroughMove> mvStack) {
if (currDepth >= mvStack.size()) {
mvStack.add(new ScoredBreakthroughMove());
} // auto grow
boolean toMaximize = (brd.getWho() == GameState.Who.HOME);
boolean isTerminal = terminalValue(brd, mvStack.get(currDepth));
if (isTerminal) {;
} else if (currDepth == depthLimit) {
mvStack.get(currDepth).set(evalBoard(brd));
} else {
double bestScore = (toMaximize ? Double.NEGATIVE_INFINITY : Double.POSITIVE_INFINITY);
ScoredBreakthroughMove bestMove = mvStack.get(currDepth);
if ((currDepth + 1) >= mvStack.size()) {
mvStack.add(new ScoredBreakthroughMove());
} // auto grow
ScoredBreakthroughMove nextMove = mvStack.get((currDepth + 1));
bestMove.set(bestScore);
ArrayList<ScoredBreakthroughMove> possMvs = getPossibleMoves(brd);
Collections.shuffle(possMvs);
for (int i = 0; i < possMvs.size(); i++) {
BreakthroughMove currMv = possMvs.get(i);
// copy original occupant
char curOccupant = brd.board[currMv.endingRow][currMv.endingCol];
// Make move on board
brd.makeMove(currMv);
// Check out worth of this move
minimax(brd, currDepth + 1, mvStack);
// Undo the move
brd.undoMove(currMv, curOccupant);
// Check out the results, relative to what we've seen before
if (toMaximize && nextMove.score > bestMove.score) {
bestMove.set(
currMv.startRow, currMv.startCol, currMv.endingRow, currMv.endingCol, nextMove.score);
} else if (!toMaximize && nextMove.score < bestMove.score) {
bestMove.set(
currMv.startRow, currMv.startCol, currMv.endingRow, currMv.endingCol, nextMove.score);
}
}
}
return mvStack.get(currDepth);
}
/**
* Is the game complete at this brd state? If it is, the evaluation values for these boards is
* recorded (i.e., 0 for a draw +X, for a HOME win and -X for an AWAY win.
*
* @param brd Breakthrough board to be examined
* @param mv where to place the score information; move is irrelevant
* @return true if the brd is in a terminal state
*/
protected boolean terminalValue(GameState brd, ScoredBreakthroughMove mv) {
GameState.Status status = brd.getStatus();
boolean isTerminal = true;
if (status == GameState.Status.HOME_WIN) {
mv.set(MAX_EVAL_SCORE);
} else if (status == GameState.Status.AWAY_WIN) {
mv.set(-MAX_EVAL_SCORE);
} else if (status == GameState.Status.DRAW) {
mv.set(0);
} else {
isTerminal = false;
}
return isTerminal;
}
/**
* Get an ArrayList of all possible moves that we can make given state.
*
* @param state
* @return
*/
protected ArrayList<ScoredBreakthroughMove> getPossibleMoves(GameState state) {
BreakthroughState board = (BreakthroughState) state;
ArrayList<ScoredBreakthroughMove> list = new ArrayList<ScoredBreakthroughMove>();
ScoredBreakthroughMove mv = new ScoredBreakthroughMove();
int dir = state.who == GameState.Who.HOME ? +1 : -1;
for (int r = 0; r < BreakthroughState.N; r++) {
for (int c = 0; c < BreakthroughState.N; c++) {
mv.startRow = r;
mv.startCol = c;
mv.endingRow = r + dir;
mv.endingCol = c;
if (board.moveOK(mv)) {
list.add((ScoredBreakthroughMove) mv.clone());
}
mv.endingRow = r + dir;
mv.endingCol = c + 1;
if (board.moveOK(mv)) {
list.add((ScoredBreakthroughMove) mv.clone());
}
mv.endingRow = r + dir;
mv.endingCol = c - 1;
if (board.moveOK(mv)) {
list.add((ScoredBreakthroughMove) mv.clone());
}
}
}
return list;
}