// Alpha beta searching with transposition table
private double AlphaBeta(int[,] board, int color, int depth = 0, double alpha = double.NegativeInfinity, double beta = double.PositiveInfinity)
{
nodeCount += 1;
evaluator.SetBoard(board);
// Return evaluation value if reaching depth = depthMax or terminal node
if (depth >= currentDepthMax)
{
return(evaluator.Evaluate(selfColor));
}
// Return evaluation when game ends
List <Pos> newOptions = evaluator.Availables(color);
List <Pos> oppNewOptions = evaluator.Availables(StoneColor.OppColor(color));
if (newOptions.Count == 0 && oppNewOptions.Count == 0)
{
int selfStones = evaluator.CountStones(selfColor);
int oppStones = evaluator.CountStones(StoneColor.OppColor(selfColor));
if (selfStones > oppStones)
{
return(double.PositiveInfinity);
}
else if (selfStones < oppStones)
{
return(double.NegativeInfinity);
}
else
{
return(evaluator.Evaluate(selfColor));
}
}
// When only the opponent can put stone, go to next depth
if (newOptions.Count == 0)
{
depth += 1;
color = StoneColor.OppColor(color);
newOptions = oppNewOptions;
}
// Expand board and store the all child boards in children list
// Associate the child and the action of that time
List <int[, ]> children = new List <int[, ]>();
Dictionary <int[, ], Pos> actionChildTable = new Dictionary <int[, ], Pos>();
foreach (Pos action in newOptions)
{
Board childBoard = new Board();
childBoard.SetBoard(board);
childBoard.UpdateBoard(action, color);
children.Add(childBoard.GetBoard());
actionChildTable.Add(childBoard.GetBoard(), action);
}
// Sort children in order of the score
// In descending order when self turn and in ascending order when opponent turn
st.Start();
if (depth <= 3)
{
children = OrderBoards(children, color);
}
st.Stop();
// Alpha beta searching
if (color == selfColor)
{
// In self turn, search max value of children
double score = double.NegativeInfinity;
foreach (int[,] child in children)
{
// Check if the child is stored in transposition table and the node type is EXACT
// If it does, set the value for the score
// If not, start alpha-beta-searching in next depth and store the score
string childHash = BoardToHash(child);
if (transpositionTable.ContainsKey(childHash) && transpositionTable[childHash].Depth >= currentDepthMax && transpositionTable[childHash].NodeType == "EXACT")
{
transpositionCutCount += 1;
score = transpositionTable[childHash].Score;
}
else
{
score = AlphaBeta(child, StoneColor.OppColor(color), depth + 1, alpha, beta);
transpositionTable[childHash] = new TranspositionTableEntry(child, currentDepthMax, score);
}
if (score > alpha)
{
alpha = score;
// Get best action
if (depth == 0)
{
foreach (KeyValuePair <int[, ], Pos> kvp in actionChildTable)
{
if (kvp.Key.Cast <int>().SequenceEqual(child.Cast <int>()))
{
bestAction = kvp.Value;
}
}
}
}
// Beta cut
if (alpha >= beta)
{
betaCutCount += 1;
break;
}
}
return(alpha);
}
else
{
// If the opponent turn, search minimum value of children
double score = double.PositiveInfinity;
foreach (int[,] child in children)
{
string childHash = BoardToHash(child);
if (transpositionTable.ContainsKey(childHash) && transpositionTable[childHash].Depth >= currentDepthMax && transpositionTable[childHash].NodeType == "EXACT")
{
transpositionCutCount += 1;
score = transpositionTable[childHash].Score;
}
else
{
score = AlphaBeta(child, StoneColor.OppColor(color), depth + 1, alpha, beta);
transpositionTable[childHash] = new TranspositionTableEntry(child, currentDepthMax, score);
}
beta = Math.Min(beta, score);
// Alpha cut
if (beta <= alpha)
{
alphaCutCount += 1;
break;
}
}
return(beta);
}
}
// Whether the stone at the position is imreversible
bool IsConfirmed(Pos pos, int color)
{
// Return false if there is not self stone at the position
if (board[pos.y, pos.x] != color)
{
return(false);
}
// Check if the position is connected to the corners only with own stones
// Vertical line
// (0, pos.x) ~ (pos.y-1, pos.x) are self stones or (pos.y+1, pos.x) ~ (boarSize-1, pos.x) are self stones
if (pos.x == 0 || pos.x == boardSize - 1)
{
bool upperFlag = true;
bool lowerFlag = true;
for (int j = 0; j < pos.y; j++)
{
if (board[j, pos.x] != color)
{
upperFlag = false;
break;
}
}
for (int j = pos.y + 1; j < boardSize; j++)
{
if (board[j, pos.x] != color)
{
lowerFlag = false;
break;
}
}
return(upperFlag || lowerFlag);
}
// Horizontal line
// (pos.y, 0) ~ (pos.y, pos.x-1) are self stones or (pos.y, pos.x+1) ~ (pos.y, boardSize-1) are self stones
if (pos.y == 0 || pos.y == boardSize - 1)
{
bool leftFlag = true;
bool rightFlag = true;
for (int i = 0; i < pos.x; i++)
{
if (board[pos.y, i] != color)
{
leftFlag = false;
break;
}
}
for (int i = pos.x + 1; i < boardSize; i++)
{
if (board[pos.y, i] != color)
{
rightFlag = false;
break;
}
}
return(leftFlag || rightFlag);
}
return(false);
}