/// <summary>
/// Look up a position's hash in the table.
/// </summary>
/// <param name="hash">The hash to search for.</param>
/// <param name="entry">The found entry.</param>
/// <returns>True if the lookup was successful and the out-param contains the correct record.</returns>
public bool TryGetValue(ulong hash, out TTEntry entry) {
entry = table[GetIndex(hash)];
if (null == entry) return false;
if (entry.Hash == hash) return true;
return false;
}
/// <summary>
/// Submit an entry to the tablebase. It will decide for itself whether it keeps it or prefers the previous record.
/// </summary>
public void Write(TTEntry entry) {
//CURRENT STRATEGY: "Replace with higher depth"
int index = GetIndex(entry.Hash);
TTEntry oldEntry = table[index];
if ((null == oldEntry) || (entry.Depth > oldEntry.Depth)) table[index] = entry;
}
/// <summary>
/// Recursively calculates the value of the current position, up to the given depth.
/// </summary>
/// <param name="move">The last move. Win is checked in its neighborhood.</param>
/// <param name="depth">The remaining available depth.</param>
/// <param name="color">The color of the player to move. 1 ~ black, -1 ~ white.</param>
/// <param name="alpha">The lower boundary on "interesting" scores.</param>
/// <param name="beta">The upper boundary on "interesting" scores.</param>
private int negamax(Square move, int depth, int color, int alpha, int beta)
{
Square localBestMove = move;
int origAlpha = alpha;
//TT lookup
TTEntry ttEntry;
if (tt.TryGetValue(currentHash, out ttEntry))
{
localBestMove = ttEntry.BestMove; //for ordering purposes, just take the best move regardless of depth. For the rest, depth does matter.
if (ttEntry.Depth >= depth)
{
switch (ttEntry.Flag)
{
case TTFlag.Exact:
return(ttEntry.Score);
case TTFlag.Upper:
if (ttEntry.Score < beta)
{
beta = ttEntry.Score;
}
break;
case TTFlag.Lower:
if (ttEntry.Score > alpha)
{
alpha = ttEntry.Score;
}
break;
default:
break;
}
if (alpha >= beta)
{
return(ttEntry.Score);
}
}
}
//if state is terminal, return:
if (checkWin(move, color == -1))
{
return(-12345678); //someone won, so the score [calculated always for black] is +inf * (previous color). We return (color to move)*(score for black) --> always -inf
}
if (moveCount >= Gamestate.maxMoves)
{
return(0); //draw, return 0
}
if (depth <= 0)
{
int result = color * evaluate(); //not a win, not a draw, but reached maximum depth (0 remaining depth), return evaluation
return(result);
}
//otherwise keep searching children
int value = -12345678;
int candidateValue;
foreach (Square child in GenerateShuffledMoves(localBestMove))
{
if (stopSearch)
{
return(value); //do not visit any more children if search is stopped
}
incrementBoard(child, color == 1);
candidateValue = -negamax(child, depth - 1, -color, -beta, -alpha);
decrementBoard(child);
if (candidateValue > value)
{
value = candidateValue;
localBestMove = child;
}
if (value > alpha)
{
alpha = value;
}
if (alpha >= beta)
{
break;
}
}
if (stopSearch)
{
return(value); //do not update TT if search is stopped
}
//TT update
TTFlag flag;
if (value <= origAlpha)
{
flag = TTFlag.Upper;
}
else if (value >= beta)
{
flag = TTFlag.Lower;
}
else
{
flag = TTFlag.Exact;
}
ttEntry = new TTEntry(currentHash, localBestMove, depth, value, flag);
tt.Write(ttEntry);
return(value);
}
