/** Does a 1-ply search.
For each move in the consider set, if the move is a win, returns
true and the move. If the move is a loss, prune it out of the
consider set if there are non-losing moves in the consider set.
If all moves are losing, perform no pruning, search will resist.
Returns true if there is a win, false otherwise.
@todo Is it true that MoHex will resist in the strongest way
possible?
*/
bool MoHexPlayer::PerformPreSearch(HexBoard& brd, HexColor color,
bitset_t& consider, double maxTime,
PointSequence& winningSequence)
{
bitset_t losing;
HexColor other = !color;
PointSequence seq;
bool foundWin = false;
SgTimer elapsed;
Resistance resist;
resist.Evaluate(brd);
std::vector<HexPoint> moves;
SortConsiderSet(consider, resist, moves);
for (std::size_t i = 0; i < moves.size() && !foundWin; ++i)
{
if (elapsed.GetTime() > maxTime)
{
LogInfo() << "PreSearch: max time reached "
<< '(' << i << '/' << moves.size() << ").\n";
break;
}
brd.PlayMove(color, moves[i]);
seq.push_back(moves[i]);
if (EndgameUtil::IsLostGame(brd, other)) // Check for winning move
{
winningSequence = seq;
foundWin = true;
}
else if (EndgameUtil::IsWonGame(brd, other))
losing.set(moves[i]);
seq.pop_back();
brd.UndoMove();
}
// Abort if we found a one-move win
if (foundWin)
return true;
// Backing up cannot cause this to happen, right?
BenzeneAssert(!EndgameUtil::IsDeterminedState(brd, color));
// Use the backed-up ice info to shrink the moves to consider
if (m_backup_ice_info)
{
bitset_t new_consider
= EndgameUtil::MovesToConsider(brd, color) & consider;
if (new_consider.count() < consider.count())
{
consider = new_consider;
LogFine() << "$$$$$$ new moves to consider $$$$$$"
<< brd.Write(consider) << '\n';
}
}
// Subtract any losing moves from the set we consider, unless all of them
// are losing (in which case UCT search will find which one resists the
// loss well).
if (losing.any())
{
if (BitsetUtil::IsSubsetOf(consider, losing))
LogInfo() << "************************************\n"
<< " All UCT root children are losing!!\n"
<< "************************************\n";
else
{
LogFine() << "Removed losing moves: " << brd.Write(losing) << '\n';
consider = consider - losing;
}
}
BenzeneAssert(consider.any());
LogInfo() << "Moves to consider:\n" << brd.Write(consider) << '\n';
return false;
}
