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#include "chess.h"

#include "data.h"

/* last modified 05/08/14 */

/*

 *******************************************************************************

 *                                                                             *

 *   Search() is the recursive routine used to implement the alpha/beta        *

 *   negamax search (similar to minimax but simpler to code.)  Search() is     *

 *   called whenever there is "depth" remaining so that all moves are subject  *

 *   to searching.  Search() recursively calls itself so long as there is at   *

 *   least one ply of depth left, otherwise it calls Quiesce() instead.        *

 *                                                                             *

 *******************************************************************************

 */

int Search(TREE * RESTRICT tree, int alpha, int beta, int side, int depth,

    int ply, int do_null) {

  ROOT_MOVE temp_rm;

  uint64_t start_nodes = tree->nodes_searched;

  int first_tried = 0, moves_searched = 0, repeat = 0;

  int original_alpha = alpha, value = 0, t_beta = beta;

  int extend, reduce, max_reduce, i;

  int pv_node = alpha != beta - 1;

/*

 ************************************************************

 *                                                          *

 *  Step 1.  Check to see if we have searched enough nodes  *

 *  that it is time to peek at how much time has been used, *

 *  or if is time to check for operator keyboard input.     *

 *  This is usually enough nodes to force a time/input      *

 *  check about once per second, except when the target     *

 *  time per move is very small, in which case we try to    *

 *  check the time more frequently.                         *

 *                                                          *

 *  Note that we check input or time-out in thread 0.  This *

 *  makes the code simpler and eliminates some problematic  *

 *  race conditions.                                        *

 *                                                          *

 ************************************************************

 */

#if defined(NODES)

  if (--temp_search_nodes <= 0) {

    abort_search = 1;

    return 0;

  }

#endif

  if (tree->thread_id == 0) {

    if (--next_time_check <= 0) {

      next_time_check = nodes_between_time_checks;

      if (TimeCheck(tree, 1)) {

        abort_search = 1;

        return 0;

      }

      if (CheckInput()) {

        Interrupt(ply);

        if (abort_search)

          return 0;

      }

    }

  }

  if (ply >= MAXPLY - 1)

    return beta;

/*

 ************************************************************

 *                                                          *

 *  Step 2.  Check for draw by repetition, which includes   *

 *  50 move draws also.  This is the quickest way to get    *

 *  out of further searching, with minimal effort.  This    *

 *  and the next two steps are skipped for moves at the     *

 *  root (ply = 1).                                         *

 *                                                          *

 ************************************************************

 */

  if (ply > 1) {

    if ((repeat = Repeat(tree, ply))) {

      value = DrawScore(side);

      if (value < beta)

        SavePV(tree, ply, 0);

#if defined(TRACE)

      if (ply <= trace_level)

        printf("draw by repetition detected, ply=%d.\n", ply);

#endif

      return value;

    }

/*

 ************************************************************

 *                                                          *

 *  Step 3.  Check the transposition/refutation (hash)      *

 *  table to see if we have searched this position          *

 *  previously and still have the results available.  We    *

 *  might get a real score, or a bound, or perhaps only a   *

 *  good move to try first.  The possible results are:      *

 *                                                          *

 *  1. HashProbe() returns "HASH_HIT".  This terminates     *

 *  the search instantly and we simply return the value     *

 *  found in the hash table.  This value is simply the      *

 *  value we found when we did a real search in this        *

 *  position previously, and HashProbe() verifies that the  *

 *  value is useful based on draft and current bounds.      *

 *                                                          *

 *  2. HashProbe() returns "AVOID_NULL_MOVE" which means    *

 *  the hashed score/bound was no good, but it indicated    *

 *  that trying a null-move in this position would be a     *

 *  waste of time since it will likely fail low, not high.  *

 *                                                          *

 *  3. HashProbe() returns "HASH_MISS" when forces us to    *

 *  do a normal search to resolve this node.                *

 *                                                          *

 ************************************************************

 */

    switch (HashProbe(tree, ply, depth, side, alpha, beta, &value)) {

      case HASH_HIT:

        return value;

      case AVOID_NULL_MOVE:

        do_null = 0;

      case HASH_MISS:

        break;

    }

/*

 ************************************************************

 *                                                          *

 *  Step 4.  Now it's time to try a probe into the endgame  *

 *  tablebase files.  This is done if we notice that there  *

 *  are 6 or fewer pieces left on the board.  EGTB_use      *

 *  tells us how many pieces to probe on.  Note that this   *

 *  can be zero when trying to swindle the opponent, so     *

 *  that no probes are done since we know it is a draw.     *

 *  This is another way to get out of the search quickly,   *

 *  but not as quickly as the previous steps since this can *

 *  result in an I/O operation.                             *

 *                                                          *

 *  Note that in "swindle mode" this can be turned off by   *

 *  Iterate() setting "EGTB_use = 0" so that we won't probe *

 *  the EGTBs since we are searching only the root moves    *

 *  that lead to a draw and we want to play the move that   *

 *  makes the draw more difficult to reach by the opponent  *

 *  to give him a chance to make a mistake.                 *

 *                                                          *

 *  Another special case is that we slightly fudge the      *

 *  score for draws.  In a normal circumstance, draw=0.00   *

 *  since it is "equal".  However, here we add 0.01 if      *

 *  white has more material, or subtract 0.01 if black has  *

 *  more material, since in a drawn KRP vs KR we would      *

 *  prefer to have the KRP side since the opponent can make *

 *  a mistake and convert the draw to a loss.               *

 *                                                          *

 ************************************************************

 */

#if !defined(NOEGTB)

    if (depth > 6 && TotalAllPieces <= EGTB_use &&

        Castle(ply, white) + Castle(ply, black) == 0 &&

        (CaptureOrPromote(tree->curmv[ply - 1]) || ply < 3)) {

      int egtb_value;

      tree->egtb_probes++;

      if (EGTBProbe(tree, ply, side, &egtb_value)) {

        tree->egtb_probes_successful++;

        alpha = egtb_value;

        if (MateScore(alpha))

          alpha += (alpha > 0) ? -ply + 1 : ply;

        else if (alpha == 0) {

          alpha = DrawScore(side);

          if (Material > 0)

            alpha += (side) ? 1 : -1;

          else if (Material < 0)

            alpha -= (side) ? 1 : -1;

        }

        if (alpha < beta)

          SavePV(tree, ply, 2);

        return alpha;

      }

    }

#endif

/*

 ************************************************************

 *                                                          *

 *  Step 5.  We now know there is no quick way to get out   *

 *  of here, which leaves one more possibility, although it *

 *  does require s search, but to a reduced depth.  We      *

 *  try a null move to see if we can get a quick cutoff     *

 *  with only a little work.  This operates as follows.     *

 *  Instead of making a legal move, the side on move passes *

 *  and does nothing.  The resulting position is searched   *

 *  to a shallower depth than normal (usually 3 plies less  *

 *  but settable by the user.) This will result in a cutoff *

 *  if our position is very good, but it produces the       *

 *  cutoff much quicker since the search is far shallower   *

 *  than a normal search that would also be likely to fail  *

 *  high.                                                   *

 *                                                          *

 *  This is skipped for any of the following reasons:       *

 *                                                          *

 *  1.  The side on move is in check.  The null move        *

 *      results in an illegal position.                     *

 *  2.  No more than one null move can appear in succession *

 *      as all this does is burn 6 plies of depth.          *

 *  3.  The side on move has only pawns left, which makes   *

 *      zugzwang positions more likely.                     *

 *  4.  The transposition table probe found an entry that   *

 *      indicates that a null-move search will not fail     *

 *      high, so we avoid the wasted effort.                *

 *                                                          *

 ************************************************************

 */

    tree->inchk[ply + 1] = 0;

    tree->last[ply] = tree->last[ply - 1];

    if (do_null && !pv_node && depth > 1 && !tree->inchk[ply]

        && TotalPieces(side, occupied)) {

      uint64_t save_hash_key;

      tree->curmv[ply] = 0;

      tree->phase[ply] = NULL_MOVE;

#if defined(TRACE)

      if (ply <= trace_level)

        Trace(tree, ply, depth, side, beta - 1, beta, "Search1", NULL_MOVE);

#endif

      tree->status[ply + 1] = tree->status[ply];

      Reversible(ply + 1) = 0;

      save_hash_key = HashKey;

      if (EnPassant(ply + 1)) {

        HashEP(EnPassant(ply + 1));

        EnPassant(ply + 1) = 0;

      }

      if (depth - null_depth - 1 > 0)

        value =

            -Search(tree, -beta, -beta + 1, Flip(side),

            depth - null_depth - 1, ply + 1, NO_NULL);

      else

        value = -Quiesce(tree, -beta, -beta + 1, Flip(side), ply + 1, 1);

      HashKey = save_hash_key;

      if (abort_search || tree->stop)

        return 0;

      if (value >= beta) {

        HashStore(tree, ply, depth, side, LOWER, value, tree->hash_move[ply]);

        return value;

      }

    }

/*

 ************************************************************

 *                                                          *

 *  Step 6.  This step is rarely executed.  It is used when *

 *  there is no best move from the hash table, and this is  *

 *  a PV node, since we need a good move to search first.   *

 *  While killers moves are good, they are not quite good   *

 *  enough.  the simplest solution is to try a shallow      *

 *  search (depth-2) to get a move.  note that when we call *

 *  Search() with depth-2, it, too, will not have a hash    *

 *  move, and will therefore recursively continue this      *

 *  process, hence the name "internal iterative deepening." *

 *                                                          *

 ************************************************************

 */

    tree->next_status[ply].phase = HASH_MOVE;

    if (!tree->hash_move[ply] && depth >= 6 && do_null && ply > 1) {

      if (pv_node) {

        do {

          tree->curmv[ply] = 0;

          if (depth - 2 > 0)

            value = Search(tree, alpha, beta, side, depth - 2, ply, DO_NULL);

          else

            value = Quiesce(tree, alpha, beta, side, ply, 1);

          if (abort_search || tree->stop)

            break;

          if (value > alpha) {

            if (value < beta) {

              if ((int) tree->pv[ply - 1].pathl > ply)

                tree->hash_move[ply] = tree->pv[ply - 1].path[ply];

            } else

              tree->hash_move[ply] = tree->curmv[ply];

            tree->last[ply] = tree->last[ply - 1];

            tree->next_status[ply].phase = HASH_MOVE;

          }

        } while (0);

      }

    }

  }

/*  

 ************************************************************

 *                                                          *

 *  Step 7.  Now iterate through the move list and search   *

 *  the resulting positions.  Note that Search() culls any  *

 *  move that is not legal by using Check().  The special   *

 *  case is that we must find one legal move to search to   *

 *  confirm that it's not a mate or draw.                   *

 *                                                          *

 *  We have three possible procedures we call here, one is  *

 *  specific to ply=1 (NextRootMove()), the second is a     *

 *  specific routine to escape check (NextEvasion()) and    *

 *  the last is the normal NextMove() procedure that does   *

 *  the usual move ordering stuff.                          *

 *                                                          *

 *  Special case:  if we have searched one move at root,    *

 *  and the returned score == alpha, we want to get out of  *

 *  here and return to Iterate() where the search bounds    *

 *  will be adjusted.  Otherwise we would search all root   *

 *  moves and possibly fail low after expending a sig-      *

 *  nificant amount of time.                                *

 *                                                          *

 ************************************************************

 */

  tree->next_status[ply].phase = HASH_MOVE;

  while (1) {

    if (ply > 1)

      tree->phase[ply] =

          (tree->inchk[ply]) ? NextEvasion(tree, ply, side) : NextMove(tree,

          ply, side);

    else if (moves_searched == 1 && alpha == original_alpha)

      break;

    else

      tree->phase[ply] = NextRootMove(tree, tree, side);

    if (!tree->phase[ply])

      break;

#if defined(TRACE)

    if (ply <= trace_level)

      Trace(tree, ply, depth, side, alpha, beta, "Search2", tree->phase[ply]);

#endif

    MakeMove(tree, ply, tree->curmv[ply], side);

    tree->nodes_searched++;

    if (tree->inchk[ply] || !Check(side))

      do {

        if (++moves_searched == 1)

          first_tried = tree->curmv[ply];

/*

 ************************************************************

 *                                                          *

 *  Step 7a.  If the move to be made checks the opponent,   *

 *  then we need to remember that he's in check and also    *

 *  extend the depth by one ply for him to get out.  Note   *

 *  that if the move gives check, it is not a candidate for *

 *  either depth reduction or forward-pruning.              *

 *                                                          *

 *  We do not extend unsafe checking moves (as indicated by *

 *  Swap(), a SEE algorithm, since these are usually a      *

 *  waste of time and simply blow up the tree search space. *

 *                                                          *

 ************************************************************

 */

        extend = 0;

        reduce = 0;

        if (Check(Flip(side))) {

          tree->inchk[ply + 1] = 1;

          if (SwapO(tree, tree->curmv[ply], side) <= 0) {

            extend = check_depth;

            tree->extensions_done++;

          }

        } else

          tree->inchk[ply + 1] = 0;

/*

 ************************************************************

 *                                                          *

 *  Step 7b.  Now for the forward-pruning stuff.  The idea  *

 *  here is based on the old FUTILITY idea, where if the    *

 *  current material + a fudge factor is lower than alpha,  *

 *  then there is little promise in searching this move to  *

 *  make up for the material deficit.  This is not done if  *

 *  we chose to extend this move in the previous step.      *

 *                                                          *

 *  This is a useful idea in today's 20+ ply searches, as   *

 *  when we near the tips, if we are too far behind in      *

 *  material, there is little time left to recover it and   *

 *  moves that don't bring us closer to a reasonable        *

 *  material balance can safely be skipped.  It is much     *

 *  more dangerous in shallow searches.                     *

 *                                                          *

 *  We have an array of pruning margin values that are      *

 *  indexed by depth (remaining plies left until we drop    *

 *  into the quiescence search) and which increase with     *

 *  depth since more depth means a greater chance of        *

 *  bringing the score back up to alpha or beyond.  If the  *

 *  current material + the bonus is less than alpha, we     *

 *  simply avoid searching this move at all, and skip to    *

 *  the next move without expending any more effort.  Note  *

 *  that this is classic forward-pruning and can certainly  *

 *  introduce errors into the search.  However, cluster     *

 *  testing has shown that this improves play in real       *

 *  games.  The current implementation only prunes in the   *

 *  last 5 plies before quiescence, although this can be    *

 *  tuned with the "eval" command changing the "pruning     *

 *  depth" value to something other than 6 (test is for     *

 *  depth < pruning depth, current value is 6 which prunes  *

 *  in last 5 plies only).  Testing shows no benefit in     *

 *  larger values than 6, although this might change in     *

 *  future versions as other things are modified.           *

 *                                                          *

 *  Exception:                                              *

 *                                                          *

 *    We do not prune if we are safely pushing a passed     *

 *    pawn to the 6th rank, where it becomes very dangerous *

 *    since it can promote in two more moves.               *

 *                                                          *

 ************************************************************

 */

        if (tree->phase[ply] == REMAINING_MOVES && !tree->inchk[ply]

            && !extend && moves_searched > 1) {

          if (depth < pruning_depth &&

              MaterialSTM(side) + pruning_margin[depth] <= alpha) {

            if (Piece(tree->curmv[ply]) != pawn ||

                !Passed(To(tree->curmv[ply]), side)

                || rankflip[side][Rank(To(tree->curmv[ply]))] < RANK6) {

              tree->moves_fpruned++;

              continue;

            }

          }

/*

 ************************************************************

 *                                                          *

 *  Step 7c.  Now it's time to try to reduce the search     *

 *  depth if the move appears to be "poor".  To reduce the  *

 *  search, the following requirements must be met:         *

 *                                                          *

 *  (1) We must be in the REMAINING_MOVES part of the move  *

 *      ordering, so that we have nearly given up on        *

 *      failing high on any move.                           *

 *                                                          *

 *  (2) We must not be too close to the horizon (this is    *

 *      the LMR_remaining_depth value).                     *

 *                                                          *

 *  (3) The current move must not be a checking move and    *

 *      the side to move can not be in check.               *

 *                                                          *

 ************************************************************

 */

          if (Piece(tree->curmv[ply]) != pawn ||

              !Passed(To(tree->curmv[ply]), side)

              || rankflip[side][Rank(To(tree->curmv[ply]))] < RANK6) {

            reduce = LMR_min_reduction;

            if (moves_searched > 3)

              reduce = LMR_max_reduction;

            max_reduce = Max(depth - 1 - LMR_remaining_depth, 0);

            if (reduce > max_reduce)

              reduce = max_reduce;

            if (reduce)

              tree->reductions_done++;

          }

        }

/*  

 ************************************************************

 *                                                          *

 *  Step 7d.  We have determined whether the depth is to    *

 *  be changed by an extension or a reduction.  If we get   *

 *  to this point, then the move is not being pruned.  So   *

 *  off we go to a recursive search/quiescence call to work *

 *  our way toward a terminal node.                         *

 *                                                          *

 *  There is one special-case to handle.  If the depth was  *

 *  reduced, and Search() returns a value >= beta then      *

 *  accepting that is risky (we reduced the move as we      *

 *  thought it was bad and expected it to fail low) so we   *

 *  repeat the search using the original (non-reduced)      *

 *  depth to see if the fail-high happens again.            *

 *                                                          *

 ************************************************************

 */

        if (depth + extend - reduce - 1 > 0) {

          value =

              -Search(tree, -t_beta, -alpha, Flip(side),

              depth + extend - reduce - 1, ply + 1, DO_NULL);

          if (value > alpha && reduce)

            value =

                -Search(tree, -t_beta, -alpha, Flip(side), depth - 1, ply + 1,

                DO_NULL);

        } else

          value = -Quiesce(tree, -t_beta, -alpha, Flip(side), ply + 1, 1);

        if (abort_search || tree->stop)

          break;

/*

 ************************************************************

 *                                                          *

 *  Step 7e.  This is the PVS re-search code.  If we reach  *

 *  this point and value > alpha and value < beta, then     *

 *  this can not be a null-window search.  We have to re-   *

 *  search the position with the original beta value (not   *

 *  alpha+1 as is the usual case in PVS) to see if it still *

 *  fails high before we treat this as a real fail-high and *

 *  back up the value to the previous ply.                  *

 *                                                          *

 *  Special case:  ply == 1.                                *

 *                                                          *

 *  In this case, we need to clean up and then move the     *

 *  best move to the top of the root move list, and return  *

 *  back to Iterate() to let it produce the usual informa-  *

 *  tive output and re-start the search with a new beta     *

 *  value.  We also reset the failhi_delta back to 16,      *

 *  since an earlier fail-high or fail low in this          *

 *  iteration could have left it at a large value.          *

 *                                                          *

 *  Last step is to build a usable PV in case this move     *

 *  fails low on the re-search, because we do want to play  *

 *  this move no matter what happens.                       *

 *                                                          *

 ************************************************************

 */

        if (value > alpha && value < beta && moves_searched > 1) {

          if (ply == 1) {

            alpha = value;

            UnmakeMove(tree, ply, tree->curmv[ply], side);

            root_beta = alpha;

            failhi_delta = 16;

            for (i = 0; i < n_root_moves; i++)

              if (tree->curmv[1] == root_moves[i].move)

                break;

            if (i < n_root_moves) {

              temp_rm = root_moves[i];

              for (; i > 0; i--)

                root_moves[i] = root_moves[i - 1];

              root_moves[0] = temp_rm;

            }

            root_moves[0].bm_age = 4;

            tree->pv[1].path[1] = tree->curmv[1];

            tree->pv[1].pathl = 2;

            tree->pv[1].pathh = 0;

            tree->pv[1].pathd = iteration_depth;

            tree->pv[0] = tree->pv[1];

            return alpha;

          }

          if (depth + extend - 1 > 0)

            value =

                -Search(tree, -beta, -alpha, Flip(side), depth + extend - 1,

                ply + 1, DO_NULL);

          else

            value = -Quiesce(tree, -beta, -alpha, Flip(side), ply + 1, 1);

          if (abort_search || tree->stop)

            break;

        }

/*  

 ************************************************************

 *                                                          *

 *  Step 7f.  We have completed the search/re-search and we *

 *  we have the final score.  Now we need to check for a    *

 *  fail-high which terminates this search instantly since  *

 *  no further searching is required.  On a fail high, we   *

 *  need to update the killer moves, and hash table before  *

 *  we return.                                              *

 *                                                          *

 *  If ply == 1, we call Output() which will dump the new   *

 *  PV.  But but we need to back up the PV to ply=0 so that *

 *  it will be available to tell main() which move to make. *

 *                                                          *

 ************************************************************

 */

        if (value > alpha) {

          alpha = value;

          if (ply == 1) {

            tree->pv[1].pathv = value;

            Output(tree, beta);

            tree->pv[0] = tree->pv[1];

          }

          if (value >= beta) {

            Killer(tree, ply, tree->curmv[ply]);

            UnmakeMove(tree, ply, tree->curmv[ply], side);

            HashStore(tree, ply, depth, side, LOWER, value, tree->curmv[ply]);

            tree->fail_highs++;

            if (moves_searched == 1)

              tree->fail_high_first_move++;

            return value;

          }

        }

        t_beta = alpha + 1;

      } while (0);

    UnmakeMove(tree, ply, tree->curmv[ply], side);

    if (abort_search || tree->stop)

      return 0;

/*

 ************************************************************

 *                                                          *

 *  Step 7g.  If are doing an SMP search, and we have idle  *

 *  processors, now is the time to get them involved.  We   *

 *  have now satisfied the "young brothers wait" condition  *

 *  since we have searched one move.  All that is left is   *

 *  to check the split constraints to see if we are an      *

 *  acceptable split point.                                 *

 *                                                          *

 *    (1) We can't split within N plies of the frontier     *

 *        nodes to avoid excessive split overhead.          *

 *                                                          *

 *    (2) We can't split until at least M nodes have been   *

 *        searched since this thread was last split, to     *

 *        avoid splitting too often, mainly in endgames.    *

 *                                                          *

 *    (3) We have to have searched one legal move to avoid  *

 *        splitting at a node where we have no legal moves  *

 *        (the first move tried might have been illegal as  *

 *        in when we encounter a stalemate).                *

 *                                                          *

 *    (4) If we are at ply=1, we can't split unless the     *

 *        smp_split_at_root flag is set to 1, AND the next  *

 *        move in the ply=1 move list is not flagged as     *

 *        "do not search in parallel" which happens when    *

 *        this move was a best move in the last couple of   *

 *        searches and we want all processors on it at once *

 *        to get a score back quicker.                      *

 *                                                          *

 *    (5) if the variable smp_split is > 0, we have idle    *

 *        threads that can help, however if smp_split < 0,  *

 *        we are already doing a split on another thread    *

 *        so there is no point in waiting to try one here.  *

 *                                                          *

 *  SearchParallel() primarily contains steps 7 through 7f  *

 *  which is the main search loop.  We do the final clean-  *

 *  up below when either we finish the search normally or   *

 *  a parallel search completes and returns to this point.  *

 *                                                          *

 *  Special case:  we do not split if we are at ply=1 and   *

 *  alpha == original_alpha.  That means the first move     *

 *  failed low, and we are going to exit search and return  *

 *  to Iterate() to report this.                            *

 *                                                          *

 *  One potential problem is that multiple threads can get  *

 *  to this point at the same time, and they all stack up   *

 *  waiting to grab the lock_smp lock that protects the     *

 *  split operation.  I now have a new lock, lock_split,    *

 *  to try to limit this wasted time.  A thread now has to  *

 *  acquire that lock, and then it tests the smp_split      *

 *  to see if a split STILL needs to be done.  If not, we   *

 *  release the lock and move on, rather than waiting on    *

 *  main lock_smp lock.                                     *

 *                                                          *

 *  If we acquire the lock_split lock AND we notice that    *

 *  smp_split is still set to 1, we quickly set smp_split   *

 *  to zero (-1) so that other threads will bug out rather  *

 *  than trying to split and end up in a queue behind us,   *

 *  waiting while we split and they try to split and fail.  *

 *  We release lock_split to eliminate the log-jam of       *

 *  threads waiting to split and get them back into their   *

 *  normal searches, and jump right into Thread().          *

 *                                                          *

 *  The smp_split = -1 has a complex meaning, but simply    *

 *  stated it means "split currently in progress".  Here,   *

 *  that means "do not attempt a split since we are already *

 *  in the middle of one."  It also tells individual        *

 *  threads to not change smp_split to 1 when they become   *

 *  idle, because with a split in progress, it is likely we *

 *  will pick them up automatically.                        *

 *                                                          *

 ************************************************************

 */

#if (CPUS > 1)

    if (smp_split > 0 && depth >= smp_min_split_depth && moves_searched &&

        tree->nodes_searched - start_nodes > smp_split_nodes && (ply > 1 ||

            (smp_split_at_root && NextRootMoveParallel() &&

                alpha != original_alpha)))

      do {

        Lock(lock_split);

        if (smp_split <= 0) {

          Unlock(lock_split);

          break;

        }

        smp_split = -1;

        Unlock(lock_split);

        tree->alpha = alpha;

        tree->beta = beta;

        tree->value = alpha;

        tree->side = side;

        tree->ply = ply;

        tree->depth = depth;

        tree->moves_searched = moves_searched;

        if (Thread(tree)) {

          if (abort_search || tree->stop)

            return 0;

          if (tree->thread_id == 0 && CheckInput())

            Interrupt(ply);

          value = tree->value;

          if (value > alpha) {

            if (value >= beta) {

              HashStore(tree, ply, depth, side, LOWER, value, tree->cutmove);

              return value;

            }

            alpha = value;

            break;

          }

        }

      } while (0);

#endif

  }

/*

 ************************************************************

 *                                                          *

 *  Step 8.  All moves have been searched.  If none were    *

 *  legal, return either MATE or DRAW depending on whether  *

 *  the side to move is in check or not.                    *

 *                                                          *

 ************************************************************

 */

  if (abort_search || tree->stop)

    return 0;

  if (moves_searched == 0) {

    value = (Check(side)) ? -(MATE - ply) : DrawScore(side);

    if (value >= alpha && value < beta) {

      SavePV(tree, ply, 0);

#if defined(TRACE)

      if (ply <= trace_level)

        printf("Search() no moves!  ply=%d\n", ply);

#endif

    }

    return value;

  } else {

    int bestmove, type;

    bestmove =

        (alpha == original_alpha) ? first_tried : tree->pv[ply].path[ply];

    type = (alpha == original_alpha) ? UPPER : EXACT;

    if (repeat == 2 && alpha != -(MATE - ply - 1)) {

      value = DrawScore(side);

      if (value < beta)

        SavePV(tree, ply, 0);

#if defined(TRACE)

      if (ply <= trace_level)

        printf("draw by 50 move rule detected, ply=%d.\n", ply);

#endif

      return value;

    } else if (alpha != original_alpha) {

      tree->pv[ply - 1] = tree->pv[ply];

      tree->pv[ply - 1].path[ply - 1] = tree->curmv[ply - 1];

      Killer(tree, ply, tree->pv[ply].path[ply]);

    }

    HashStore(tree, ply, depth, side, type, alpha, bestmove);

    return alpha;

  }

}

/* last modified 05/08/14 */

/*

 *******************************************************************************

 *                                                                             *

 *   SearchParallel() is the recursive routine used to implement alpha/beta    *

 *   negamax search using parallel threads.  When this code is called, the     *

 *   first move has already been searched, so all that is left is to search    *

 *   the remainder of the moves and then return.  Note that the hash table and *

 *   such can't be modified here since this only represents a part of the      *

 *   search at this ply.  All of that is deferred until we return and reach    *

 *   the original instance of Search() where we have the complete results from *

 *   all the threads that are helping here.                                    *

 *                                                                             *

 *******************************************************************************

 */

int SearchParallel(TREE * RESTRICT tree, int alpha, int beta, int value,

    int side, int depth, int ply) {

  ROOT_MOVE temp_rm;

  int extend, reduce, max_reduce, i;

/*

 ************************************************************

 *                                                          *

 *  Step 7.  Now we continue to iterate through the move    *

 *  list and search the resulting positions.  Note that     *

 *  SearchParallel() culls any move that is not legal by    *

 *  using Check().  The special case mentioned in Search()  *

 *  is not an issue here as we don't do a parallel split    *

 *  until we have searched one legal move.                  *

 *                                                          *

 *  We have three possible procedures we call here, one is  *

 *  specific to ply=1 (NextRootMove()), the second is a     *

 *  specific routine to escape check (NextEvasion()) and    *

 *  the last is the normal NextMove() procedure that does   *

 *  the usual move ordering stuff.                          *

 *                                                          *

 ************************************************************

 */

  while (1) {

    Lock(tree->parent->lock);

    if (ply > 1)

      tree->phase[ply] =

          (tree->inchk[ply]) ? NextEvasion((TREE *) tree->parent, ply,

          side) : NextMove((TREE *)

          tree->parent, ply, side);

    else

      tree->phase[ply] = NextRootMove(tree->parent, tree, side);

    tree->curmv[ply] = tree->parent->curmv[ply];

    Unlock(tree->parent->lock);

    if (!tree->phase[ply])

      break;

#if defined(TRACE)

    if (ply <= trace_level)

      Trace(tree, ply, depth, side, alpha, beta, "SearchParallel",

          tree->phase[ply]);

#endif

    MakeMove(tree, ply, tree->curmv[ply], side);

    tree->nodes_searched++;

    if (tree->inchk[ply] || !Check(side))

      do {

        tree->parent->moves_searched++;

/*

 ************************************************************

 *                                                          *

 *  Step 7a.  If the move to be made checks the opponent,   *

 *  then we need to remember that he's in check and also    *

 *  extend the depth by one ply for him to get out.  Note   *

 *  that if the move gives check, it is not a candidate for *

 *  either depth reduction or forward-pruning.              *

 *                                                          *

 *  We do not extend unsafe checking moves (as indicated by *

 *  Swap(), a SEE algorithm, since these are usually a      *

 *  waste of time and simply blow up the tree search space. *

 *                                                          *

 ************************************************************

 */

        extend = 0;

        reduce = 0;

        if (Check(Flip(side))) {

          tree->inchk[ply + 1] = 1;

          if (SwapO(tree, tree->curmv[ply], side) <= 0) {

            extend = check_depth;

            tree->extensions_done++;