Subversion Repositories Games.Chess Giants

#include "chess.h"

#include "data.h"

/* last modified 01/10/16 */

/*

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

 *                                                                             *

 *   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 ply, int depth, int wtm, int alpha,

    int beta, int in_check, int do_null) {

  int repeat = 0, value = 0, pv_node = alpha != beta - 1, n_depth;

  int searched[256];

/*

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

 *                                                          *

 *  Timeout.  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 (search_nodes && --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;

/*

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

 *                                                          *

 *  Draws.  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 four steps are skipped for moves at the    *

 *  root (ply = 1).                                         *

 *                                                          *

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

 */

  if (ply > 1) {

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

      if (repeat == 1 || !in_check) {

        value = DrawScore(wtm);

        if (value < beta)

          SavePV(tree, ply, 3);

#if defined(TRACE)

        if (ply <= trace_level)

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

#endif

        return value;

      }

    }

/*

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

 *                                                          *

 *  Mate distance pruning.  If we have found a mate, we can *

 *  stop if we are too deep to find a shorter mate.  This   *

 *  only affects the size of the tree in positions where    *

 *  there are forced mates.  It does not change the outcome *

 *  of the search at all, just the time it takes.           *

 *                                                          *

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

 */

    alpha = Max(alpha, -MATE + ply - 1);

    beta = Min(beta, MATE - ply);

    if (alpha >= beta)

      return alpha;

/*

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

 *                                                          *

 *  Trans/Ref.  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 ProbeTransRef() 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, wtm, alpha, beta, &value)) {

      case HASH_HIT:

        return value;

      case AVOID_NULL_MOVE:

        do_null = 0;

      case HASH_MISS:

        break;

    }

/*

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

 *                                                          *

 *  EGTBs.  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 AND the move at *

 *  the previous ply was a capture.  If it was not, then we *

 *  would have already probed the EGTBs so if it was a miss *

 *  when we probed then, it will also miss here.  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 > EGTB_depth && TotalAllPieces <= EGTB_use &&

        !Castle(ply, white) && !Castle(ply, black) &&

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

      int egtb_value;

      tree->egtb_probes++;

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

        tree->egtb_hits++;

        alpha = egtb_value;

        if (MateScore(alpha))

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

        else if (alpha == 0) {

          alpha = DrawScore(wtm);

          if (MaterialSTM(wtm) > 0)

            alpha += 1;

          else if (MaterialSTM(wtm) < 0)

            alpha -= 1;

        }

        if (alpha < beta)

          SavePV(tree, ply, 2);

        return alpha;

      }

    }

#endif

/*

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

 *                                                          *

 *  Null-move.  We now know there is no quick way to get    *

 *  out of here, which leaves one more possibility,         *

 *  although it does require a 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 (see       *

 *  below).  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.                 *

 *                                                          *

 *  The reduction amount starts off at null_depth (normally *

 *  set to 3 but the user can change this via the crafty    *

 *  personality command) but is then increased as follows:  *

 *                                                          *

 *     R = null_depth + depth / null_divisor                *

 *                                                          *

 *  null_divisor defaults to 6, but this can also be set    *

 *  by the user to try more/less aggressive settings.       *

 *                                                          *

 *  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 2x 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->last[ply] = tree->last[ply - 1];

    n_depth = (TotalPieces(wtm, occupied) > 9 || n_root_moves > 17 ||

            depth > 3) ? 1 : 3;

    if (do_null && !pv_node && depth > n_depth && !in_check &&

        TotalPieces(wtm, occupied)) {

      uint64_t save_hash_key;

      int R = null_depth + depth / null_divisor;

      tree->curmv[ply] = 0;

#if defined(TRACE)

      if (ply <= trace_level)

        Trace(tree, ply, depth, wtm, value - 1, value, "SearchNull", serial,

            NULL_MOVE, 0);

#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;

      }

      tree->null_done[Min(R, 15)]++;

      if (depth - R - 1 > 0)

        value =

            -Search(tree, ply + 1, depth - R - 1, Flip(wtm), -beta, -beta + 1,

            0, NO_NULL);

      else

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

      HashKey = save_hash_key;

      if (abort_search || tree->stop)

        return 0;

      if (value >= beta) {

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

        return value;

      }

    }

/*

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

 *                                                          *

 *  IID.  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;

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

      tree->curmv[ply] = 0;

      if (depth - 2 > 0)

        value =

            Search(tree, ply, depth - 2, wtm, alpha, beta, in_check, DO_NULL);

      else

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

      if (abort_search || tree->stop)

        return 0;

      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;

      }

    }

  }

/*

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

 *                                                          *

 *  Search moves.  Now we call SearchMoveList() to interate *

 *  through the move list and search each new position.     *

 *  Note that this is done in a separate procedure because  *

 *  this is also the code that is used to do the parallel   *

 *  search.                                                 *

 *                                                          *

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

 */

  searched[0] = 0;

  value =

      SearchMoveList(tree, ply, depth, wtm, alpha, beta, searched, in_check,

      repeat, serial);

  return value;

}

/* last modified 09/28/15 */

/*

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

 *                                                                             *

 *   SearchMoveList() is the recursive routine used to implement the main      *

 *   search loop.  This code is responsible for iterating through the move     *

 *   list until it encounters a condition that ends the search at this ply.    *

 *   At that point it simply returns the current negamax value to the caller   *

 *   to handle as necessary.                                                   *

 *                                                                             *

 *   The "mode" flag indicates which of the following conditions apply here    *

 *   which directly controls parts of the search.                              *

 *                                                                             *

 *      mode = serial   ->  this is a serial search.                           *

 *                                                                             *

 *      mode = parallel ->  this is a parallel search, which implies that this *

 *                          is a partial search which means we do NOT want to  *

 *                          do any trans/ref updating and we also need to take *

 *                          care about locking things that are being updated   *

 *                          by more than one thread in parallel.               *

 *                                                                             *

 *   When mode = parallel, this code performs the same function as the old     *

 *   SearchParallel() code, except that it is the main search loop for the     *

 *   program, there is no longer any duplicated code.  This is called by the   *

 *   normal Search() function and by ThreadWait() where idle processes wait    *

 *   for work and then call this procedure to search a subset of the moves at  *

 *   this ply (in parallel with other threads).                                *

 *                                                                             *

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

 */

int SearchMoveList(TREE * RESTRICT tree, int ply, int depth, int wtm,

    int alpha, int beta, int searched[], int in_check, int repeat, int mode) {

  TREE *current;

  int extend, reduce, check, original_alpha = alpha, t_beta;

  int i, value = 0, pv_node = alpha != beta - 1, status, order;

  int moves_done = 0, phase, bestmove, type;

/*

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

 *                                                          *

 *  Basic initialization before we begin the loop through   *

 *  the move list.  If this is a parallel search, we have   *

 *  already searched one move, so we set t_beta to alpha+1  *

 *  to set up for a normal PVS search (for moves 2-n)       *

 *  instead of using alpha,beta for the first move as we do *

 *  in a normal search.  Also, if this is a serial search,  *

 *  we are fixing to search the first move so we set the    *

 *  searched move counter to zero, where in a parallel      *

 *  search this has already been done and we leave it alone *

 *  here.                                                   *

 *                                                          *

 *  We also set <current> to tree for a serial search, and  *

 *  to tree->parent for a parallel search since we need to  *

 *  share the move list at split nodes.                     *

 *                                                          *

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

 */

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

  if (mode == parallel) {

    current = tree->parent;

    t_beta = alpha + 1;

  } else {

    current = tree;

    t_beta = beta;

  }

/*

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

 *                                                          *

 *  Iterate.  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 call NextMove() which will generate moves in the     *

 *  normal way (captures, killers, etc) or it will use the  *

 *  GenerateEvasions() generator if we are in check.  For   *

 *  the special case of ply=1, we use NextRootMove() since  *

 *  the ply=1 move list has been generated and the order is *

 *  updated as each search iteration is executed.           *

 *                                                          *

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

 */

  while (1) {

    if (ply == 1 && moves_done == 1 && alpha == original_alpha &&

        mode == serial)

      break;

    if (mode == parallel)

      Lock(current->lock);

    order = (ply > 1) ? NextMove(current, ply, depth, wtm, in_check) :

      NextRootMove(current, tree, wtm);

    phase = current->phase[ply];

    if (mode == parallel) {

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

      Unlock(current->lock);

    }

    if (!order)

      break;

#if defined(TRACE)

    if (ply <= trace_level)

      Trace(tree, ply, depth, wtm, alpha, beta, "SearchMoveList", mode, phase,

          order);

#endif

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

    tree->nodes_searched++;

    status = ILLEGAL;

    if (in_check || !Check(wtm))

      do {

        searched[0]++;

        moves_done++;

        status = LEGAL;

        searched[searched[0]] = tree->curmv[ply];

/*

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

 *                                                          *

 *  Check.  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.         *

 *                                                          *

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

 *  the SEE algorithm), since these are usually a waste of  *

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

 *                                                          *

 *  Note that extending here disables any potential foward  *

 *  pruning or reductions for this move.                    *

 *                                                          *

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

 */

        extend = 0;

        reduce = 0;

        if (Check(Flip(wtm))) {

          check = 1;

          if (SEEO(tree, wtm,

                  tree->curmv[ply]) - pcval[Captured(tree->curmv[ply])] <=

              0) {

            extend = check_depth;

            tree->extensions_done++;

          }

        } else

          check = 0;

/*

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

 *                                                          *

 *  Futility.  First attempt at forward pruning based on    *

 *  the futility idea.                                      *

 *                                                          *

 *  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 6 plies before quiescence, although this can be    *

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

 *  depth" value to something other than 7 (test is for     *

 *  depth < pruning depth, current value is 7 which prunes  *

 *  in last 6 plies only).  Testing shows no benefit in     *

 *  larger values than 7, 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.               *

 *                                                          *

 *  All pruning and reduction code is skipped if any of the *

 *  following are true:                                     *

 *                                                          *

 *  (1) side on move is in check.                           *

 *                                                          *

 *  (2) move has not already been flagged for a search      *

 *      extension.                                          *

 *                                                          *

 *  (3) this is not the first move at this ply.             *

 *                                                          *

 *  (4) we are in the REMAINING phase, which means that a   *

 *      cutoff is not very likely.                          *

 *                                                          *

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

 */

        if (!in_check && !extend && order > 1 && phase >= HISTORY &&

            !(PawnPush(wtm, tree->curmv[ply]))) {

          if (!pv_node && depth < pruning_depth &&

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

            tree->moves_fpruned++;

            break;

          }

/*

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

 *                                                          *

 *  LMP.  Final attempt at forward pruning based on the     *

 *  "late move pruning" idea (a take-off on LMR).           *

 *                                                          *

 *  The basic idea here is that once we have searched a     *

 *  significant number of moves at a ply, it becomes less   *

 *  and less likely that any of the moves left will produce *

 *  a cutoff.  If the move appears to be simple (not a      *

 *  check, etc) then we simply skip it, once the move count *

 *  has been satisfied.  At present, we only do this in the *

 *  last two plies although this might be changed in the    *

 *  future.                                                 *

 *                                                          *

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

 */

          if (!pv_node && alpha > -MATE + 300 && depth < movecnt_depth &&

              !CaptureOrPromote(tree->curmv[ply]) &&

              order > movecnt_pruning[depth]) {

            tree->moves_mpruned++;

            break;

          }

/*

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

 *                                                          *

 *  LMR.  Now it's time to try to reduce the search depth   *

 *  if the move appears to be "poor" because it appears     *

 *  later in the move list.                                 *

 *                                                          *

 *  The reduction is variable and is done via a table look- *

 *  up that uses a function based on remaining depth (more  *

 *  depth remaining, the larger the reduction) and the      *

 *  number of moves searched (more moves searched, the      *

 *  larger the reduction).  The "shape" of this reduction   *

 *  formula is user-settable via the "lmr" command.         *

 *                                                          *

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

 */

          reduce = LMR[Min(depth, 31)][Min(order, 63)];

          tree->LMR_done[reduce]++;

        }

/*

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

 *                                                          *

 *  Now do the PVS search on the current move.              *

 *                                                          *

 *  Note that we do the usual alpha/beta cutoff tests here  *

 *  but we only set an indicator that is used after we have *

 *  called Unmake().  This cleaned up the exit from search  *

 *  and makes it easier to understand when there is only    *

 *  one point where this is done, without needing multiple  *

 *  Unmake() calls when there are different exit points.    *

 *                                                          *

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

 */

        value =

            SearchMove(tree, ply, depth, wtm, alpha, t_beta, beta, extend,

            reduce, check);

        if (value > alpha) {

          status = IN_WINDOW;

          if (value >= beta)

            status = FAIL_HIGH;

          if (mode == parallel && ply == 1)

            status = FAIL_HIGH;

        }

      } while (0);

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

    if (abort_search || tree->stop)

      break;

/*

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

 *                                                          *

 *  Test 1.  When we get here, we have made a move,         *

 *  searched it (and re-searched if necessary/appropriate), *

 *  and the move has been unmade so that the board is in a  *

 *  correct state.                                          *

 *                                                          *

 *  If status = FAIL_HIGH, the search failed high.  The     *

 *  first thing to handle is the case where we are at       *

 *  ply=1, which is a special case.  If we are going to     *

 *  fail high here and terminate the search immediately, we *

 *  need to build the fail-high PV to back up to Iterate()  *

 *  so it will produce the correct output and widen the     *

 *  alpha/beta window.                                      *

 *                                                          *

 *  We then check to see if this is a parallel search.  If  *

 *  so then we are done here, but we need to tell all of    *

 *  the siblings that are helping at this split point that  *

 *  they should immediately stop searching here since we    *

 *  don't need their results.                               *

 *                                                          *

 *  Otherwise we update the killer moves and history        *

 *  counters and store the fail-high information in the     *

 *  trans/ref table for future use if we happen to reach    *

 *  this position again.                                    *

 *                                                          *

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

 */

    if (status == FAIL_HIGH) {

      if (ply == 1) {

        if (!tree->stop) {

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

          tree->pv[1].pathl = 2;

          tree->pv[1].pathh = 0;

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

          tree->pv[0] = tree->pv[1];

        }

      }

#if (CPUS > 1)

      if (mode == parallel) {

        Lock(lock_smp);

        Lock(tree->parent->lock);

        if (!tree->stop) {

          int proc;

          parallel_aborts++;

          for (proc = 0; proc < (int) smp_max_threads; proc++) // Pierre-Marie Baty -- added type cast

            if (tree->parent->siblings[proc] && proc != tree->thread_id)

              ThreadStop(tree->parent->siblings[proc]);

        }

        Unlock(tree->parent->lock);

        Unlock(lock_smp);

        return value;

      }

#endif

      tree->fail_highs++;

      if (order == 1)

        tree->fail_high_first_move++;

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

      History(tree, ply, depth, wtm, tree->curmv[ply], searched);

      return beta;

/*

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

 *                                                          *

 *  Test 2.  If status = IN_WINDOW, this is a search that   *

 *  improved alpha without failing high.  We simply update  *

 *  alpha and continue searching moves.                     *

 *                                                          *

 *  Special case:  If ply = 1 in a normal search, we have   *

 *  a best move and score that just changed.  We need to    *

 *  update the root move list by adding the PV and the      *

 *  score, and then we look to make sure this new "best     *

 *  move" is not actually worse than the best we have found *

 *  so far this iteration.  If it is worse, we restore the  *

 *  best move and score from the real best move so our      *

 *  search window won't be out of whack, which would let    *

 *  moves with scores in between this bad move and the best *

 *  move fail high, cause re-searches, and waste time.      *

 *                                                          *

 *  If this is ply = 1, we display the PV to keep the user  *

 *  informed.                                               *

 *                                                          *

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

 */

    } else if (status == IN_WINDOW) {

      alpha = value;

      if (ply == 1 && mode == serial) {

        tree->pv[1].pathv = value;

        tree->pv[0] = tree->pv[1];

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

          if (root_moves[i].move == tree->pv[1].path[1]) {

            root_moves[i].path = tree->pv[1];

            root_moves[i].path.pathv = alpha;

          }

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

          if (value < root_moves[i].path.pathv) {

            value = root_moves[i].path.pathv;

            alpha = value;

            tree->pv[0] = root_moves[i].path;

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

          }

        Output(tree);

        failhi_delta = 16;

        faillo_delta = 16;

      }

    }

/*

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

 *                                                          *

 *  Test 3.  If status = ILLEGAL, this search was given an  *

 *  illegal move and no search was done, we skip any        *

 *  updating and simply select the next move to search.     *

 *                                                          *

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

 */

    else if (status == ILLEGAL)

      continue;

    t_beta = alpha + 1;

/*

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

 *                                                          *

 *  SMP.  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 at least one move.  All that is  *

 *  left is to check the split constraints to see if this   *

 *  is 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 N 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, which means we want to get *

 *        them involved quickly, OR if this node is an      *

 *        acceptable "gratuitous-split" point by being far  *

 *        enough from the tips of the tree to avoid         *

 *        excessive overhead.                               *

 *                                                          *

 *  We use this code recursively to perform a parallel      *

 *  search at this ply.  But when we finish a partial piece *

 *  of the search in parallel, we don't need to update any  *

 *  search data structures, we will defer that until all of *

 *  parallel threads complete and return back into this     *

 *  code after the parallel search has been collapsed back  *

 *  to one instance of search at this ply.                  *

 *                                                          *

 *  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.                            *

 *                                                          *

 *  In Generation II, multiple threads can reach this point *

 *  at the same time.  We allow multiple threads to split   *

 *  at the same time, but then the idle threads will choose *

 *  to join the thread with the most attractive split point *

 *  rather than just taking pot-luck.  The only limitation  *

 *  on a thread adding a split point here is that if the    *

 *  thread already has enough joinable split points have    *

 *  not been joined yet, we do not incur the overhead of    *

 *  creating another split point until the existing split   *

 *  point is completed or a thread joins at at that point.  *

 *                                                          *

 *  We do not lock anything here, as the split operation    *

 *  only affects thread-local data.  When the split is done *

 *  then the ThreadJoin() function will acquire the lock    *

 *  needed to avoid race conditions during the join op-     *

 *  eration.                                                *

 *                                                          *

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

 */

#if (CPUS > 1)

    if (mode == serial && moves_done && smp_threads &&

        ThreadSplit(tree, ply, depth, alpha, original_alpha, moves_done))

      do {

        tree->alpha = alpha;

        tree->beta = beta;

        tree->value = alpha;

        tree->wtm = wtm;

        tree->ply = ply;

        tree->depth = depth;

        tree->in_check = in_check;

        tree->searched = searched;

        if (Split(tree)) {

          if (abort_search || tree->stop)

            return 0;

          value = tree->value;

          if (value > alpha) {

            if (ply == 1)

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

            if (value >= beta) {

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

              return value;

            }

            alpha = value;

            break;

          }

        }

      } while (0);

#endif

  }

/*

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

 *                                                          *

 *  SMP Cleanup.  If we are doing an SMP search, there are  *

 *  no "end-of-search" things to do.  We have searched all  *

 *  the remaining moves at this ply in parallel, and now    *

 *  return and let the original search that started this    *

 *  sub-tree clean up, do the tests for mate/stalemate,     *

 *  update the hash table, etc.                             *

 *                                                          *

 *  As we return, we end back up in Thread() where we       *

 *  started, which then copies the best score/etc back to   *

 *  the parent thread.                                      *

 *                                                          *

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

 */