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  1. #include "chess.h"
  2. #include "data.h"
  3. /* last modified 12/21/09 */
  4. /*
  5.  *******************************************************************************
  6.  *                                                                             *
  7.  *   Drawn() is used to answer the question "is this position a hopeless       *
  8.  *   draw?"  Several considerations are included in making this decision, but  *
  9.  *   the most basic one is simply the remaining material for each side.  If    *
  10.  *   either side has pawns, it's not a draw.  With no pawns, equal material is *
  11.  *   a draw.  Otherwise, the superior side must have enough material to be     *
  12.  *   able to force a mate.                                                     *
  13.  *                                                                             *
  14.  *   Drawn() has 0 possible return values:                                     *
  15.  *                                                                             *
  16.  *   0:  Game is not a draw by rule or drawish in nature.                      *
  17.  *                                                                             *
  18.  *   1:  Game is a "technical draw" where material is even with no pawns and   *
  19.  *       the search is also returning draw scores as the best outcome.         *
  20.  *                                                                             *
  21.  *   2:  Game is an "actual draw" by FIDE rules, such as KB vs K, where mate   *
  22.  *       can't be forced, even with worst possible play by one side.           *
  23.  *                                                                             *
  24.  *******************************************************************************
  25.  */
  26. int Drawn(TREE * RESTRICT tree, int value) {
  27. /*
  28.  ************************************************************
  29.  *                                                          *
  30.  *  If either side has pawns, the game is not a draw for    *
  31.  *  lack of material.                                       *
  32.  *                                                          *
  33.  ************************************************************
  34.  */
  35.   if (TotalPieces(white, pawn) || TotalPieces(black, pawn))
  36.     return 0;
  37. /*
  38.  ************************************************************
  39.  *                                                          *
  40.  *  If the score suggests a mate has been found, this is    *
  41.  *  not a draw.                                             *
  42.  *                                                          *
  43.  ************************************************************
  44.  */
  45.   if (MateScore(value))
  46.     return 0;
  47. /*
  48.  ************************************************************
  49.  *                                                          *
  50.  *  If neither side has pawns, and one side has some sort   *
  51.  *  of material superiority, then determine if the winning  *
  52.  *  side has enough material to win.                        *
  53.  *                                                          *
  54.  ************************************************************
  55.  */
  56.   if (TotalPieces(white, occupied) + TotalPieces(black, occupied) < 4)
  57.     return 2;
  58.   if (TotalPieces(white, occupied) < 5 && TotalPieces(black, occupied) < 5)
  59.     return 1;
  60.   if (TotalPieces(white, occupied) == 5 || TotalPieces(white, occupied) > 6)
  61.     return 0;
  62.   if (TotalPieces(black, occupied) == 5 || TotalPieces(black, occupied) > 6)
  63.     return 0;
  64.   if ((TotalPieces(white, occupied) == 6 && !Bishops(white) && Material > 0)
  65.       || (TotalPieces(black, occupied) == 6 && !Bishops(black)
  66.           && Material < 0))
  67.     return 1;
  68. /*
  69.  ************************************************************
  70.  *                                                          *
  71.  *  The search result must indicate a draw also, otherwise  *
  72.  *  it could be a tactical win or loss, so we will skip     *
  73.  *  calling an equal material (no pawns) position a draw to *
  74.  *  make sure there is no mate that the search sees.        *
  75.  *                                                          *
  76.  *  If neither side has pawns, then one side must have some *
  77.  *  sort of material superiority, otherwise it is a draw.   *
  78.  *                                                          *
  79.  ************************************************************
  80.  */
  81.   if (value != DrawScore(game_wtm))
  82.     return 0;
  83.   if (TotalPieces(white, occupied) == TotalPieces(black, occupied))
  84.     return 1;
  85.   return 0;
  86. }
  87.