From 2570cd28716702ecfc78d650b63a9a90e21397c6 Mon Sep 17 00:00:00 2001
From: Peter Osterlund <peterosterlund2@gmail.com>
Date: Mon, 11 Sep 2017 23:03:59 +0200
Subject: [PATCH] DroidFish: Improve mate finding in endgames.

Include the early_mate code from
https://github.com/syzygy1/Stockfish/tree/early_mate
---
 DroidFish/jni/stockfish/search.cpp | 42 +++++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 9 deletions(-)

diff --git a/DroidFish/jni/stockfish/search.cpp b/DroidFish/jni/stockfish/search.cpp
index 9f6b511..a638027 100644
--- a/DroidFish/jni/stockfish/search.cpp
+++ b/DroidFish/jni/stockfish/search.cpp
@@ -541,7 +541,7 @@ namespace {
     Key posKey;
     Move ttMove, move, excludedMove, bestMove;
     Depth extension, newDepth;
-    Value bestValue, value, ttValue, eval;
+    Value bestValue, value, ttValue, eval, maxValue;
     bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture;
     Piece movedPiece;
@@ -553,6 +553,7 @@ namespace {
     moveCount = quietCount = ss->moveCount = 0;
     ss->statScore = 0;
     bestValue = -VALUE_INFINITE;
+    maxValue = VALUE_INFINITE;
     ss->ply = (ss-1)->ply + 1;
 
     // Check for the available remaining time
@@ -641,7 +642,7 @@ namespace {
             && !pos.can_castle(ANY_CASTLING))
         {
             TB::ProbeState err;
-            TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
+            TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
 
             if (err != TB::ProbeState::FAIL)
             {
@@ -649,15 +650,30 @@ namespace {
 
                 int drawScore = TB::UseRule50 ? 1 : 0;
 
-                value =  v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
-                       : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply
-                                        :  VALUE_DRAW + 2 * v * drawScore;
+                value =  wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
+                       : wdl >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply
+                                          :  VALUE_DRAW + 2 * wdl * drawScore;
 
-                tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
-                          std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
-                          MOVE_NONE, VALUE_NONE, TT.generation());
+                Bound b =  wdl < -drawScore ? BOUND_UPPER
+                         : wdl >  drawScore ? BOUND_LOWER : BOUND_EXACT;
 
-                return value;
+                if (    b == BOUND_EXACT
+                    || (b == BOUND_LOWER ? value >= beta : value <= alpha))
+                {
+                    tte->save(posKey, value_to_tt(value, ss->ply), b,
+                              std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
+                              MOVE_NONE, VALUE_NONE, TT.generation());
+
+                    return value;
+                }
+
+                if (PvNode)
+                {
+                    if (b == BOUND_LOWER)
+                        bestValue = value, alpha = std::max(alpha, bestValue);
+                    else
+                        maxValue = value;
+                }
             }
         }
     }
@@ -1120,6 +1136,9 @@ moves_loop: // When in check search starts from here
              && is_ok((ss-1)->currentMove))
         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
 
+    if (PvNode)
+        bestValue = std::min(bestValue, maxValue);
+
     if (!excludedMove)
         tte->save(posKey, value_to_tt(bestValue, ss->ply),
                   bestValue >= beta ? BOUND_LOWER :
@@ -1616,4 +1635,9 @@ void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves)
         TB::Score =  TB::Score > VALUE_DRAW ?  VALUE_MATE - MAX_PLY - 1
                    : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
                                             :  VALUE_DRAW;
+
+    // Since root_probe() and root_probe_wdl() dirty the root move scores,
+    // we reset them to -VALUE_INFINITE
+    for (RootMove& rm : rootMoves)
+        rm.score = -VALUE_INFINITE;
 }