Syzygy tablebases

Adds support for Syzygy tablebases to Stockfish.  See
the Readme for information on using the tablebases.

Tablebase support can be enabled/disabled at the Makefile
level as well, by setting syzygy=yes or syzygy=no.

Big/little endian are both supported.

No functional change (if Tablebases are not used).

Resolves #6

Readme.md

@@ -12,6 +12,8 @@ to one search thread, so it is therefore recommended to inspect the value of
 the *Threads* UCI parameter, and to make sure it equals the number of CPU
 cores on your computer.
 
+This version of Stockfish has support for Syzygybases.
+
 
 ### Files
 
@@ -25,6 +27,58 @@ This distribution of Stockfish consists of the following files:
     that can be used to compile Stockfish on Unix-like systems.
 
 
+### Syzygybases
+
+**Configuration**
+
+Syzygybases are configured using the UCI options "SyzygyPath",
+"SyzygyProbeDepth", "Syzygy50MoveRule" and "SyzygyProbeLimit".
+
+The option "SyzygyPath" should be set to the directory or directories that
+contain the .rtbw and .rtbz files. Multiple directories should be
+separated by ";" on Windows and by ":" on Unix-based operating systems.
+**Do not use spaces around the ";" or ":".**
+
+Example: `C:\tablebases\wdl345;C:\tablebases\wdl6;D:\tablebases\dtz345;D:\tablebases\dtz6`
+
+It is recommended to store .rtbw files on an SSD. There is no loss in
+storing the .rtbz files on a regular HD.
+
+Increasing the "SyzygyProbeDepth" option lets the engine probe less
+aggressively. Set this option to a higher value if you experience too much
+slowdown (in terms of nps) due to TB probing.
+
+Set the "Syzygy50MoveRule" option to false if you want tablebase positions
+that are drawn by the 50-move rule to count as win or loss. This may be useful
+for correspondence games (because of tablebase adjudication).
+
+The "SyzygyProbeLimit" option should normally be left at its default value.
+
+**What to expect**  
+If the engine is searching a position that is not in the tablebases (e.g.
+a position with 7 pieces), it will access the tablebases during the search.
+If the engine reports a very large score (typically 123.xx), this means
+that it has found a winning line into a tablebase position.
+
+If the engine is given a position to search that is in the tablebases, it
+will use the tablebases at the beginning of the search to preselect all
+good moves, i.e. all moves that preserve the win or preserve the draw while
+taking into account the 50-move rule.
+It will then perform a search only on those moves. **The engine will not move
+immediately**, unless there is only a single good move. **The engine likely
+will not report a mate score even if the position is known to be won.**
+
+It is therefore clear that behaviour is not identical to what one might
+be used to with Nalimov tablebases. There are technical reasons for this
+difference, the main technical reason being that Nalimov tablebases use the
+DTM metric (distance-to-mate), while Syzygybases use a variation of the
+DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move
+counter). This special metric is one of the reasons that Syzygybases are
+more compact than Nalimov tablebases, while still storing all information
+needed for optimal play and in addition being able to take into account
+the 50-move rule.
+
+
 ### Compiling it yourself
 
 On Unix-like systems, it should be possible to compile Stockfish

src/Makefile

@@ -75,6 +75,12 @@ bsfq = no
 popcnt = no
 sse = no
 pext = no
+syzygy = yes
+
+ifeq ($(syzygy),yes)
+	OBJS += syzygy/tbprobe.o
+	CXXFLAGS += -DSYZYGY
+endif
 
 ### 2.2 Architecture specific
 
@@ -398,7 +404,7 @@ install:
 	-strip $(BINDIR)/$(EXE)
 
 clean:
-	$(RM) $(EXE) $(EXE).exe *.o .depend *~ core bench.txt *.gcda
+	$(RM) $(EXE) $(EXE).exe *.o .depend *~ core bench.txt *.gcda ./syzygy/*.o
 
 default:
 	help

src/main.cpp

@@ -27,6 +27,10 @@
 #include "tt.h"
 #include "uci.h"
 
+#ifdef SYZYGY
+#include "syzygy/tbprobe.h"
+#endif
+
 int main(int argc, char* argv[]) {
 
   std::cout << engine_info() << std::endl;
@@ -40,6 +44,9 @@ int main(int argc, char* argv[]) {
   Pawns::init();
   Threads.init();
   TT.resize(Options["Hash"]);
+#ifdef SYZYGY
+  Tablebases::init(Options["SyzygyPath"]);
+#endif
 
   UCI::loop(argc, argv);
 

src/position.h

@@ -24,9 +24,9 @@
 #include <cstddef>
 
 #include "bitboard.h"
+#include "bitcount.h"
 #include "types.h"
 
-
 /// The checkInfo struct is initialized at c'tor time and keeps info used
 /// to detect if a move gives check.
 class Position;
@@ -100,6 +100,7 @@ public:
   bool empty(Square s) const;
   template<PieceType Pt> int count(Color c) const;
   template<PieceType Pt> const Square* list(Color c) const;
+  int total_piece_count() const;
 
   // Castling
   int can_castle(Color c) const;
@@ -166,6 +167,7 @@ public:
   uint64_t nodes_searched() const;
   void set_nodes_searched(uint64_t n);
   bool is_draw() const;
+  int rule50_count() const;
 
   // Position consistency check, for debugging
   bool pos_is_ok(int* step = NULL) const;
@@ -352,6 +354,14 @@ inline int Position::game_ply() const {
   return gamePly;
 }
 
+inline int Position::rule50_count() const {
+  return st->rule50;
+}
+
+inline int Position::total_piece_count() const {
+  return HasPopCnt ? popcount<Full>(pieces()) : pieceCount[WHITE][ALL_PIECES];
+}
+
 inline bool Position::opposite_bishops() const {
 
   return   pieceCount[WHITE][BISHOP] == 1
@@ -402,6 +412,8 @@ inline void Position::put_piece(Square s, Color c, PieceType pt) {
   byColorBB[c] |= s;
   index[s] = pieceCount[c][pt]++;
   pieceList[c][pt][index[s]] = s;
+  if (!HasPopCnt)
+      pieceCount[WHITE][ALL_PIECES]++;
 }
 
 inline void Position::move_piece(Square from, Square to, Color c, PieceType pt) {
@@ -432,6 +444,8 @@ inline void Position::remove_piece(Square s, Color c, PieceType pt) {
   index[lastSquare] = index[s];
   pieceList[c][pt][index[lastSquare]] = lastSquare;
   pieceList[c][pt][pieceCount[c][pt]] = SQ_NONE;
+  if (!HasPopCnt)
+      pieceCount[WHITE][ALL_PIECES]--;
 }
 
 #endif // #ifndef POSITION_H_INCLUDED

src/search.cpp

@@ -34,6 +34,10 @@
 #include "tt.h"
 #include "uci.h"
 
+#ifdef SYZYGY
+#include "syzygy/tbprobe.h"
+#endif
+
 namespace Search {
 
   volatile SignalsType Signals;
@@ -42,6 +46,12 @@ namespace Search {
   Position RootPos;
   Time::point SearchTime;
   StateStackPtr SetupStates;
+  int TBCardinality;
+  uint64_t TBHits;
+  bool RootInTB;
+  bool TB50MoveRule;
+  Depth TBProbeDepth;
+  Value TBScore;
 }
 
 using std::string;
@@ -181,6 +191,8 @@ template uint64_t Search::perft<true>(Position& pos, Depth depth);
 void Search::think() {
 
   TimeMgr.init(Limits, RootPos.game_ply(), RootPos.side_to_move());
+  TBHits = TBCardinality = 0;
+  RootInTB = false;
 
   int cf = Options["Contempt"] * PawnValueEg / 100; // From centipawns
   DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(cf);
@@ -195,6 +207,60 @@ void Search::think() {
   }
   else
   {
+#ifdef SYZYGY
+      // Check Tablebases at root
+      int piecesCnt = RootPos.total_piece_count();
+      TBCardinality = Options["SyzygyProbeLimit"];
+      TBProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
+      if (TBCardinality > Tablebases::TBLargest)
+      {
+          TBCardinality = Tablebases::TBLargest;
+          TBProbeDepth = 0 * ONE_PLY;
+      }
+      TB50MoveRule = Options["Syzygy50MoveRule"];
+
+      if (piecesCnt <= TBCardinality)
+      {
+          TBHits = RootMoves.size();
+
+          // If the current root position is in the tablebases then RootMoves
+          // contains only moves that preserve the draw or win.
+          RootInTB = Tablebases::root_probe(RootPos, TBScore);
+
+          if (RootInTB)
+          {
+              TBCardinality = 0; // Do not probe tablebases during the search
+
+              // It might be a good idea to mangle the hash key (xor it
+              // with a fixed value) in order to "clear" the hash table of
+              // the results of previous probes. However, that would have to
+              // be done from within the Position class, so we skip it for now.
+
+              // Optional: decrease target time.
+          }
+          else // If DTZ tables are missing, use WDL tables as a fallback
+          {
+              // Filter out moves that do not preserve a draw or win
+              RootInTB = Tablebases::root_probe_wdl(RootPos, TBScore);
+
+              // Only probe during search if winning
+              if (TBScore <= VALUE_DRAW)
+                  TBCardinality = 0;
+          }
+
+          if (!RootInTB)
+          {
+              TBHits = 0;
+          }
+          else if (!TB50MoveRule)
+          {
+              TBScore = TBScore > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
+                      : TBScore < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
+                      : TBScore;
+          }
+      }
+#endif
+
       for (size_t i = 0; i < Threads.size(); ++i)
           Threads[i]->maxPly = 0;
 
@@ -486,6 +552,39 @@ namespace {
         return ttValue;
     }
 
+#ifdef SYZYGY
+    // Step 4a. Tablebase probe
+    if (   !RootNode
+        && pos.total_piece_count() <= TBCardinality
+        && ( pos.total_piece_count() < TBCardinality || depth >= TBProbeDepth )
+        && pos.rule50_count() == 0)
+    {
+        int found, v = Tablebases::probe_wdl(pos, &found);
+
+        if (found)
+        {
+            TBHits++;
+
+            if (TB50MoveRule) {
+                value = v < -1 ? -VALUE_MATE + MAX_PLY + ss->ply
+                      : v >  1 ?  VALUE_MATE - MAX_PLY - ss->ply
+                      : VALUE_DRAW + 2 * v;
+            }
+            else
+            {
+                value = v < 0 ? -VALUE_MATE + MAX_PLY + ss->ply
+                      : v > 0 ?  VALUE_MATE - MAX_PLY - ss->ply
+                      : VALUE_DRAW;
+            }
+
+            TT.store(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
+                     std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY), MOVE_NONE, VALUE_NONE);
+
+            return value;
+        }
+    }
+#endif
+
     // Step 5. Evaluate the position statically and update parent's gain statistics
     if (inCheck)
     {
@@ -1352,15 +1451,25 @@ moves_loop: // When in check and at SpNode search starts from here
         Depth d = updated ? depth : depth - ONE_PLY;
         Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
 
+        bool tb = RootInTB;
+        if (tb)
+        {
+            if (abs(v) >= VALUE_MATE - MAX_PLY)
+                tb = false;
+            else
+                v = TBScore;
+        }
+
         if (ss.rdbuf()->in_avail()) // Not at first line
             ss << "\n";
 
         ss << "info depth " << d / ONE_PLY
            << " seldepth "  << selDepth
            << " multipv "   << i + 1
-           << " score "     << (i == PVIdx ? UCI::format_value(v, alpha, beta) : UCI::format_value(v))
+           << " score "     << ((!tb && i == PVIdx) ? UCI::format_value(v, alpha, beta) : UCI::format_value(v))
            << " nodes "     << pos.nodes_searched()
            << " nps "       << pos.nodes_searched() * 1000 / elapsed
+           << " tbhits "    << TBHits
            << " time "      << elapsed
            << " pv";
 

src/syzygy/tbcore.h

@@ -0,0 +1,157 @@
+/*
+  Copyright (c) 2011-2013 Ronald de Man
+*/
+
+#ifndef TBCORE_H
+#define TBCORE_H
+
+#ifndef __WIN32__
+#include <pthread.h>
+#define SEP_CHAR ':'
+#define FD int
+#define FD_ERR -1
+#else
+#include <windows.h>
+#define SEP_CHAR ';'
+#define FD HANDLE
+#define FD_ERR INVALID_HANDLE_VALUE
+#endif
+
+#ifndef __WIN32__
+#define LOCK_T pthread_mutex_t
+#define LOCK_INIT(x) pthread_mutex_init(&(x), NULL)
+#define LOCK(x) pthread_mutex_lock(&(x))
+#define UNLOCK(x) pthread_mutex_unlock(&(x))
+#else
+#define LOCK_T HANDLE
+#define LOCK_INIT(x) do { x = CreateMutex(NULL, FALSE, NULL); } while (0)
+#define LOCK(x) WaitForSingleObject(x, INFINITE)
+#define UNLOCK(x) ReleaseMutex(x)
+#endif
+
+#define WDLSUFFIX ".rtbw"
+#define DTZSUFFIX ".rtbz"
+#define WDLDIR "RTBWDIR"
+#define DTZDIR "RTBZDIR"
+#define TBPIECES 6
+
+typedef unsigned long long uint64;
+typedef unsigned int uint32;
+typedef unsigned char ubyte;
+typedef unsigned short ushort;
+
+const ubyte WDL_MAGIC[4] = { 0x71, 0xe8, 0x23, 0x5d };
+const ubyte DTZ_MAGIC[4] = { 0xd7, 0x66, 0x0c, 0xa5 };
+
+#define TBHASHBITS 10
+
+struct TBHashEntry;
+
+typedef uint64 base_t;
+
+struct PairsData {
+  char *indextable;
+  ushort *sizetable;
+  ubyte *data;
+  ushort *offset;
+  ubyte *symlen;
+  ubyte *sympat;
+  int blocksize;
+  int idxbits;
+  int min_len;
+  base_t base[1]; // C++ complains about base[]...
+};
+
+struct TBEntry {
+  char *data;
+  uint64 key;
+  uint64 mapping;
+  ubyte ready;
+  ubyte num;
+  ubyte symmetric;
+  ubyte has_pawns;
+} __attribute__((__may_alias__));
+
+struct TBEntry_piece {
+  char *data;
+  uint64 key;
+  uint64 mapping;
+  ubyte ready;
+  ubyte num;
+  ubyte symmetric;
+  ubyte has_pawns;
+  ubyte enc_type;
+  struct PairsData *precomp[2];
+  int factor[2][TBPIECES];
+  ubyte pieces[2][TBPIECES];
+  ubyte norm[2][TBPIECES];
+};
+
+struct TBEntry_pawn {
+  char *data;
+  uint64 key;
+  uint64 mapping;
+  ubyte ready;
+  ubyte num;
+  ubyte symmetric;
+  ubyte has_pawns;
+  ubyte pawns[2];
+  struct {
+    struct PairsData *precomp[2];
+    int factor[2][TBPIECES];
+    ubyte pieces[2][TBPIECES];
+    ubyte norm[2][TBPIECES];
+  } file[4];
+};
+
+struct DTZEntry_piece {
+  char *data;
+  uint64 key;
+  uint64 mapping;
+  ubyte ready;
+  ubyte num;
+  ubyte symmetric;
+  ubyte has_pawns;
+  ubyte enc_type;
+  struct PairsData *precomp;
+  int factor[TBPIECES];
+  ubyte pieces[TBPIECES];
+  ubyte norm[TBPIECES];
+  ubyte flags; // accurate, mapped, side
+  ushort map_idx[4];
+  ubyte *map;
+};
+
+struct DTZEntry_pawn {
+  char *data;
+  uint64 key;
+  uint64 mapping;
+  ubyte ready;
+  ubyte num;
+  ubyte symmetric;
+  ubyte has_pawns;
+  ubyte pawns[2];
+  struct {
+    struct PairsData *precomp;
+    int factor[TBPIECES];
+    ubyte pieces[TBPIECES];
+    ubyte norm[TBPIECES];
+  } file[4];
+  ubyte flags[4];
+  ushort map_idx[4][4];
+  ubyte *map;
+};
+
+struct TBHashEntry {
+  uint64 key;
+  struct TBEntry *ptr;
+};
+
+struct DTZTableEntry {
+  uint64 key1;
+  uint64 key2;
+  struct TBEntry *entry;
+};
+
+#endif
+

src/syzygy/tbprobe.cpp

@@ -0,0 +1,825 @@
+/*
+  Copyright (c) 2013 Ronald de Man
+  This file may be redistributed and/or modified without restrictions.
+
+  tbprobe.cpp contains the Stockfish-specific routines of the
+  tablebase probing code. It should be relatively easy to adapt
+  this code to other chess engines.
+*/
+
+#include "../position.h"
+#include "../movegen.h"
+#include "../bitboard.h"
+#include "../search.h"
+#include "../bitcount.h"
+
+#include "tbprobe.h"
+#include "tbcore.h"
+
+#include "tbcore.cpp"
+
+namespace Zobrist {
+  extern Key psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
+}
+
+int Tablebases::TBLargest = 0;
+
+// Given a position with 6 or fewer pieces, produce a text string
+// of the form KQPvKRP, where "KQP" represents the white pieces if
+// mirror == 0 and the black pieces if mirror == 1.
+static void prt_str(Position& pos, char *str, int mirror)
+{
+  Color color;
+  PieceType pt;
+  int i;
+  
+  color = !mirror ? WHITE : BLACK;
+  for (pt = KING; pt >= PAWN; --pt)
+    for (i = popcount<Max15>(pos.pieces(color, pt)); i > 0; i--)
+      *str++ = pchr[6 - pt];
+  *str++ = 'v';
+  color = ~color;
+  for (pt = KING; pt >= PAWN; --pt)
+    for (i = popcount<Max15>(pos.pieces(color, pt)); i > 0; i--)
+      *str++ = pchr[6 - pt];
+  *str++ = 0;
+}
+
+// Given a position, produce a 64-bit material signature key.
+// If the engine supports such a key, it should equal the engine's key.
+static uint64 calc_key(Position& pos, int mirror)
+{
+  Color color;
+  PieceType pt;
+  int i;
+  uint64 key = 0;
+
+  color = !mirror ? WHITE : BLACK;
+  for (pt = PAWN; pt <= KING; ++pt)
+    for (i = popcount<Max15>(pos.pieces(color, pt)); i > 0; i--)
+      key ^= Zobrist::psq[WHITE][pt][i - 1];
+  color = ~color;
+  for (pt = PAWN; pt <= KING; ++pt)
+    for (i = popcount<Max15>(pos.pieces(color, pt)); i > 0; i--)
+      key ^= Zobrist::psq[BLACK][pt][i - 1];
+
+  return key;
+}
+
+// Produce a 64-bit material key corresponding to the material combination
+// defined by pcs[16], where pcs[1], ..., pcs[6] is the number of white
+// pawns, ..., kings and pcs[9], ..., pcs[14] is the number of black
+// pawns, ..., kings.
+static uint64 calc_key_from_pcs(int *pcs, int mirror)
+{
+  int color;
+  PieceType pt;
+  int i;
+  uint64 key = 0;
+
+  color = !mirror ? 0 : 8;
+  for (pt = PAWN; pt <= KING; ++pt)
+    for (i = 0; i < pcs[color + pt]; i++)
+      key ^= Zobrist::psq[WHITE][pt][i];
+  color ^= 8;
+  for (pt = PAWN; pt <= KING; ++pt)
+    for (i = 0; i < pcs[color + pt]; i++)
+      key ^= Zobrist::psq[BLACK][pt][i];
+
+  return key;
+}
+
+bool is_little_endian() {
+  union {
+    int i;
+    char c[sizeof(int)];
+  } x;
+  x.i = 1;
+  return x.c[0] == 1;
+}
+
+static ubyte decompress_pairs(struct PairsData *d, uint64 idx)
+{
+  static const bool isLittleEndian = is_little_endian();
+  return isLittleEndian ? decompress_pairs<true >(d, idx)
+                        : decompress_pairs<false>(d, idx);
+}
+
+// probe_wdl_table and probe_dtz_table require similar adaptations.
+static int probe_wdl_table(Position& pos, int *success)
+{
+  struct TBEntry *ptr;
+  struct TBHashEntry *ptr2;
+  uint64 idx;
+  uint64 key;
+  int i;
+  ubyte res;
+  int p[TBPIECES];
+
+  // Obtain the position's material signature key.
+  key = pos.material_key();
+
+  // Test for KvK.
+  if (key == (Zobrist::psq[WHITE][KING][0] ^ Zobrist::psq[BLACK][KING][0]))
+    return 0;
+
+  ptr2 = TB_hash[key >> (64 - TBHASHBITS)];
+  for (i = 0; i < HSHMAX; i++)
+    if (ptr2[i].key == key) break;
+  if (i == HSHMAX) {
+    *success = 0;
+    return 0;
+  }
+
+  ptr = ptr2[i].ptr;
+  if (!ptr->ready) {
+    LOCK(TB_mutex);
+    if (!ptr->ready) {
+      char str[16];
+      prt_str(pos, str, ptr->key != key);
+      if (!init_table_wdl(ptr, str)) {
+        ptr2[i].key = 0ULL;
+        *success = 0;
+        UNLOCK(TB_mutex);
+        return 0;
+      }
+      // Memory barrier to ensure ptr->ready = 1 is not reordered.
+      __asm__ __volatile__ ("" ::: "memory");
+      ptr->ready = 1;
+    }
+    UNLOCK(TB_mutex);
+  }
+
+  int bside, mirror, cmirror;
+  if (!ptr->symmetric) {
+    if (key != ptr->key) {
+      cmirror = 8;
+      mirror = 0x38;
+      bside = (pos.side_to_move() == WHITE);
+    } else {
+      cmirror = mirror = 0;
+      bside = !(pos.side_to_move() == WHITE);
+    }
+  } else {
+    cmirror = pos.side_to_move() == WHITE ? 0 : 8;
+    mirror = pos.side_to_move() == WHITE ? 0 : 0x38;
+    bside = 0;
+  }
+
+  // p[i] is to contain the square 0-63 (A1-H8) for a piece of type
+  // pc[i] ^ cmirror, where 1 = white pawn, ..., 14 = black king.
+  // Pieces of the same type are guaranteed to be consecutive.
+  if (!ptr->has_pawns) {
+    struct TBEntry_piece *entry = (struct TBEntry_piece *)ptr;
+    ubyte *pc = entry->pieces[bside];
+    for (i = 0; i < entry->num;) {
+      Bitboard bb = pos.pieces((Color)((pc[i] ^ cmirror) >> 3),
+                                      (PieceType)(pc[i] & 0x07));
+      do {
+        p[i++] = pop_lsb(&bb);
+      } while (bb);
+    }
+    idx = encode_piece(entry, entry->norm[bside], p, entry->factor[bside]);
+    res = decompress_pairs(entry->precomp[bside], idx);
+  } else {
+    struct TBEntry_pawn *entry = (struct TBEntry_pawn *)ptr;
+    int k = entry->file[0].pieces[0][0] ^ cmirror;
+    Bitboard bb = pos.pieces((Color)(k >> 3), (PieceType)(k & 0x07));
+    i = 0;
+    do {
+      p[i++] = pop_lsb(&bb) ^ mirror;
+    } while (bb);
+    int f = pawn_file(entry, p);
+    ubyte *pc = entry->file[f].pieces[bside];
+    for (; i < entry->num;) {
+      bb = pos.pieces((Color)((pc[i] ^ cmirror) >> 3),
+                                    (PieceType)(pc[i] & 0x07));
+      do {
+        p[i++] = pop_lsb(&bb) ^ mirror;
+      } while (bb);
+    }
+    idx = encode_pawn(entry, entry->file[f].norm[bside], p, entry->file[f].factor[bside]);
+    res = decompress_pairs(entry->file[f].precomp[bside], idx);
+  }
+
+  return ((int)res) - 2;
+}
+
+static int probe_dtz_table(Position& pos, int wdl, int *success)
+{
+  struct TBEntry *ptr;
+  uint64 idx;
+  int i, res;
+  int p[TBPIECES];
+
+  // Obtain the position's material signature key.
+  uint64 key = pos.material_key();
+
+  if (DTZ_table[0].key1 != key && DTZ_table[0].key2 != key) {
+    for (i = 1; i < DTZ_ENTRIES; i++)
+      if (DTZ_table[i].key1 == key) break;
+    if (i < DTZ_ENTRIES) {
+      struct DTZTableEntry table_entry = DTZ_table[i];
+      for (; i > 0; i--)
+        DTZ_table[i] = DTZ_table[i - 1];
+      DTZ_table[0] = table_entry;
+    } else {
+      struct TBHashEntry *ptr2 = TB_hash[key >> (64 - TBHASHBITS)];
+      for (i = 0; i < HSHMAX; i++)
+        if (ptr2[i].key == key) break;
+      if (i == HSHMAX) {
+        *success = 0;
+        return 0;
+      }
+      ptr = ptr2[i].ptr;
+      char str[16];
+      int mirror = (ptr->key != key);
+      prt_str(pos, str, mirror);
+      if (DTZ_table[DTZ_ENTRIES - 1].entry)
+        free_dtz_entry(DTZ_table[DTZ_ENTRIES-1].entry);
+      for (i = DTZ_ENTRIES - 1; i > 0; i--)
+        DTZ_table[i] = DTZ_table[i - 1];
+      load_dtz_table(str, calc_key(pos, mirror), calc_key(pos, !mirror));
+    }
+  }
+
+  ptr = DTZ_table[0].entry;
+  if (!ptr) {
+    *success = 0;
+    return 0;
+  }
+
+  int bside, mirror, cmirror;
+  if (!ptr->symmetric) {
+    if (key != ptr->key) {
+      cmirror = 8;
+      mirror = 0x38;
+      bside = (pos.side_to_move() == WHITE);
+    } else {
+      cmirror = mirror = 0;
+      bside = !(pos.side_to_move() == WHITE);
+    }
+  } else {
+    cmirror = pos.side_to_move() == WHITE ? 0 : 8;
+    mirror = pos.side_to_move() == WHITE ? 0 : 0x38;
+    bside = 0;
+  }
+
+  if (!ptr->has_pawns) {
+    struct DTZEntry_piece *entry = (struct DTZEntry_piece *)ptr;
+    if ((entry->flags & 1) != bside && !entry->symmetric) {
+      *success = -1;
+      return 0;
+    }
+    ubyte *pc = entry->pieces;
+    for (i = 0; i < entry->num;) {
+      Bitboard bb = pos.pieces((Color)((pc[i] ^ cmirror) >> 3),
+                                    (PieceType)(pc[i] & 0x07));
+      do {
+        p[i++] = pop_lsb(&bb);
+      } while (bb);
+    }
+    idx = encode_piece((struct TBEntry_piece *)entry, entry->norm, p, entry->factor);
+    res = decompress_pairs(entry->precomp, idx);
+
+    if (entry->flags & 2)
+      res = entry->map[entry->map_idx[wdl_to_map[wdl + 2]] + res];
+
+    if (!(entry->flags & pa_flags[wdl + 2]) || (wdl & 1))
+      res *= 2;
+  } else {
+    struct DTZEntry_pawn *entry = (struct DTZEntry_pawn *)ptr;
+    int k = entry->file[0].pieces[0] ^ cmirror;
+    Bitboard bb = pos.pieces((Color)(k >> 3), (PieceType)(k & 0x07));
+    i = 0;
+    do {
+      p[i++] = pop_lsb(&bb) ^ mirror;
+    } while (bb);
+    int f = pawn_file((struct TBEntry_pawn *)entry, p);
+    if ((entry->flags[f] & 1) != bside) {
+      *success = -1;
+      return 0;
+    }
+    ubyte *pc = entry->file[f].pieces;
+    for (; i < entry->num;) {
+      bb = pos.pieces((Color)((pc[i] ^ cmirror) >> 3),
+                            (PieceType)(pc[i] & 0x07));
+      do {
+        p[i++] = pop_lsb(&bb) ^ mirror;
+      } while (bb);
+    }
+    idx = encode_pawn((struct TBEntry_pawn *)entry, entry->file[f].norm, p, entry->file[f].factor);
+    res = decompress_pairs(entry->file[f].precomp, idx);
+
+    if (entry->flags[f] & 2)
+      res = entry->map[entry->map_idx[f][wdl_to_map[wdl + 2]] + res];
+
+    if (!(entry->flags[f] & pa_flags[wdl + 2]) || (wdl & 1))
+      res *= 2;
+  }
+
+  return res;
+}
+
+// Add underpromotion captures to list of captures.
+static ExtMove *add_underprom_caps(Position& pos, ExtMove *stack, ExtMove *end)
+{
+  ExtMove *moves, *extra = end;
+
+  for (moves = stack; moves < end; moves++) {
+    Move move = moves->move;
+    if (type_of(move) == PROMOTION && !pos.empty(to_sq(move))) {
+      (*extra++).move = (Move)(move - (1 << 12));
+      (*extra++).move = (Move)(move - (2 << 12));
+      (*extra++).move = (Move)(move - (3 << 12));
+    }
+  }
+
+  return extra;
+}
+
+static int probe_ab(Position& pos, int alpha, int beta, int *success)
+{
+  int v;
+  ExtMove stack[64];
+  ExtMove *moves, *end;
+  StateInfo st;
+
+  // Generate (at least) all legal non-ep captures including (under)promotions.
+  // It is OK to generate more, as long as they are filtered out below.
+  if (!pos.checkers()) {
+    end = generate<CAPTURES>(pos, stack);
+    // Since underpromotion captures are not included, we need to add them.
+    end = add_underprom_caps(pos, stack, end);
+  } else
+    end = generate<EVASIONS>(pos, stack);
+
+  CheckInfo ci(pos);
+
+  for (moves = stack; moves < end; moves++) {
+    Move capture = moves->move;
+    if (!pos.capture(capture) || type_of(capture) == ENPASSANT
+                        || !pos.legal(capture, ci.pinned))
+      continue;
+    pos.do_move(capture, st, ci, pos.gives_check(capture, ci));
+    v = -probe_ab(pos, -beta, -alpha, success);
+    pos.undo_move(capture);
+    if (*success == 0) return 0;
+    if (v > alpha) {
+      if (v >= beta) {
+        *success = 2;
+        return v;
+      }
+      alpha = v;
+    }
+  }
+
+  v = probe_wdl_table(pos, success);
+  if (*success == 0) return 0;
+  if (alpha >= v) {
+    *success = 1 + (alpha > 0);
+    return alpha;
+  } else {
+    *success = 1;
+    return v;
+  }
+}
+
+// Probe the WDL table for a particular position.
+// If *success != 0, the probe was successful.
+// The return value is from the point of view of the side to move:
+// -2 : loss
+// -1 : loss, but draw under 50-move rule
+//  0 : draw
+//  1 : win, but draw under 50-move rule
+//  2 : win
+int Tablebases::probe_wdl(Position& pos, int *success)
+{
+  int v;
+
+  *success = 1;
+  v = probe_ab(pos, -2, 2, success);
+
+  // If en passant is not possible, we are done.
+  if (pos.ep_square() == SQ_NONE)
+    return v;
+  if (!(*success)) return 0;
+
+  // Now handle en passant.
+  int v1 = -3;
+  // Generate (at least) all legal en passant captures.
+  ExtMove stack[192];
+  ExtMove *moves, *end;
+  StateInfo st;
+
+  if (!pos.checkers())
+    end = generate<CAPTURES>(pos, stack);
+  else
+    end = generate<EVASIONS>(pos, stack);
+
+  CheckInfo ci(pos);
+
+  for (moves = stack; moves < end; moves++) {
+    Move capture = moves->move;
+    if (type_of(capture) != ENPASSANT
+          || !pos.legal(capture, ci.pinned))
+      continue;
+    pos.do_move(capture, st, ci, pos.gives_check(capture, ci));
+    int v0 = -probe_ab(pos, -2, 2, success);
+    pos.undo_move(capture);
+    if (*success == 0) return 0;
+    if (v0 > v1) v1 = v0;
+  }
+  if (v1 > -3) {
+    if (v1 >= v) v = v1;
+    else if (v == 0) {
+      // Check whether there is at least one legal non-ep move.
+      for (moves = stack; moves < end; moves++) {
+        Move capture = moves->move;
+        if (type_of(capture) == ENPASSANT) continue;
+        if (pos.legal(capture, ci.pinned)) break;
+      }
+      if (moves == end && !pos.checkers()) {
+        end = generate<QUIETS>(pos, end);
+        for (; moves < end; moves++) {
+          Move move = moves->move;
+          if (pos.legal(move, ci.pinned))
+            break;
+        }
+      }
+      // If not, then we are forced to play the losing ep capture.
+      if (moves == end)
+        v = v1;
+    }
+  }
+
+  return v;
+}
+
+// This routine treats a position with en passant captures as one without.
+static int probe_dtz_no_ep(Position& pos, int *success)
+{
+  int wdl, dtz;
+
+  wdl = probe_ab(pos, -2, 2, success);
+  if (*success == 0) return 0;
+
+  if (wdl == 0) return 0;
+
+  if (*success == 2)
+    return wdl == 2 ? 1 : 101;
+
+  ExtMove stack[192];
+  ExtMove *moves, *end = NULL;
+  StateInfo st;
+  CheckInfo ci(pos);
+
+  if (wdl > 0) {
+    // Generate at least all legal non-capturing pawn moves
+    // including non-capturing promotions.
+    if (!pos.checkers())
+      end = generate<NON_EVASIONS>(pos, stack);
+    else
+      end = generate<EVASIONS>(pos, stack);
+
+    for (moves = stack; moves < end; moves++) {
+      Move move = moves->move;
+      if (type_of(pos.moved_piece(move)) != PAWN || pos.capture(move)
+                || !pos.legal(move, ci.pinned))
+        continue;
+      pos.do_move(move, st, ci, pos.gives_check(move, ci));
+      int v = -probe_ab(pos, -2, -wdl + 1, success);
+      pos.undo_move(move);
+      if (*success == 0) return 0;
+      if (v == wdl)
+        return v == 2 ? 1 : 101;
+    }
+  }
+
+  dtz = 1 + probe_dtz_table(pos, wdl, success);
+  if (*success >= 0) {
+    if (wdl & 1) dtz += 100;
+    return wdl >= 0 ? dtz : -dtz;
+  }
+
+  if (wdl > 0) {
+    int best = 0xffff;
+    for (moves = stack; moves < end; moves++) {
+      Move move = moves->move;
+      if (pos.capture(move) || type_of(pos.moved_piece(move)) == PAWN
+                || !pos.legal(move, ci.pinned))
+        continue;
+      pos.do_move(move, st, ci, pos.gives_check(move, ci));
+      int v = -Tablebases::probe_dtz(pos, success);
+      pos.undo_move(move);
+      if (*success == 0) return 0;
+      if (v > 0 && v + 1 < best)
+        best = v + 1;
+    }
+    return best;
+  } else {
+    int best = -1;
+    if (!pos.checkers())
+      end = generate<NON_EVASIONS>(pos, stack);
+    else
+      end = generate<EVASIONS>(pos, stack);
+    for (moves = stack; moves < end; moves++) {
+      int v;
+      Move move = moves->move;
+      if (!pos.legal(move, ci.pinned))
+        continue;
+      pos.do_move(move, st, ci, pos.gives_check(move, ci));
+      if (st.rule50 == 0) {
+        if (wdl == -2) v = -1;
+        else {
+          v = probe_ab(pos, 1, 2, success);
+          v = (v == 2) ? 0 : -101;
+        }
+      } else {
+        v = -Tablebases::probe_dtz(pos, success) - 1;
+      }
+      pos.undo_move(move);
+      if (*success == 0) return 0;
+      if (v < best)
+        best = v;
+    }
+    return best;
+  }
+}
+
+static int wdl_to_dtz[] = {
+  -1, -101, 0, 101, 1
+};
+
+// Probe the DTZ table for a particular position.
+// If *success != 0, the probe was successful.
+// The return value is from the point of view of the side to move:
+//         n < -100 : loss, but draw under 50-move rule
+// -100 <= n < -1   : loss in n ply (assuming 50-move counter == 0)
+//         0        : draw
+//     1 < n <= 100 : win in n ply (assuming 50-move counter == 0)
+//   100 < n        : win, but draw under 50-move rule
+//
+// The return value n can be off by 1: a return value -n can mean a loss
+// in n+1 ply and a return value +n can mean a win in n+1 ply. This
+// cannot happen for tables with positions exactly on the "edge" of
+// the 50-move rule.
+//
+// This implies that if dtz > 0 is returned, the position is certainly
+// a win if dtz + 50-move-counter <= 99. Care must be taken that the engine
+// picks moves that preserve dtz + 50-move-counter <= 99.
+//
+// If n = 100 immediately after a capture or pawn move, then the position
+// is also certainly a win, and during the whole phase until the next
+// capture or pawn move, the inequality to be preserved is
+// dtz + 50-movecounter <= 100.
+//
+// In short, if a move is available resulting in dtz + 50-move-counter <= 99,
+// then do not accept moves leading to dtz + 50-move-counter == 100.
+//
+int Tablebases::probe_dtz(Position& pos, int *success)
+{
+  *success = 1;
+  int v = probe_dtz_no_ep(pos, success);
+
+  if (pos.ep_square() == SQ_NONE)
+    return v;
+  if (*success == 0) return 0;
+
+  // Now handle en passant.
+  int v1 = -3;
+
+  ExtMove stack[192];
+  ExtMove *moves, *end;
+  StateInfo st;
+
+  if (!pos.checkers())
+    end = generate<CAPTURES>(pos, stack);
+  else
+    end = generate<EVASIONS>(pos, stack);
+  CheckInfo ci(pos);
+
+  for (moves = stack; moves < end; moves++) {
+    Move capture = moves->move;
+    if (type_of(capture) != ENPASSANT
+                || !pos.legal(capture, ci.pinned))
+      continue;
+    pos.do_move(capture, st, ci, pos.gives_check(capture, ci));
+    int v0 = -probe_ab(pos, -2, 2, success);
+    pos.undo_move(capture);
+    if (*success == 0) return 0;
+    if (v0 > v1) v1 = v0;
+  }
+  if (v1 > -3) {
+    v1 = wdl_to_dtz[v1 + 2];
+    if (v < -100) {
+      if (v1 >= 0)
+        v = v1;
+    } else if (v < 0) {
+      if (v1 >= 0 || v1 < 100)
+        v = v1;
+    } else if (v > 100) {
+      if (v1 > 0)
+        v = v1;
+    } else if (v > 0) {
+      if (v1 == 1)
+        v = v1;
+    } else if (v1 >= 0) {
+      v = v1;
+    } else {
+      for (moves = stack; moves < end; moves++) {
+        Move move = moves->move;
+        if (type_of(move) == ENPASSANT) continue;
+        if (pos.legal(move, ci.pinned)) break;
+      }
+      if (moves == end && !pos.checkers()) {
+        end = generate<QUIETS>(pos, end);
+        for (; moves < end; moves++) {
+          Move move = moves->move;
+          if (pos.legal(move, ci.pinned))
+            break;
+        }
+      }
+      if (moves == end)
+        v = v1;
+    }
+  }
+
+  return v;
+}
+
+// Check whether there has been at least one repetition of positions
+// since the last capture or pawn move.
+static int has_repeated(StateInfo *st)
+{
+  while (1) {
+    int i = 4, e = std::min(st->rule50, st->pliesFromNull);
+    if (e < i)
+      return 0;
+    StateInfo *stp = st->previous->previous;
+    do {
+      stp = stp->previous->previous;
+      if (stp->key == st->key)
+        return 1;
+      i += 2;
+    } while (i <= e);
+    st = st->previous;
+  }
+}
+
+static Value wdl_to_Value[5] = {
+  -VALUE_MATE + MAX_PLY + 1,
+  VALUE_DRAW - 2,
+  VALUE_DRAW,
+  VALUE_DRAW + 2,
+  VALUE_MATE - MAX_PLY - 1
+};
+
+// Use the DTZ tables to filter out moves that don't preserve the win or draw.
+// If the position is lost, but DTZ is fairly high, only keep moves that
+// maximise DTZ.
+//
+// A return value false indicates that not all probes were successful and that
+// no moves were filtered out.
+bool Tablebases::root_probe(Position& pos, Value& TBScore)
+{
+  int success;
+
+  int dtz = probe_dtz(pos, &success);
+  if (!success) return false;
+
+  StateInfo st;
+  CheckInfo ci(pos);
+
+  // Probe each move.
+  for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+    Move move = Search::RootMoves[i].pv[0];
+    pos.do_move(move, st, ci, pos.gives_check(move, ci));
+    int v = 0;
+    if (pos.checkers() && dtz > 0) {
+      ExtMove s[192];
+      if (generate<LEGAL>(pos, s) == s)
+        v = 1;
+    }
+    if (!v) {
+      if (st.rule50 != 0) {
+        v = -Tablebases::probe_dtz(pos, &success);
+        if (v > 0) v++;
+        else if (v < 0) v--;
+      } else {
+        v = -Tablebases::probe_wdl(pos, &success);
+        v = wdl_to_dtz[v + 2];
+      }
+    }
+    pos.undo_move(move);
+    if (!success) return false;
+    Search::RootMoves[i].score = (Value)v;
+  }
+
+  // Obtain 50-move counter for the root position.
+  // In Stockfish there seems to be no clean way, so we do it like this:
+  int cnt50 = st.previous->rule50;
+
+  // Use 50-move counter to determine whether the root position is
+  // won, lost or drawn.
+  int wdl = 0;
+  if (dtz > 0)
+    wdl = (dtz + cnt50 <= 100) ? 2 : 1;
+  else if (dtz < 0)
+    wdl = (-dtz + cnt50 <= 100) ? -2 : -1;
+
+  // Determine the score to report to the user.
+  TBScore = wdl_to_Value[wdl + 2];
+  // If the position is winning or losing, but too few moves left, adjust the
+  // score to show how close it is to winning or losing.
+  // NOTE: int(PawnValueEg) is used as scaling factor in score_to_uci().
+  if (wdl == 1 && dtz <= 100)
+    TBScore = (Value)(((200 - dtz - cnt50) * int(PawnValueEg)) / 200);
+  else if (wdl == -1 && dtz >= -100)
+    TBScore = -(Value)(((200 + dtz - cnt50) * int(PawnValueEg)) / 200);
+
+  // Now be a bit smart about filtering out moves.
+  size_t j = 0;
+  if (dtz > 0) { // winning (or 50-move rule draw)
+    int best = 0xffff;
+    for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+      int v = Search::RootMoves[i].score;
+      if (v > 0 && v < best)
+        best = v;
+    }
+    int max = best;
+    // If the current phase has not seen repetitions, then try all moves
+    // that stay safely within the 50-move budget, if there are any.
+    if (!has_repeated(st.previous) && best + cnt50 <= 99)
+      max = 99 - cnt50;
+    for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+      int v = Search::RootMoves[i].score;
+      if (v > 0 && v <= max)
+        Search::RootMoves[j++] = Search::RootMoves[i];
+    }
+  } else if (dtz < 0) { // losing (or 50-move rule draw)
+    int best = 0;
+    for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+      int v = Search::RootMoves[i].score;
+      if (v < best)
+        best = v;
+    }
+    // Try all moves, unless we approach or have a 50-move rule draw.
+    if (-best * 2 + cnt50 < 100)
+      return true;
+    for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+      if (Search::RootMoves[i].score == best)
+        Search::RootMoves[j++] = Search::RootMoves[i];
+    }
+  } else { // drawing
+    // Try all moves that preserve the draw.
+    for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+      if (Search::RootMoves[i].score == 0)
+        Search::RootMoves[j++] = Search::RootMoves[i];
+    }
+  }
+  Search::RootMoves.resize(j, Search::RootMove(MOVE_NONE));
+
+  return true;
+}
+
+// Use the WDL tables to filter out moves that don't preserve the win or draw.
+// This is a fallback for the case that some or all DTZ tables are missing.
+//
+// A return value false indicates that not all probes were successful and that
+// no moves were filtered out.
+bool Tablebases::root_probe_wdl(Position& pos, Value& TBScore)
+{
+  int success;
+
+  int wdl = Tablebases::probe_wdl(pos, &success);
+  if (!success) return false;
+  TBScore = wdl_to_Value[wdl + 2];
+
+  StateInfo st;
+  CheckInfo ci(pos);
+
+  int best = -2;
+
+  // Probe each move.
+  for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+    Move move = Search::RootMoves[i].pv[0];
+    pos.do_move(move, st, ci, pos.gives_check(move, ci));
+    int v = -Tablebases::probe_wdl(pos, &success);
+    pos.undo_move(move);
+    if (!success) return false;
+    Search::RootMoves[i].score = (Value)v;
+    if (v > best)
+      best = v;
+  }
+
+  size_t j = 0;
+  for (size_t i = 0; i < Search::RootMoves.size(); i++) {
+    if (Search::RootMoves[i].score == best)
+      Search::RootMoves[j++] = Search::RootMoves[i];
+  }
+  Search::RootMoves.resize(j, Search::RootMove(MOVE_NONE));
+
+  return true;
+}
+

src/syzygy/tbprobe.h

@@ -0,0 +1,16 @@
+#ifndef TBPROBE_H
+#define TBPROBE_H
+
+namespace Tablebases {
+
+extern int TBLargest;
+
+void init(const std::string& path);
+int probe_wdl(Position& pos, int *success);
+int probe_dtz(Position& pos, int *success);
+bool root_probe(Position& pos, Value& TBScore);
+bool root_probe_wdl(Position& pos, Value& TBScore);
+
+}
+
+#endif

src/types.h

@@ -175,8 +175,8 @@ enum Value {
   VALUE_INFINITE  = 32001,
   VALUE_NONE      = 32002,
 
-  VALUE_MATE_IN_MAX_PLY  =  VALUE_MATE - MAX_PLY,
+  VALUE_MATE_IN_MAX_PLY  =  VALUE_MATE - 2 * MAX_PLY,
-  VALUE_MATED_IN_MAX_PLY = -VALUE_MATE + MAX_PLY,
+  VALUE_MATED_IN_MAX_PLY = -VALUE_MATE + 2 * MAX_PLY,
 
   VALUE_ENSURE_INTEGER_SIZE_P = INT_MAX,
   VALUE_ENSURE_INTEGER_SIZE_N = INT_MIN,

src/uci.cpp

@@ -227,7 +227,7 @@ string UCI::format_value(Value v, Value alpha, Value beta) {
 
   stringstream ss;
 
-  if (abs(v) < VALUE_MATE_IN_MAX_PLY)
+  if (abs(v) < VALUE_MATE - MAX_PLY)
       ss << "cp " << v * 100 / PawnValueEg;
   else
       ss << "mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;

src/ucioption.cpp

@@ -27,6 +27,10 @@
 #include "tt.h"
 #include "uci.h"
 
+#ifdef SYZYGY
+#include "syzygy/tbprobe.h"
+#endif
+
 using std::string;
 
 UCI::OptionsMap Options; // Global object
@@ -38,6 +42,9 @@ void on_logger(const Option& o) { start_logger(o); }
 void on_threads(const Option&) { Threads.read_uci_options(); }
 void on_hash_size(const Option& o) { TT.resize(o); }
 void on_clear_hash(const Option&) { TT.clear(); }
+#ifdef SYZYGY
+void on_tb_path(const Option& o) { Tablebases::init(o); }
+#endif
 
 
 /// Our case insensitive less() function as required by UCI protocol
@@ -65,6 +72,12 @@ void init(OptionsMap& o) {
   o["Minimum Thinking Time"] << Option(20, 0, 5000);
   o["Slow Mover"]            << Option(80, 10, 1000);
   o["UCI_Chess960"]          << Option(false);
+#ifdef SYZYGY
+  o["SyzygyPath"]            << Option("<empty>", on_tb_path);
+  o["SyzygyProbeDepth"]      << Option(1, 1, 100);
+  o["Syzygy50MoveRule"]      << Option(true);
+  o["SyzygyProbeLimit"]      << Option(6, 0, 6);
+#endif
 }