/*
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.
*/
#define NOMINMAX
#include <algorithm>
#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::MaxCardinality = 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.
#ifdef _MSC_VER
_ReadWriteBarrier();
#else
__asm__ __volatile__ ("" ::: "memory");
#endif
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, 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, 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, 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, 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, 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, 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, Search::RootMoveVector& rootMoves, Value& score)
{
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 < rootMoves.size(); i++) {
Move move = rootMoves[i].pv[0];
pos.do_move(move, st, 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;
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.
score = 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)
score = (Value)(((200 - dtz - cnt50) * int(PawnValueEg)) / 200);
else if (wdl == -1 && dtz >= -100)
score = -(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 < rootMoves.size(); i++) {
int v = 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 < rootMoves.size(); i++) {
int v = rootMoves[i].score;
if (v > 0 && v <= max)
rootMoves[j++] = rootMoves[i];
}
} else if (dtz < 0) { // losing (or 50-move rule draw)
int best = 0;
for (size_t i = 0; i < rootMoves.size(); i++) {
int v = 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 < rootMoves.size(); i++) {
if (rootMoves[i].score == best)
rootMoves[j++] = rootMoves[i];
}
} else { // drawing
// Try all moves that preserve the draw.
for (size_t i = 0; i < rootMoves.size(); i++) {
if (rootMoves[i].score == 0)
rootMoves[j++] = rootMoves[i];
}
}
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, Search::RootMoveVector& rootMoves, Value& score)
{
int success;
int wdl = Tablebases::probe_wdl(pos, &success);
if (!success) return false;
score = wdl_to_Value[wdl + 2];
StateInfo st;
CheckInfo ci(pos);
int best = -2;
// Probe each move.
for (size_t i = 0; i < rootMoves.size(); i++) {
Move move = rootMoves[i].pv[0];
pos.do_move(move, st, pos.gives_check(move, ci));
int v = -Tablebases::probe_wdl(pos, &success);
pos.undo_move(move);
if (!success) return false;
rootMoves[i].score = (Value)v;
if (v > best)
best = v;
}
size_t j = 0;
for (size_t i = 0; i < rootMoves.size(); i++) {
if (rootMoves[i].score == best)
rootMoves[j++] = rootMoves[i];
}
rootMoves.resize(j, Search::RootMove(MOVE_NONE));
return true;
}