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do not change cursed wins to draws #5805

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do not change cursed wins to draws #5805

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7 changes: 4 additions & 3 deletions src/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1952,6 +1952,7 @@ void syzygy_extend_pv(const OptionsMap& options,

auto t_start = std::chrono::steady_clock::now();
int moveOverhead = int(options["Move Overhead"]);
bool rule50 = bool(options["Syzygy50MoveRule"]);

// Do not use more than moveOverhead / 2 time, if time management is active
auto time_abort = [&t_start, &moveOverhead, &limits]() -> bool {
Expand Down Expand Up @@ -1989,7 +1990,7 @@ void syzygy_extend_pv(const OptionsMap& options,
pos.do_move(pvMove, st);

// Do not allow for repetitions or drawing moves along the PV in TB regime
if (config.rootInTB && pos.is_draw(ply))
if (config.rootInTB && ((rule50 && pos.is_draw(ply)) || pos.has_repeated()))
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pos.is_draw(ply) and pos.has_repeated() are different in how they flag 2 and 3 fold repetitions. However, it might be that pos.has_repeated() but that needs some motivation.

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Yes, I tried minimal changes at first, and did not want to add a new function to the code base at this stage. We may eventually have to, but I do not know.

{
pos.undo_move(pvMove);
ply--;
Expand All @@ -2008,7 +2009,7 @@ void syzygy_extend_pv(const OptionsMap& options,
// Step 2, now extend the PV to mate, as if the user explored syzygy-tables.info
// using top ranked moves (minimal DTZ), which gives optimal mates only for simple
// endgames e.g. KRvK.
while (!pos.is_draw(0))
while (!((rule50 && pos.is_draw(0)) || pos.has_repeated()))
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could be similar.

{
if (time_abort())
break;
Expand Down Expand Up @@ -2061,7 +2062,7 @@ void syzygy_extend_pv(const OptionsMap& options,
// We adjust the score to match the found PV. Note that a TB loss score can be
// displayed if the engine did not find a drawing move yet, but eventually search
// will figure it out (e.g. 1kq5/q2r4/5K2/8/8/8/8/7Q w - - 96 1 )
if (pos.is_draw(0))
if (rule50 && pos.is_draw(0))
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Also here not necessarily equivalent with 3-folds, not sure we can have that case.

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@robertnurnberg robertnurnberg Jan 20, 2025 •
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Since pos.is_draw(0) would flag any position after 50moves without capture or pawn move as draw, we have to guard this here by rule50 to avoid cursed wins to be changed to draws. I believe this is the only place that is strictly necessary to fix the bug. The other changes try to maintain the nice PV extension also for !rule50, without success so far it seems.

v = VALUE_DRAW;

// Undo the PV moves
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