Fully comply 3 fold repetition

Since many simplification patches are added, we performed a test on them all and passed.
Task: Big Simplification
Book: 3mvs 140-200_150560
TC: 10+0.1
Total/Win/Draw/Lose: 6784 / 1254 / 4341 / 1189
PTNML: 21 / 654 / 1989 / 695 / 33
WinRate: 50.48%
ELO: 2.84[-1.60, 7.23]
LOS: 89.55
LLR: 3.46[-2.94, 2.94]

README.md

@@ -152,6 +152,9 @@ change them via a chess GUI. This is a list of available UCI options in Pikafish
   * #### Rule60
     Use 60 move rule in gameplay.
 
+  * #### Strict3Fold
+    Not only use 3 fold repetition at root nodes, but also use 3 fold at non-root nodes. Enable this will help get better analyses results but lose elo.
+
   * #### Debug Log File
     Write all communication to and from the engine into a text file.
 

src/position.cpp

@@ -937,6 +937,7 @@ bool Position::rule_judge(Value& result, int ply) const {
 
     if (end >= 4 && filter[st->key])
     {
+        int cnt = 1;
         StateInfo* stp = st->previous->previous;
         bool perpetualThem = st->checkersBB && stp->checkersBB;
         bool perpetualUs = st->previous->checkersBB && stp->previous->checkersBB;
@@ -946,8 +947,9 @@ bool Position::rule_judge(Value& result, int ply) const {
             stp = stp->previous->previous;
             perpetualThem &= bool(stp->checkersBB);
 
-            // Return a score if a position repeats once earlier.
-            if (stp->key == st->key)
+            // Return a score if a position repeats once earlier but strictly
+            // after the root, or repeats twice before or at the root.
+            if (stp->key == st->key && ++cnt == (!Strict3Fold && ply > i ? 2 : 3))
             {
                 if (perpetualThem || perpetualUs)
                 {
@@ -963,6 +965,7 @@ bool Position::rule_judge(Value& result, int ply) const {
                 rollback.set_chase_info(i);
 
                 // Chasing detection
+                cnt = 1;
                 stp = st->previous->previous;
                 uint16_t chaseThem = st->chased & stp->chased;
                 uint16_t chaseUs = st->previous->chased & stp->previous->chased;
@@ -974,8 +977,9 @@ bool Position::rule_judge(Value& result, int ply) const {
                         chaseThem &= stp->previous->previous->chased;
                     stp = stp->previous->previous;
 
-                    // Return a score if a position repeats once earlier.
-                    if (stp->key == st->key)
+                    // Return a score if a position repeats once earlier but strictly
+                    // after the root, or repeats twice before or at the root.
+                    if (stp->key == st->key && ++cnt == (!Strict3Fold && ply > i ? 2 : 3))
                     {
                         result = (chaseThem || chaseUs) ? (!chaseUs ? mate_in(ply) : !chaseThem ? mated_in(ply) : VALUE_DRAW) : VALUE_DRAW;
                         return true;

src/uci.h

@@ -81,6 +81,7 @@ Move to_move(const Position& pos, std::string& str);
 
 extern UCI::OptionsMap Options;
 extern bool UseRule60;
+extern bool Strict3Fold;
 
 } // namespace Stockfish
 

src/ucioption.cpp

@@ -34,6 +34,7 @@ namespace Stockfish {
 
 UCI::OptionsMap Options; // Global object
 bool UseRule60 = true;
+bool Strict3Fold = false;
 
 namespace UCI {
 
@@ -43,6 +44,7 @@ void on_hash_size(const Option& o) { TT.resize(size_t(o)); }
 void on_logger(const Option& o) { start_logger(o); }
 void on_threads(const Option& o) { Threads.set(size_t(o)); }
 void on_rule60(const Option& o) { UseRule60 = bool(o); }
+void on_strict3fold(const Option& o) { Strict3Fold = bool(o); }
 void on_eval_file(const Option& ) { Eval::NNUE::init(); }
 
 /// Our case insensitive less() function as required by UCI protocol
@@ -70,6 +72,7 @@ void init(OptionsMap& o) {
   o["Slow Mover"]            << Option(100, 10, 1000);
   o["nodestime"]             << Option(0, 0, 10000);
   o["Rule60"]                << Option(true, on_rule60);
+  o["Strict3Fold"]           << Option(false, on_strict3fold);
   o["UCI_LimitStrength"]     << Option(false);
   o["UCI_Elo"]               << Option(1350, 1350, 2850);
   o["UCI_WDLCentipawn"]      << Option(true);