Introduce evaluation damping based on the half-move clock.

src/search.cpp

@@ -466,7 +466,7 @@ Value Worker::search(
     if (!is_in_check) {
         correction      = m_td.history.get_correction(pos);
         raw_eval        = tt_data && !is_mate_score(tt_data->eval) ? tt_data->eval : evaluate(pos);
-        ss->static_eval = raw_eval + correction;
+        ss->static_eval = adj_shuffle(pos, raw_eval) + correction;
         improving = (ss - 2)->static_eval != -VALUE_INF && ss->static_eval > (ss - 2)->static_eval;
 
         if (!tt_data) {
@@ -952,7 +952,7 @@ Value Worker::quiesce(const Position& pos, Stack* ss, Value alpha, Value beta, i
     if (!is_in_check) {
         correction  = m_td.history.get_correction(pos);
         raw_eval    = tt_data && !is_mate_score(tt_data->eval) ? tt_data->eval : evaluate(pos);
-        static_eval = raw_eval + correction;
+        static_eval = adj_shuffle(pos, raw_eval) + correction;
 
         if (!tt_data) {
             m_searcher.tt.store(pos, ply, raw_eval, Move::none(), -VALUE_INF, 0, ttpv, Bound::None);
@@ -1045,5 +1045,19 @@ Value Worker::evaluate(const Position& pos) {
     return -VALUE_INF;  // Not implemented in tune mode
 #endif
 }
+
+Value Worker::adj_shuffle(const Position& pos, Value value) {
+    // During datagen, use raw evals only.
+    // source: chef.
+    if (m_searcher.settings.datagen) {
+        return value;
+    }
+
+    // Scale down the value when the fifty-move counter is high.
+    i32 clock = pos.get_50mr_counter();
+    value     = value * (200 - clock) / 200;
+
+    return value;
+}
 }  // namespace Search
 }  // namespace Clockwork

src/search.hpp

@@ -193,6 +193,7 @@ class alignas(128) Worker {
     template<bool IS_MAIN>
     Value quiesce(const Position& pos, Stack* ss, Value alpha, Value beta, i32 ply);
     Value evaluate(const Position& pos);
+    Value adj_shuffle(const Position& pos, Value value);
     bool  check_tm_hard_limit();
 };