handle insufficient material draws

no measurable elo gain

include/board.h

@@ -438,6 +438,24 @@ struct board{
     return turn() ? is_passed_push_<color::white>(mv) : is_passed_push_<color::black>(mv);
   }
 
+  template<color c>
+  size_t side_num_pieces() const {
+    return man_.us<c>().pawn().count() + 
+           man_.us<c>().knight().count() +
+           man_.us<c>().bishop().count() +
+           man_.us<c>().rook().count() +
+           man_.us<c>().queen().count() +
+           man_.us<c>().king().count();
+  }
+
+  size_t num_pieces() const {
+    return side_num_pieces<color::white>() + side_num_pieces<color::black>();
+  }
+
+  bool is_trivially_drawn() const {
+    return (num_pieces() == 2) || ((num_pieces() == 3) && (man_.white.knight() | man_.white.bishop() | man_.black.knight() | man_.black.bishop()).any());
+  }
+
   template<color c>
   board forward_(const move& mv) const {
     auto cpy = *this;

include/thread_worker.h

@@ -78,6 +78,7 @@ struct thread_worker{
     if(all_list.size() == 0 && is_check){ return mate_score<T>; }
     if(all_list.size() == 0) { return draw_score<T>; }
     if(ss.is_three_fold(bd.hash())){ return draw_score<T>; }
+    if(bd.is_trivially_drawn()){ return draw_score<T>; }
 
     const auto list = is_check ? all_list : all_list.loud();
     auto orderer = move_orderer(move_orderer_data{move::null(), move::null(), move::null(), &bd, list, &hh_.us(bd.turn())});
@@ -131,6 +132,7 @@ struct thread_worker{
     if(list.size() == 0 && is_check){ return make_result(mate_score<T>, move::null()); }
     if(list.size() == 0) { return make_result(draw_score<T>, move::null()); }
     if(ss.is_three_fold(bd.hash())){ return make_result(draw_score<T>, move::null()); }
+    if(!is_root && bd.is_trivially_drawn()){ return make_result(draw_score<T>, move::null()); }
 
     // don't drop into qsearch if in check
     if(is_check && depth <= 0){ depth = 1; }
@@ -175,7 +177,7 @@ struct thread_worker{
     // step 6. static null move pruning
     const bool snm_prune = 
       !is_root && !is_pv && 
-      !is_check && 
+      !is_check &&
       depth <= constants_ -> snmp_depth() &&
       static_eval > beta + constants_ -> snmp_margin<T>(improving, depth) &&
       static_eval > mate_score<T>;