StateInfo is usually allocated on the stack by search()

And passed in do_move(), this ensures maximum efficiency and
speed and at the same time unlimited move numbers.

The draw back is that to handle Position init we need to
reserve a StateInfo inside Position itself and use at
init time and when copying from another Position.

After lazy SMP we don't need anymore this gimmick and we can
get rid of this special case and always pass an external
StateInfo to Position object.

Also rewritten and simplified Position constructors.

Verified it does not regress with a 3 threads SMP test:
ELO: -0.00 +-12.7 (95%) LOS: 50.0%
Total: 1000 W: 173 L: 173 D: 654

No functional change.

src/position.h

@@ -23,7 +23,9 @@
 
 #include <cassert>
 #include <cstddef>  // For offsetof()
+#include <memory>   // For std::unique_ptr
 #include <string>
+#include <vector>
 
 #include "bitboard.h"
 #include "types.h"
@@ -75,6 +77,8 @@ struct StateInfo {
   StateInfo* previous;
 };
 
+typedef std::unique_ptr<std::vector<StateInfo>> StateListPtr;
+
 
 /// Position class stores information regarding the board representation as
 /// pieces, side to move, hash keys, castling info, etc. Important methods are
@@ -86,14 +90,12 @@ class Position {
 public:
   static void init();
 
-  Position() = default; // To define the global object RootPos
+  Position() = default;
   Position(const Position&) = delete;
-  Position(const Position& pos, Thread* th) { *this = pos; thisThread = th; }
-  Position(const std::string& f, bool c960, Thread* th) { set(f, c960, th); }
-  Position& operator=(const Position&); // To assign RootPos from UCI
+  Position& operator=(const Position&) = delete;
 
   // FEN string input/output
-  void set(const std::string& fenStr, bool isChess960, Thread* th);
+  Position& set(const std::string& fenStr, bool isChess960, StateInfo* si, Thread* th);
   const std::string fen() const;
 
   // Position representation
@@ -178,7 +180,6 @@ public:
 
 private:
   // Initialization helpers (used while setting up a position)
-  void clear();
   void set_castling_right(Color c, Square rfrom);
   void set_state(StateInfo* si) const;
 
@@ -200,7 +201,6 @@ private:
   int castlingRightsMask[SQUARE_NB];
   Square castlingRookSquare[CASTLING_RIGHT_NB];
   Bitboard castlingPath[CASTLING_RIGHT_NB];
-  StateInfo startState;
   uint64_t nodes;
   int gamePly;
   Color sideToMove;