Fix incorrect draw detection

In this position we should have draw for repetition:

position fen rnbqkbnr/2pppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 moves g1f3 g8f6 f3g1
go infinite

But latest patch broke it.

Actually we had two(!) very subtle bugs, the first is that Position::set()
clears the passed state and in particular 'previous' member, so
that on passing setupStates, 'previous' pointer was reset.

Second bug is even more subtle: SetupStates was based on std::vector
as container, but when vector grows, std::vector copies all its contents
to a new location invalidating all references to its entries. Because
all StateInfo records are linked by 'previous' pointer, this made pointers
go stale upon adding more element to setupStates. So revert to use a
std::deque that ensures references are preserved when pushing back new
elements.

No functional change.

src/benchmark.cpp

@@ -148,7 +148,7 @@ void benchmark(const Position& current, istream& is) {
 
   for (size_t i = 0; i < fens.size(); ++i)
   {
-      StateListPtr states(new std::vector<StateInfo>(1));
+      StateListPtr states(new std::deque<StateInfo>(1));
       pos.set(fens[i], Options["UCI_Chess960"], &states->back(), Threads.main());
 
       cerr << "\nPosition: " << i + 1 << '/' << fens.size() << endl;

src/position.h

@@ -23,6 +23,7 @@
 
 #include <cassert>
 #include <cstddef>  // For offsetof()
+#include <deque>
 #include <memory>   // For std::unique_ptr
 #include <string>
 #include <vector>
@@ -77,7 +78,8 @@ struct StateInfo {
   StateInfo* previous;
 };
 
-typedef std::unique_ptr<std::vector<StateInfo>> StateListPtr;
+// In a std::deque references to elements are unaffected upon resizing
+typedef std::unique_ptr<std::deque<StateInfo>> StateListPtr;
 
 
 /// Position class stores information regarding the board representation as

src/thread.cpp

@@ -190,6 +190,8 @@ void ThreadPool::start_thinking(const Position& pos, StateListPtr& states,
   if (states.get())
       setupStates = std::move(states); // Ownership transfer, states is now empty
 
+  StateInfo tmp = setupStates->back();
+
   for (Thread* th : Threads)
   {
       th->maxPly = 0;
@@ -198,5 +200,7 @@ void ThreadPool::start_thinking(const Position& pos, StateListPtr& states,
       th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);
   }
 
+  setupStates->back() = tmp; // Restore st->previous, cleared by Position::set()
+
   main()->start_searching();
 }

src/uci.cpp

@@ -42,7 +42,7 @@ namespace {
   // A list to keep track of the position states along the setup moves (from the
   // start position to the position just before the search starts). Needed by
   // 'draw by repetition' detection.
-  StateListPtr States(new std::vector<StateInfo>(1));
+  StateListPtr States(new std::deque<StateInfo>(1));
 
 
   // position() is called when engine receives the "position" UCI command.
@@ -68,7 +68,7 @@ namespace {
     else
         return;
 
-    States = StateListPtr(new std::vector<StateInfo>(1));
+    States = StateListPtr(new std::deque<StateInfo>(1));
     pos.set(fen, Options["UCI_Chess960"], &States->back(), Threads.main());
 
     // Parse move list (if any)