Small touches to book.cpp

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.cpp

@@ -361,8 +361,8 @@ Book::~Book() {
 }
 
 
-/// Book::close() closes the file only if it is open, otherwise
-/// we can end up in a little mess due to how std::ifstream works.
+/// Book::close() closes the file only if it is open, otherwise the call fails
+/// and the failbit internal state flag is set.
 
 void Book::close() {
 
@@ -374,21 +374,20 @@ void Book::close() {
 }
 
 
-/// Book::open() opens a book file with a given file name
+/// Book::open() opens a book file with a given name
 
 void Book::open(const string& fileName) {
 
   // Close old file before opening the new
   close();
 
-  bookFile.open(fileName.c_str(), ifstream::in | ifstream::binary);
+  bookFile.open(fileName.c_str(), ifstream::in | ifstream::binary |ios::ate);
 
   // Silently return when asked to open a non-exsistent file
   if (!bookFile.is_open())
       return;
 
-  // Get the book size in number of entries
-  bookFile.seekg(0, ios::end);
+  // Get the book size in number of entries, we are already at the file end
   bookSize = long(bookFile.tellg()) / sizeof(BookEntry);
 
   if (!bookFile.good())
@@ -402,44 +401,34 @@ void Book::open(const string& fileName) {
 }
 
 
-/// Book::get_move() gets a book move for a given position. Returns
-/// MOVE_NONE if no book move is found. If findBestMove is true then
-/// return always the highest rated book move.
+/// Book::probe() gets a book move for a given position. Returns MOVE_NONE
+/// if no book move is found. If findBest is true then returns always the
+/// highest rated move otherwise chooses randomly based on the move score.
 
-Move Book::get_move(const Position& pos, bool findBestMove) {
+Move Book::probe(const Position& pos, bool findBest) {
 
   if (!bookSize || !bookFile.is_open())
       return MOVE_NONE;
 
   BookEntry entry;
-  int bookMove = MOVE_NONE;
-  unsigned score, scoresSum = 0, bestScore = 0;
+  unsigned scoresSum = 0, bestScore = 0, bookMove = 0;
   uint64_t key = book_key(pos);
+  int idx = first_entry(key) - 1;
 
   // Choose a book move among the possible moves for the given position
-  for (int idx = first_entry(key); idx < bookSize; idx++)
+  while (++idx < bookSize && (entry = read_entry(idx), entry.key == key))
   {
-      entry = read_entry(idx);
+      scoresSum += entry.count;
 
-      if (entry.key != key)
-          break;
-
-      score = entry.count;
-
-      if (!findBestMove)
-      {
-          // Choose book move according to its score. If a move has a very
-          // high score it has higher probability to be choosen than a move
-          // with lower score. Note that first entry is always chosen.
-          scoresSum += score;
-          if (RKiss.rand<unsigned>() % scoresSum < score)
-              bookMove = entry.move;
-      }
-      else if (score > bestScore)
-      {
-          bestScore = score;
+      // Choose book move according to its score. If a move has a very
+      // high score it has higher probability to be choosen than a move
+      // with lower score. Note that first entry is always chosen.
+      if (   RKiss.rand<unsigned>() % scoresSum < entry.count
+          || (findBest && entry.count > bestScore))
           bookMove = entry.move;
-      }
+
+      if (entry.count > bestScore)
+          bestScore = entry.count;
   }
 
   if (!bookMove)
@@ -458,12 +447,12 @@ Move Book::get_move(const Position& pos, bool findBestMove) {
   int promotion = (bookMove >> 12) & 7;
 
   if (promotion)
-      bookMove = int(make_promotion_move(move_from(Move(bookMove)),
-                                         move_to(Move(bookMove)),
-                                         PieceType(promotion + 1)));
+      bookMove = make_promotion_move(move_from(Move(bookMove)),
+                                     move_to(Move(bookMove)),
+                                     PieceType(promotion + 1));
   // Verify the book move is legal
   for (MoveList<MV_LEGAL> ml(pos); !ml.end(); ++ml)
-      if ((ml.move() & ~(3 << 14)) == bookMove) // Mask out special flags
+      if (unsigned(ml.move() & ~(3 << 14)) == bookMove) // Mask out special flags
           return ml.move();
 
   return MOVE_NONE;

src/book.h

@@ -44,7 +44,7 @@ public:
   ~Book();
   void open(const std::string& fileName);
   void close();
-  Move get_move(const Position& pos, bool findBestMove);
+  Move probe(const Position& pos, bool findBestMove);
   const std::string name() const { return bookName; }
 
 private:

src/search.cpp

@@ -391,7 +391,7 @@ bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]) {
       if (Options["Book File"].value<string>() != book.name())
           book.open(Options["Book File"].value<string>());
 
-      Move bookMove = book.get_move(pos, Options["Best Book Move"].value<bool>());
+      Move bookMove = book.probe(pos, Options["Best Book Move"].value<bool>());
       if (bookMove != MOVE_NONE)
       {
           if (Limits.ponder)