Improve grammar of comments

closes https://github.com/official-stockfish/Stockfish/pull/4801

No functional change

src/bitboard.h

@@ -209,7 +209,7 @@ template<> inline int distance<Square>(Square x, Square y) { return SquareDistan
 
 inline int edge_distance(File f) { return std::min(f, File(FILE_H - f)); }
 
-/// attacks_bb(Square) returns the pseudo attacks of the give piece type
+/// attacks_bb(Square) returns the pseudo attacks of the given piece type
 /// assuming an empty board.
 
 template<PieceType Pt>

src/movegen.h

@@ -56,8 +56,9 @@ inline bool operator<(const ExtMove& f, const ExtMove& s) {
 template<GenType>
 ExtMove* generate(const Position& pos, ExtMove* moveList);
 
-/// The MoveList struct is a simple wrapper around generate(). It sometimes comes
+/// The MoveList struct wraps the generate() function and returns a convenient
-/// in handy to use this class instead of the low level generate() function.
+/// list of moves. Using MoveList is sometimes preferable to directly calling
+/// the lower level generate() function.
 template<GenType T>
 struct MoveList {
 

src/movepick.cpp

@@ -55,11 +55,11 @@ namespace {
 } // namespace
 
 
-/// Constructors of the MovePicker class. As arguments we pass information
+/// Constructors of the MovePicker class. As arguments, we pass information
-/// to help it to return the (presumably) good moves first, to decide which
+/// to help it return the (presumably) good moves first, to decide which
 /// moves to return (in the quiescence search, for instance, we only want to
-/// search captures, promotions, and some checks) and how important good move
+/// search captures, promotions, and some checks) and how important a good
-/// ordering is at the current node.
+/// move ordering is at the current node.
 
 /// MovePicker constructor for the main search
 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,

src/position.cpp

@@ -102,8 +102,9 @@ std::ostream& operator<<(std::ostream& os, const Position& pos) {
 }
 
 
-// Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
+// Implements Marcel van Kervinck's cuckoo algorithm to detect repetition of positions 
-// situations. Description of the algorithm in the following paper:
+// for 3-fold repetition draws. The algorithm uses two hash tables with Zobrist hashes to 
+// allow fast detection of recurring positions. For details see:
 // http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
 
 // First and second hash functions for indexing the cuckoo tables
@@ -549,7 +550,7 @@ bool Position::legal(Move m) const {
 
 
 /// Position::pseudo_legal() takes a random move and tests whether the move is
-/// pseudo legal. It is used to validate moves from TT that can be corrupted
+/// pseudo-legal. It is used to validate moves from TT that can be corrupted
 /// due to SMP concurrent access or hash position key aliasing.
 
 bool Position::pseudo_legal(const Move m) const {
@@ -565,7 +566,7 @@ bool Position::pseudo_legal(const Move m) const {
       return checkers() ? MoveList<    EVASIONS>(*this).contains(m)
                         : MoveList<NON_EVASIONS>(*this).contains(m);
 
-  // Is not a promotion, so promotion piece must be empty
+  // Is not a promotion, so the promotion piece must be empty
   assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
 
   // If the 'from' square is not occupied by a piece belonging to the side to
@@ -603,7 +604,7 @@ bool Position::pseudo_legal(const Move m) const {
   {
       if (type_of(pc) != KING)
       {
-          // Double check? In this case a king move is required
+          // Double check? In this case, a king move is required
           if (more_than_one(checkers()))
               return false;
 
@@ -611,7 +612,7 @@ bool Position::pseudo_legal(const Move m) const {
           if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
               return false;
       }
-      // In case of king moves under check we have to remove king so as to catch
+      // In case of king moves under check we have to remove the king so as to catch
       // invalid moves like b1a1 when opposite queen is on c1.
       else if (attackers_to(to, pieces() ^ from) & pieces(~us))
           return false;
@@ -1134,7 +1135,7 @@ bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
       }
 
       else // KING
-           // If we "capture" with the king but opponent still has attackers,
+           // If we "capture" with the king but the opponent still has attackers,
            // reverse the result.
           return (attackers & ~pieces(stm)) ? res ^ 1 : res;
   }
@@ -1265,7 +1266,7 @@ void Position::flip() {
 
 
 /// Position::pos_is_ok() performs some consistency checks for the
-/// position object and raises an asserts if something wrong is detected.
+/// position object and raise an assert if something wrong is detected.
 /// This is meant to be helpful when debugging.
 
 bool Position::pos_is_ok() const {

src/search.cpp

@@ -100,10 +100,12 @@ namespace {
     return VALUE_DRAW - 1 + Value(thisThread->nodes & 0x2);
   }
 
-  // Skill structure is used to implement strength limit. If we have an uci_elo then
+  // Skill structure is used to implement strength limit.
-  // we convert it to a suitable fractional skill level using anchoring to CCRL Elo
+  // If we have a UCI_Elo, we convert it to an appropriate skill level, anchored to the Stash engine.
-  // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for a match (TC 60+0.6)
+  // This method is based on a fit of the Elo results for games played between the master at various
-  // results spanning a wide range of k values.
+  // skill levels and various versions of the Stash engine, all ranked at CCRL.
+  // Skill 0 .. 19 now covers CCRL Blitz Elo from 1320 to 3190, approximately
+  // Reference: https://github.com/vondele/Stockfish/commit/a08b8d4e9711c20acedbfe17d618c3c384b339ec
   struct Skill {
     Skill(int skill_level, int uci_elo) {
         if (uci_elo)
@@ -272,10 +274,9 @@ void MainThread::search() {
 
 void Thread::search() {
 
-  // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
+  // Allocate stack with extra size to allow access from (ss-7) to (ss+2)
-  // The former is needed to allow update_continuation_histories(ss-1, ...),
+  // (ss-7) is needed for update_continuation_histories(ss-1, ...) which accesses (ss-6)
-  // which accesses its argument at ss-6, also near the root.
+  // (ss+2) is needed for initialization of statScore and killers
-  // The latter is needed for statScore and killer initialization.
   Stack stack[MAX_PLY+10], *ss = stack+7;
   Move  pv[MAX_PLY+1];
   Value alpha, beta, delta;
@@ -362,7 +363,7 @@ void Thread::search() {
           alpha = std::max(prev - delta,-VALUE_INFINITE);
           beta  = std::min(prev + delta, VALUE_INFINITE);
 
-          // Adjust optimism based on root move's previousScore
+          // Adjust optimism based on root move's previousScore (~4 Elo)
           int opt = 109 * prev / (std::abs(prev) + 141);
           optimism[ us] = Value(opt);
           optimism[~us] = -optimism[us];
@@ -721,7 +722,7 @@ namespace {
     }
     else if (excludedMove)
     {
-        // Providing the hint that this node's accumulator will be used often brings significant Elo gain (13 Elo)
+        // Providing the hint that this node's accumulator will be used often brings significant Elo gain (~13 Elo)
         Eval::NNUE::hint_common_parent_position(pos);
         eval = ss->staticEval;
     }
@@ -762,7 +763,7 @@ namespace {
                 : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval > (ss-4)->staticEval
                 : true;
 
-    // Step 7. Razoring (~1 Elo).
+    // Step 7. Razoring (~1 Elo)
     // If eval is really low check with qsearch if it can exceed alpha, if it can't,
     // return a fail low.
     if (eval < alpha - 456 - 252 * depth * depth)
@@ -772,7 +773,7 @@ namespace {
             return value;
     }
 
-    // Step 8. Futility pruning: child node (~40 Elo).
+    // Step 8. Futility pruning: child node (~40 Elo)
     // The depth condition is important for mate finding.
     if (   !ss->ttPv
         &&  depth < 9

src/types.h

@@ -40,7 +40,7 @@
 #include <cstdint>
 
 #if defined(_MSC_VER)
-// Disable some silly and noisy warning from MSVC compiler
+// Disable some silly and noisy warnings from MSVC compiler
 #pragma warning(disable: 4127) // Conditional expression is constant
 #pragma warning(disable: 4146) // Unary minus operator applied to unsigned type
 #pragma warning(disable: 4800) // Forcing value to bool 'true' or 'false'
@@ -405,7 +405,7 @@ constexpr Move make(Square from, Square to, PieceType pt = KNIGHT) {
   return Move(T + ((pt - KNIGHT) << 12) + (from << 6) + to);
 }
 
-/// Based on a congruential pseudo random number generator
+/// Based on a congruential pseudo-random number generator
 constexpr Key make_key(uint64_t seed) {
   return seed * 6364136223846793005ULL + 1442695040888963407ULL;
 }

src/uci.cpp

@@ -159,7 +159,7 @@ namespace {
 
 
   // bench() is called when the engine receives the "bench" command.
-  // Firstly, a list of UCI commands is set up according to the bench
+  // First, a list of UCI commands is set up according to the bench
   // parameters, then it is run one by one, printing a summary at the end.
 
   void bench(Position& pos, std::istream& args, StateListPtr& states) {
@@ -226,14 +226,14 @@ namespace {
      // Transform the eval to centipawns with limited range
      double x = std::clamp(double(v), -4000.0, 4000.0);
 
-     // Return the win rate in per mille units rounded to the nearest value
+     // Return the win rate in per mille units, rounded to the nearest integer
      return int(0.5 + 1000 / (1 + std::exp((a - x) / b)));
   }
 
 } // namespace
 
 
-/// UCI::loop() waits for a command from the stdin, parses it and then calls the appropriate
+/// UCI::loop() waits for a command from the stdin, parses it, and then calls the appropriate
 /// function. It also intercepts an end-of-file (EOF) indication from the stdin to ensure a
 /// graceful exit if the GUI dies unexpectedly. When called with some command-line arguments,
 /// like running 'bench', the function returns immediately after the command is executed.