Improve comments about DEPTH constants

Also "fix" movepicker to allow depths between CHECKS and NO_CHECKS,
which makes them easier to tweak (not that they get tweaked hardly ever)
(This was more beneficial when there was a third stage to DEPTH_QS, but
it's still an improvement now)

closes https://github.com/official-stockfish/Stockfish/pull/5205

No functional change

src/movepick.cpp

@@ -361,8 +361,8 @@ top:
         if (select<Next>([]() { return true; }))
             return *(cur - 1);
 
-        // If we did not find any move and we do not try checks, we have finished
-        if (depth != DEPTH_QS_CHECKS)
+        // If we found no move and the depth is too low to try checks, then we have finished
+        if (depth <= DEPTH_QS_NORMAL)
             return Move::none();
 
         ++stage;

src/search.cpp

@@ -733,7 +733,7 @@ Value Search::Worker::search(
         ss->staticEval = eval = to_corrected_static_eval(unadjustedStaticEval, *thisThread, pos);
 
         // Static evaluation is saved as it was before adjustment by correction history
-        tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, Move::none(),
+        tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_UNSEARCHED, Move::none(),
                   unadjustedStaticEval, tt.generation());
     }
 
@@ -1387,8 +1387,11 @@ moves_loop:  // When in check, search starts here
 }
 
 
-// Quiescence search function, which is called by the main search
-// function with zero depth, or recursively with further decreasing depth per call.
+// Quiescence search function, which is called by the main search function with zero depth, or
+// recursively with further decreasing depth per call. With depth <= 0, we "should" be using
+// static eval only, but tactical moves may confuse the static eval. To fight this horizon effect,
+// we implement this qsearch of tactical moves only.
+// See https://www.chessprogramming.org/Horizon_Effect and https://www.chessprogramming.org/Quiescence_Search
 // (~155 Elo)
 template<NodeType nodeType>
 Value Search::Worker::qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
@@ -1446,8 +1449,10 @@ Value Search::Worker::qsearch(Position& pos, Stack* ss, Value alpha, Value beta,
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
-    // Decide the replacement and cutoff priority of the qsearch TT entries
-    ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS;
+    // Note that unlike regular search, which stores literal depth, in QS we only store the
+    // current movegen stage. If in check, we search all evasions and thus store
+    // DEPTH_QS_CHECKS. (Evasions may be quiet, and _CHECKS includes quiets.)
+    ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NORMAL;
 
     // Step 3. Transposition table lookup
     posKey  = pos.key();
@@ -1499,8 +1504,8 @@ Value Search::Worker::qsearch(Position& pos, Stack* ss, Value alpha, Value beta,
             if (std::abs(bestValue) < VALUE_TB_WIN_IN_MAX_PLY && !PvNode)
                 bestValue = (3 * bestValue + beta) / 4;
             if (!ss->ttHit)
-                tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER, DEPTH_NONE,
-                          Move::none(), unadjustedStaticEval, tt.generation());
+                tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
+                          DEPTH_UNSEARCHED, Move::none(), unadjustedStaticEval, tt.generation());
 
             return bestValue;
         }
@@ -1514,16 +1519,16 @@ Value Search::Worker::qsearch(Position& pos, Stack* ss, Value alpha, Value beta,
     const PieceToHistory* contHist[] = {(ss - 1)->continuationHistory,
                                         (ss - 2)->continuationHistory};
 
-    // Initialize a MovePicker object for the current position, and prepare
-    // to search the moves. Because the depth is <= 0 here, only captures,
-    // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
-    // will be generated.
+    // Initialize a MovePicker object for the current position, and prepare to search the moves.
+    // We presently use two stages of qs movegen, first captures+checks, then captures only.
+    // (When in check, we simply search all evasions.)
+    // (Presently, having the checks stage is worth only 1 Elo, and may be removable in the near future,
+    // which would result in only a single stage of QS movegen.)
     Square     prevSq = ((ss - 1)->currentMove).is_ok() ? ((ss - 1)->currentMove).to_sq() : SQ_NONE;
     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory,
                   contHist, &thisThread->pawnHistory);
 
-    // Step 5. Loop through all pseudo-legal moves until no moves remain
-    // or a beta cutoff occurs.
+    // Step 5. Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs.
     while ((move = mp.next_move()) != Move::none())
     {
         assert(move.is_ok());

src/tt.cpp

@@ -30,6 +30,10 @@
 
 namespace Stockfish {
 
+// DEPTH_ENTRY_OFFSET exists because 1) we use `bool(depth8)` as the occupancy check, but
+// 2) we need to store negative depths for QS. (`depth8` is the only field with "spare bits":
+// we sacrifice the ability to store depths greater than 1<<8 less the offset, as asserted below.)
+
 // Populates the TTEntry with a new node's data, possibly
 // overwriting an old position. The update is not atomic and can be racy.
 void TTEntry::save(
@@ -40,14 +44,14 @@ void TTEntry::save(
         move16 = m;
 
     // Overwrite less valuable entries (cheapest checks first)
-    if (b == BOUND_EXACT || uint16_t(k) != key16 || d - DEPTH_OFFSET + 2 * pv > depth8 - 4
+    if (b == BOUND_EXACT || uint16_t(k) != key16 || d - DEPTH_ENTRY_OFFSET + 2 * pv > depth8 - 4
         || relative_age(generation8))
     {
-        assert(d > DEPTH_OFFSET);
-        assert(d < 256 + DEPTH_OFFSET);
+        assert(d > DEPTH_ENTRY_OFFSET);
+        assert(d < 256 + DEPTH_ENTRY_OFFSET);
 
         key16     = uint16_t(k);
-        depth8    = uint8_t(d - DEPTH_OFFSET);
+        depth8    = uint8_t(d - DEPTH_ENTRY_OFFSET);
         genBound8 = uint8_t(generation8 | uint8_t(pv) << 2 | b);
         value16   = int16_t(v);
         eval16    = int16_t(ev);

src/tt.h

@@ -37,12 +37,15 @@ namespace Stockfish {
 // move       16 bit
 // value      16 bit
 // eval value 16 bit
+//
+// These fields are in the same order as accessed by TT::probe(), since memory is fastest sequentially.
+// Equally, the store order in save() matches this order.
 struct TTEntry {
 
     Move  move() const { return Move(move16); }
     Value value() const { return Value(value16); }
     Value eval() const { return Value(eval16); }
-    Depth depth() const { return Depth(depth8 + DEPTH_OFFSET); }
+    Depth depth() const { return Depth(depth8 + DEPTH_ENTRY_OFFSET); }
     bool  is_pv() const { return bool(genBound8 & 0x4); }
     Bound bound() const { return Bound(genBound8 & 0x3); }
     void  save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev, uint8_t generation8);

src/types.h

@@ -187,12 +187,17 @@ constexpr Value PieceValue[PIECE_NB] = {
 using Depth = int;
 
 enum : int {
+    // The following DEPTH_ constants are used for TT entries and QS movegen stages. In regular search,
+    // TT depth is literal: the search depth (effort) used to make the corresponding TT value.
+    // In qsearch, however, TT entries only store the current QS movegen stage (which should thus compare
+    // lower than any regular search depth).
     DEPTH_QS_CHECKS = 0,
-    DEPTH_QS_NO_CHECKS = -1,
-
-    DEPTH_NONE = -6,
-
-    DEPTH_OFFSET = -7  // value used only for TT entry occupancy check
+    DEPTH_QS_NORMAL = -1,
+    // For TT entries where no searching at all was done (whether regular or qsearch) we use
+    // _UNSEARCHED, which should thus compare lower than any QS or regular depth. _ENTRY_OFFSET is used
+    // only for the TT entry occupancy check (see tt.cpp), and should thus be lower than _UNSEARCHED.
+    DEPTH_UNSEARCHED   = -6,
+    DEPTH_ENTRY_OFFSET = -7
 };
 
 // clang-format off