KnightStackSys · Oct 9, 2026
diff --git a/include/ironphoenix/tuning.hpp b/include/ironphoenix/tuning.hpp
@@ -34,6 +34,14 @@ namespace ironphoenix {
             int bishopMobility = 4;
             int rookMobility = 2;
             int queenMobility = 1;
+
+            // Development rewards pieces that leave their player's home back
+            // line. The score is tapered by remaining non-pawn material so the
+            // term naturally fades as the game leaves the opening.
+            int developmentKnight = 8;
+            int developmentBishop = 8;
+            int developmentRook = 2;
+            int developmentQueen = 0;
         };
 
         [[nodiscard]] SearchParameters& search() noexcept;

diff --git a/src/eval.cpp b/src/eval.cpp
@@ -5,10 +5,14 @@
 #include "ironphoenix/position.hpp"
 #include "ironphoenix/tuning.hpp"
 
+#include <algorithm>
+
 namespace ironphoenix::Eval {
 
     namespace {
 
+        constexpr int STARTING_NON_PAWN_MATERIAL = 13'600;
+
         IRONPHOENIX_FORCE_INLINE int evalValue(PieceType pt) noexcept {
             const auto& params = Tuning::eval();
 
@@ -34,11 +38,33 @@ namespace ironphoenix::Eval {
             }
         }
 
+        IRONPHOENIX_FORCE_INLINE int developmentWeight(PieceType pt) noexcept {
+            const auto& params = Tuning::eval();
+
+            switch (pt) {
+            case KNIGHT: return params.developmentKnight;
+            case BISHOP: return params.developmentBishop;
+            case ROOK:   return params.developmentRook;
+            case QUEEN:  return params.developmentQueen;
+            default:     return 0;
+            }
+        }
+
         IRONPHOENIX_FORCE_INLINE Bitboard teamOccupancy(const Position& pos, Color c) noexcept {
             const Color partner = static_cast<Color>(static_cast<unsigned>(c) ^ 2u);
             return pos.occupancy(c) | pos.occupancy(partner);
         }
 
+        IRONPHOENIX_FORCE_INLINE bool onHomeBackLine(Color c, Square sq) noexcept {
+            switch (c) {
+            case RED:    return Geometry::rankOf(sq) == 0;
+            case BLUE:   return Geometry::fileOf(sq) == 0;
+            case YELLOW: return Geometry::rankOf(sq) == BOARD_RANKS - 1;
+            case GREEN:  return Geometry::fileOf(sq) == BOARD_FILES - 1;
+            }
+            return false;
+        }
+
         int materialForColor(const Position& pos, Color c) noexcept {
             int material = 0;
 
@@ -93,18 +119,62 @@ namespace ironphoenix::Eval {
             return score;
         }
 
-        int playerScore(const Position& pos, Color c) noexcept {
+        int remainingNonPawnMaterial(const Position& pos) noexcept {
+            int material = 0;
+
+            for (unsigned ci = 0; ci < COLOR_NB; ++ci) {
+                const Color c = static_cast<Color>(ci);
+                for (unsigned pt = KNIGHT; pt <= QUEEN; ++pt) {
+                    const PieceType pieceType = static_cast<PieceType>(pt);
+                    material += pos.pieces(c, pieceType).popcount() * pieceValue(pieceType);
+                }
+            }
+
+            // Promotions can take the board above starting material. Development
+            // should never grow beyond its full opening weight.
+            return std::min(material, STARTING_NON_PAWN_MATERIAL);
+        }
+
+        int developmentForColor(const Position& pos, Color c, int phaseMaterial) noexcept {
+            int score = 0;
+
+            // Development is position based rather than move-history based: a
+            // surviving piece earns its bonus while it is off the player's home
+            // back line. A piece that is captured cannot continue earning it.
+            for (unsigned pt = KNIGHT; pt <= QUEEN; ++pt) {
+                const PieceType pieceType = static_cast<PieceType>(pt);
+                const int weight = developmentWeight(pieceType);
+                if (weight == 0)
+                    continue;
+
+                Bitboard pieces = pos.pieces(c, pieceType);
+                while (pieces) {
+                    const Square sq = popLsb(pieces);
+                    if (!onHomeBackLine(c, sq))
+                        score += weight;
+                }
+            }
+
+            // Taper toward zero as non-pawn material disappears. This keeps the
+            // term focused on opening development instead of rewarding arbitrary
+            // piece placement in simplified middlegames and endgames.
+            return score * phaseMaterial / STARTING_NON_PAWN_MATERIAL;
+        }
+
+        int playerScore(const Position& pos, Color c, int phaseMaterial) noexcept {
             return materialForColor(pos, c)
-                + mobilityForColor(pos, c);
+                + mobilityForColor(pos, c)
+                + developmentForColor(pos, c, phaseMaterial);
         }
 
         int handcraftedEvaluate(const Position& pos) noexcept {
             int team0 = 0;
             int team1 = 0;
+            const int phaseMaterial = remainingNonPawnMaterial(pos);
 
             for (unsigned ci = 0; ci < COLOR_NB; ++ci) {
                 const Color c = static_cast<Color>(ci);
-                const int score = playerScore(pos, c);
+                const int score = playerScore(pos, c, phaseMaterial);
 
                 if (teamOf(c) == 0)
                     team0 += score;

diff --git a/src/tuning.cpp b/src/tuning.cpp
@@ -58,7 +58,11 @@ namespace ironphoenix::Tuning {
                 || name == "EvalKnightMobility"
                 || name == "EvalBishopMobility"
                 || name == "EvalRookMobility"
-                || name == "EvalQueenMobility";
+                || name == "EvalQueenMobility"
+                || name == "EvalDevelopmentKnight"
+                || name == "EvalDevelopmentBishop"
+                || name == "EvalDevelopmentRook"
+                || name == "EvalDevelopmentQueen";
         }
 
         bool isLmrParameter(std::string_view name) noexcept {
@@ -155,6 +159,14 @@ namespace ironphoenix::Tuning {
             p.rookMobility = value;
         else if (name == "EvalQueenMobility" && inRange(value, -100, 100))
             p.queenMobility = value;
+        else if (name == "EvalDevelopmentKnight" && inRange(value, -100, 100))
+            p.developmentKnight = value;
+        else if (name == "EvalDevelopmentBishop" && inRange(value, -100, 100))
+            p.developmentBishop = value;
+        else if (name == "EvalDevelopmentRook" && inRange(value, -100, 100))
+            p.developmentRook = value;
+        else if (name == "EvalDevelopmentQueen" && inRange(value, -100, 100))
+            p.developmentQueen = value;
         else
             return false;
 
@@ -228,6 +240,10 @@ namespace ironphoenix::Tuning {
             << "option name EvalBishopMobility type spin default 4 min -100 max 100\n"
             << "option name EvalRookMobility type spin default 2 min -100 max 100\n"
             << "option name EvalQueenMobility type spin default 1 min -100 max 100\n"
+            << "option name EvalDevelopmentKnight type spin default 8 min -100 max 100\n"
+            << "option name EvalDevelopmentBishop type spin default 8 min -100 max 100\n"
+            << "option name EvalDevelopmentRook type spin default 2 min -100 max 100\n"
+            << "option name EvalDevelopmentQueen type spin default 0 min -100 max 100\n"
             << "option name ResetTuning type button\n";
     }
 
@@ -256,7 +272,11 @@ namespace ironphoenix::Tuning {
             << " EvalKnightMobility=" << e.knightMobility
             << " EvalBishopMobility=" << e.bishopMobility
             << " EvalRookMobility=" << e.rookMobility
-            << " EvalQueenMobility=" << e.queenMobility << '\n';
+            << " EvalQueenMobility=" << e.queenMobility
+            << " EvalDevelopmentKnight=" << e.developmentKnight
+            << " EvalDevelopmentBishop=" << e.developmentBishop
+            << " EvalDevelopmentRook=" << e.developmentRook
+            << " EvalDevelopmentQueen=" << e.developmentQueen << '\n';
     }
 
 } // namespace ironphoenix::Tuning