Simplify make_index

Refactor index LUT construction to simplify make_index.

Passed STC Non-Regression:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 62432 W: 16193 L: 16006 D: 30233
Ptnml(0-2): 189, 6950, 16764, 7111, 202
https://tests.stockfishchess.org/tests/view/6959985ad844c1ce7cc7eac8

closes https://github.com/official-stockfish/Stockfish/pull/6522

No functional change.

src/nnue/features/full_threats.cpp

@@ -37,26 +37,6 @@ struct HelperOffsets {
     int cumulativePieceOffset, cumulativeOffset;
 };
 
-// Information on a particular pair of pieces and whether they should be excluded
-struct PiecePairData {
-    // Layout: bits 8..31 are the index contribution of this piece pair, bits 0 and 1 are exclusion info
-    uint32_t data;
-
-    constexpr PiecePairData() :
-        data(0) {}
-
-    constexpr PiecePairData(bool      excluded_pair,
-                            bool      semi_excluded_pair,
-                            IndexType feature_index_base) :
-        data((uint32_t(excluded_pair) << 1) | (uint32_t(semi_excluded_pair && !excluded_pair))
-             | (uint32_t(feature_index_base) << 8)) {}
-
-    // lsb: excluded if from < to; 2nd lsb: always excluded
-    constexpr uint8_t excluded_pair_info() const { return static_cast<uint8_t>(data); }
-
-    constexpr IndexType feature_index_base() const { return static_cast<IndexType>(data >> 8); }
-};
-
 constexpr std::array<Piece, 12> AllPieces = {
   W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
   B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING,
@@ -173,7 +153,7 @@ constexpr auto helper_offsets = init_threat_offsets().first;
 constexpr auto offsets = init_threat_offsets().second;
 
 constexpr auto init_index_luts() {
-    std::array<std::array<PiecePairData, PIECE_NB>, PIECE_NB> indices{};
+    std::array<std::array<std::array<uint32_t, 2>, PIECE_NB>, PIECE_NB> indices{};
 
     for (Piece attacker : AllPieces)
     {
@@ -189,8 +169,10 @@ constexpr auto init_index_luts() {
                               + (color_of(attacked) * (numValidTargets[attacker] / 2) + map)
                                   * helper_offsets[attacker].cumulativePieceOffset;
 
-            bool excluded               = map < 0;
+            bool excluded                  = map < 0;
-            indices[attacker][attacked] = PiecePairData(excluded, semi_excluded, feature);
+            indices[attacker][attacked][0] = excluded ? FullThreats::Dimensions : feature;
+            indices[attacker][attacked][1] =
+              excluded || semi_excluded ? FullThreats::Dimensions : feature;
         }
     }
 
@@ -200,7 +182,7 @@ constexpr auto init_index_luts() {
 // The final index is calculated from summing data found in these two LUTs, as well
 // as offsets[attacker][from]
 
-// [attacker][attacked]
+// [attacker][attacked][from < to]
 constexpr auto index_lut1 = init_index_luts();
 // [attacker][from][to]
 constexpr auto index_lut2 = index_lut2_array();
@@ -216,17 +198,9 @@ inline sf_always_inline IndexType FullThreats::make_index(
     unsigned    attacker_oriented = attacker ^ swap;
     unsigned    attacked_oriented = attacked ^ swap;
 
-    const auto piecePairData = index_lut1[attacker_oriented][attacked_oriented];
+    return index_lut1[attacker_oriented][attacked_oriented][from_oriented < to_oriented]
-
+         + offsets[attacker_oriented][from_oriented]
-    const bool less_than = from_oriented < to_oriented;
+         + index_lut2[attacker_oriented][from_oriented][to_oriented];
-    if ((piecePairData.excluded_pair_info() + less_than) & 2)
-        return FullThreats::Dimensions;
-
-    const IndexType index = piecePairData.feature_index_base()
-                          + offsets[attacker_oriented][from_oriented]
-                          + index_lut2[attacker_oriented][from_oriented][to_oriented];
-    sf_assume(index < Dimensions);
-    return index;
 }
 
 // Get a list of indices for active features in ascending order