⚡ Make SearchResult.Moves an array and optimize its population (#986)

Make `SearchResult.Moves` an array and optimize its population from `_pvTable` using `Span`.

src/Lynx.Benchmark/InfoCommand_Benchmark.cs

@@ -98,12 +98,13 @@
 namespace Lynx.Benchmark;
 public class InfoCommand_Benchmark : BaseBenchmark
 {
-    public static IEnumerable<SearchResult> Data => new[] {
+    public static IEnumerable<SearchResult> Data =>
+    [
         new SearchResult(33592128, 27, 20, [33592128], -500, +500 ),
         new SearchResult(33592128, 27, 20, [33591088, 33975472, 112608, 480352, 109456, 477200, 47968, 495952, 187344, 417008, 24220320, 18256304], -500, +500 ),
         new SearchResult(33592128, 27, 20, [33592128, 33974432, 112608, 477200, 109456, 33976512, 166864, 412848, 44848, 536656, 101055424, 480352, 93856, 18312528, 24276512, 101390400, 43808, 544800, 177056, 19426624], -500, +500 ),
         new SearchResult(33592128, 27, 20, [352512, 684368, 345472, 691648, 344336, 675376, 353536, 699696, 345488, 701104, 344336, 709312, 345344, 706368, 346384, 699152, 345376, 689824, 344336, 688656, 345344, 697856, 344336, 705168, 345344, 714496, 338192, 671632, 345248, 678128, 353552, 714080, 346512, 693136, 345376, 676416, 352528, 717120, 345472], -500, +500),
-        };
+    ];
 
     [Benchmark(Baseline = true)]
     [ArgumentsSource(nameof(Data))]

src/Lynx.Benchmark/InfoCommand_CopilotRefactoring_Benchmark.cs

@@ -81,12 +81,13 @@
 namespace Lynx.Benchmark;
 public class InfoCommand_CopilotRefactoring_Benchmark : BaseBenchmark
 {
-    public static IEnumerable<SearchResult> Data => new[] {
+    public static IEnumerable<SearchResult> Data =>
+    [
         new SearchResult(33592128, 27, 20, [33592128], -500, +500 ),
         new SearchResult(33592128, 27, 20, [33591088, 33975472, 112608, 480352, 109456, 477200, 47968, 495952, 187344, 417008, 24220320, 18256304], -500, +500 ),
         new SearchResult(33592128, 27, 20, [33592128, 33974432, 112608, 477200, 109456, 33976512, 166864, 412848, 44848, 536656, 101055424, 480352, 93856, 18312528, 24276512, 101390400, 43808, 544800, 177056, 19426624], -500, +500 ),
         new SearchResult(33592128, 27, 20, [352512, 684368, 345472, 691648, 344336, 675376, 353536, 699696, 345488, 701104, 344336, 709312, 345344, 706368, 346384, 699152, 345376, 689824, 344336, 688656, 345344, 697856, 344336, 705168, 345344, 714496, 338192, 671632, 345248, 678128, 353552, 714080, 346512, 693136, 345376, 676416, 352528, 717120, 345472], -500, +500),
-        };
+    ];
 
     [Benchmark(Baseline = true)]
     [ArgumentsSource(nameof(Data))]
@@ -166,7 +167,7 @@ public static string SearchResultInfo_Copilot(SearchResult searchResult)
             }
 
             // Remove the trailing space
-            if (searchResult.Moves.Count > 0)
+            if (searchResult.Moves.Length > 0)
             {
                 sb.Length--;
             }

src/Lynx/Engine.cs

@@ -336,7 +336,7 @@ public void Search(GoCommand goCommand)
             }
 
             // We print best move even in case of go ponder + stop, and IDEs are expected to ignore it
-            _moveToPonder = searchResult.Moves.Count >= 2 ? searchResult.Moves[1] : null;
+            _moveToPonder = searchResult.Moves.Length >= 2 ? searchResult.Moves[1] : null;
             _engineWriter.TryWrite(BestMoveCommand.BestMove(searchResult.BestMove, _moveToPonder));
         }
         catch (Exception e)

src/Lynx/Model/SearchResult.cs

@@ -5,7 +5,7 @@ public class SearchResult
     public Move BestMove { get; init; }
     public int Evaluation { get; init; }
     public int Depth { get; set; }
-    public List<Move> Moves { get; init; }
+    public Move[] Moves { get; init; }
     public int Alpha { get; init; }
     public int Beta { get; init; }
     public int Mate { get; init; }
@@ -24,7 +24,7 @@ public class SearchResult
 
     public (int WDLWin, int WDLDraw, int WDLLoss)? WDL { get; set; } = null;
 
-    public SearchResult(Move bestMove, int evaluation, int targetDepth, List<Move> moves, int alpha, int beta, int mate = default)
+    public SearchResult(Move bestMove, int evaluation, int targetDepth, Move[] moves, int alpha, int beta, int mate = default)
     {
         BestMove = bestMove;
         Evaluation = evaluation;
@@ -34,16 +34,4 @@ public SearchResult(Move bestMove, int evaluation, int targetDepth, List<Move> m
         Beta = beta;
         Mate = mate;
     }
-
-    public SearchResult(SearchResult previousSearchResult)
-    {
-        BestMove = previousSearchResult.Moves.ElementAtOrDefault(2);
-        Evaluation = previousSearchResult.Evaluation;
-        Depth = previousSearchResult.Depth - 2;
-        DepthReached = previousSearchResult.DepthReached - 2;
-        Moves = previousSearchResult.Moves.Skip(2).ToList();
-        Alpha = previousSearchResult.Alpha;
-        Beta = previousSearchResult.Beta;
-        Mate = previousSearchResult.Mate == 0 ? 0 : (int)Math.CopySign(Math.Abs(previousSearchResult.Mate) - 1, previousSearchResult.Mate);
-    }
 }

src/Lynx/Search/IDDFS.cs

@@ -172,7 +172,7 @@ public SearchResult IDDFS(int maxDepth, int softLimitTimeBound)
             _logger.Info("Search cancellation requested after {0}ms (depth {1}, nodes {2}), best move will be returned", _stopWatch.ElapsedMilliseconds, depth, _nodes);
 #pragma warning restore S6667 // Logging in a catch clause should pass the caught exception as a parameter.
 
-            for (int i = 0; i < lastSearchResult?.Moves.Count; ++i)
+            for (int i = 0; i < lastSearchResult?.Moves.Length; ++i)
             {
                 _pVTable[i] = lastSearchResult.Moves[i];
             }
@@ -309,7 +309,9 @@ private bool OnlyOneLegalMove(ref Move firstLegalMove, [NotNullWhen(true)] out S
     private SearchResult UpdateLastSearchResult(SearchResult? lastSearchResult,
         int bestEvaluation, int alpha, int beta, int depth, int mate)
     {
-        var pvMoves = _pVTable.TakeWhile(m => m != default).ToList();
+        var pvTableSpan = _pVTable.AsSpan();
+        var pvMoves = pvTableSpan[..(pvTableSpan.IndexOf(0))].ToArray();
+
         var maxDepthReached = _maxDepthReached.LastOrDefault(item => item != default);
 
         var elapsedTime = _stopWatch.ElapsedMilliseconds;

src/Lynx/Search/OnlineTablebase.cs

@@ -15,7 +15,7 @@ public sealed partial class Engine
 
             if (tablebaseResult.BestMove != 0)
             {
-                var searchResult = new SearchResult(tablebaseResult.BestMove, evaluation: 0, targetDepth: 0, new List<Move> { tablebaseResult.BestMove }, MinValue, MaxValue, mate: tablebaseResult.MateScore)
+                var searchResult = new SearchResult(tablebaseResult.BestMove, evaluation: 0, targetDepth: 0, [tablebaseResult.BestMove], MinValue, MaxValue, mate: tablebaseResult.MateScore)
                 {
                     DepthReached = 0,
                     Nodes = 666,                // In case some guis proritize the info command with biggest depth

src/Lynx/UCI/Commands/Engine/InfoCommand.cs

@@ -109,7 +109,7 @@ public static string SearchResultInfo(SearchResult searchResult)
         }
 
         // Remove the trailing space
-        if (searchResult.Moves.Count > 0)
+        if (searchResult.Moves.Length > 0)
         {
             sb.Length--;
         }

tests/Lynx.Test/BestMove/RegressionTest.cs

@@ -88,7 +88,7 @@ public void TrashInPVTable(int depth, string fen, string notExpectedMove)
         var engine = GetEngine(fen);
         var bestResult = engine.BestMove(goCommand);
 
-        if (bestResult.Moves.Count > depth)
+        if (bestResult.Moves.Length > depth)
         {
             Assert.AreNotEqual(notExpectedMove, bestResult.Moves[depth].UCIString());
         }
@@ -250,7 +250,7 @@ public void InvalidPV(string positionCommand)
 
         var bestMove = engine.BestMove(new GoCommand($"go depth {5}"));
         Assert.Zero(bestMove.Evaluation);
-        Assert.AreEqual(1, bestMove.Moves.Count);
+        Assert.AreEqual(1, bestMove.Moves.Length);
         Assert.AreEqual("b8c7", bestMove.BestMove.UCIString());
     }