Reduce allocations in EncodingLoader.

- Avoid garbage strings from splitting on space.
- Avoid allocations from using LINQ in a hot loop.

Signed-off-by: Bradley Grainger <bgrainger@gmail.com>

Author: bgrainger

src/libs/Tiktoken.Core/CoreBPE.cs

@@ -45,7 +45,19 @@ public CoreBpe(
         Encoder = encoder;
         FastEncoder = Encoder
             .ToDictionary(
-                static x => new string(x.Key.Select(y => (char) y).ToArray()),
+#if NETSTANDARD2_1_OR_GREATER || NET6_0_OR_GREATER
+                static x =>
+                {
+                    Span<char> chars = stackalloc char[x.Key.Length];
+                    for (var i = 0; i < x.Key.Length; i++)
+                    {
+                        chars[i] = (char)x.Key[i];
+                    }
+                    return new string(chars);
+                },
+#else
+                static x => new string(x.Key.Select(static y => (char) y).ToArray()),
+#endif
                 static x => x.Value);
         SpecialTokensEncoder = specialTokensEncoder;
 

src/libs/Tiktoken.Encodings.Abstractions/EncodingLoader.cs

@@ -30,7 +30,7 @@ public static Dictionary<byte[], int> LoadEncodingFromManifestResource(
             assembly.GetManifestResourceStream(resourcePath) ??
             throw new InvalidOperationException("Resource not found.");
         using var reader = new StreamReader(stream);
-        
+
         var lines = new List<string>();
         while (reader.ReadLine() is { } line)
         {
@@ -53,7 +53,11 @@ public static Dictionary<byte[], int> LoadEncodingFromLines(
         string name)
     {
         lines = lines ?? throw new ArgumentNullException(nameof(lines));
-        
+
+#if NET7_0_OR_GREATER
+        Span<Range> tokens = stackalloc Range[3];
+        Span<byte> bytes = stackalloc byte[256];
+#endif
         var dictionary = new Dictionary<byte[], int>(new ByteArrayComparer());
         foreach (var line in lines)
         {
@@ -62,14 +66,28 @@ public static Dictionary<byte[], int> LoadEncodingFromLines(
                 continue;
             }
 
+#if NET7_0_OR_GREATER
+            var splitCount = line.AsSpan().Split(tokens, ' ');
+            if (splitCount != 2)
+            {
+                throw new FormatException($"Invalid file format: {name}");
+            }
+#else
             var tokens = line.Split(' ');
             if (tokens.Length != 2)
             {
                 throw new FormatException($"Invalid file format: {name}");
             }
+#endif
 
+#if NET7_0_OR_GREATER
+            Convert.TryFromBase64Chars(line.AsSpan(tokens[0]), bytes, out var bytesWritten);
+            var tokenBytes = bytes.Slice(0, bytesWritten).ToArray();
+            var rank = int.Parse(line.AsSpan(tokens[1]), CultureInfo.InvariantCulture);
+#else
             var tokenBytes = Convert.FromBase64String(tokens[0]);
             var rank = int.Parse(tokens[1], CultureInfo.InvariantCulture);
+#endif
             dictionary[tokenBytes] = rank;
         }