Optimize reverse lookup in query string parsing

Author: bkoelman

src/JsonApiDotNetCore/Queries/Parsing/QueryExpressionParser.cs

@@ -156,7 +156,11 @@ protected virtual void EatSingleCharacterToken(TokenKind kind)
     {
         if (!TokenStack.TryPop(out Token? token) || token.Kind != kind)
         {
-            char ch = QueryTokenizer.SingleCharacterToTokenKinds.Single(pair => pair.Value == kind).Key;
+            if (!QueryTokenizer.TokenKindToSingleCharacterLookup.TryGetValue(kind, out char ch))
+            {
+                throw new InvalidOperationException($"Token kind '{kind}' is not a single-character token.");
+            }
+
             int position = token?.Position ?? GetNextTokenPositionOrEnd();
             throw new QueryParseException($"{ch} expected.", position);
         }

src/JsonApiDotNetCore/Queries/Parsing/QueryTokenizer.cs

@@ -1,4 +1,3 @@
-using System.Collections.ObjectModel;
 using System.Text;
 using JetBrains.Annotations;
 
@@ -7,18 +6,20 @@ namespace JsonApiDotNetCore.Queries.Parsing;
 [PublicAPI]
 public sealed class QueryTokenizer
 {
-    public static readonly IReadOnlyDictionary<char, TokenKind> SingleCharacterToTokenKinds = new ReadOnlyDictionary<char, TokenKind>(
-        new Dictionary<char, TokenKind>
-        {
-            ['('] = TokenKind.OpenParen,
-            [')'] = TokenKind.CloseParen,
-            ['['] = TokenKind.OpenBracket,
-            [']'] = TokenKind.CloseBracket,
-            ['.'] = TokenKind.Period,
-            [','] = TokenKind.Comma,
-            [':'] = TokenKind.Colon,
-            ['-'] = TokenKind.Minus
-        });
+    public static readonly IReadOnlyDictionary<char, TokenKind> SingleCharacterToTokenKinds = new Dictionary<char, TokenKind>
+    {
+        ['('] = TokenKind.OpenParen,
+        [')'] = TokenKind.CloseParen,
+        ['['] = TokenKind.OpenBracket,
+        [']'] = TokenKind.CloseBracket,
+        ['.'] = TokenKind.Period,
+        [','] = TokenKind.Comma,
+        [':'] = TokenKind.Colon,
+        ['-'] = TokenKind.Minus
+    }.AsReadOnly();
+
+    internal static readonly IReadOnlyDictionary<TokenKind, char> TokenKindToSingleCharacterLookup =
+        SingleCharacterToTokenKinds.ToDictionary(pair => pair.Value, pair => pair.Key).AsReadOnly();
 
     private readonly string _source;
     private readonly StringBuilder _textBuffer = new();