Speed up FencedCodeBlock rendering

Author: MihaZupan

src/Markdig/Polyfills/FrozenCollections.cs

@@ -22,4 +22,17 @@ public static FrozenDictionary<TKey, TValue> ToFrozenDictionary<TKey, TValue>(th
     }
 }
 
+internal sealed class FrozenSet<T> : HashSet<T>
+{
+    public FrozenSet(HashSet<T> set, IEqualityComparer<T> comparer) : base(set, comparer) { }
+}
+
+internal static class FrozenSetExtensions
+{
+    public static FrozenSet<T> ToFrozenSet<T>(this HashSet<T> set, IEqualityComparer<T> comparer)
+    {
+        return new FrozenSet<T>(set, comparer);
+    }
+}
+
 #endif

src/Markdig/Renderers/Html/CodeBlockRenderer.cs

@@ -31,11 +31,27 @@ public CodeBlockRenderer() { }
     /// </summary>
     public Dictionary<string, string> BlockMapping { get; } = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
+    private FrozenSet<string>? _specialBlockMapping;
+
+    private FrozenSet<string> SpecialBlockMapping
+    {
+        get
+        {
+            return _specialBlockMapping ?? CreateNew();
+
+            FrozenSet<string> CreateNew()
+            {
+                HashSet<string> set = [.. BlocksAsDiv, .. BlockMapping.Keys];
+                return _specialBlockMapping = set.ToFrozenSet(StringComparer.OrdinalIgnoreCase);
+            }
+        }
+    }
+
     protected override void Write(HtmlRenderer renderer, CodeBlock obj)
     {
         renderer.EnsureLine();
 
-        if ((obj as FencedCodeBlock)?.Info is string info && (BlocksAsDiv.Contains(info) || BlockMapping.ContainsKey(info)))
+        if ((obj as FencedCodeBlock)?.Info is string info && SpecialBlockMapping.Contains(info))
         {
             var infoPrefix = (obj.Parser as FencedCodeBlockParser)?.InfoPrefix ??
                              FencedCodeBlockParser.DefaultInfoPrefix;