HTML tag lookup generator

TagLookupGenerator.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;
using AngleSharp.Dom;

namespace AngleSharp.Performance.Playground
{
	internal static class TagLookupGenerator
	{
		private const string _builderVar = "builder";

		public static string Generate()
		{
			const int defaultIndent = 3;
			StringBuilder builder = new StringBuilder();

			var tagsByLength
				= typeof( TagNames )
				.GetFields( BindingFlags.Public | BindingFlags.Static )
				.Select( f => new Tag { Name = f.Name, Value = f.GetValue( null ) as string } )
				.GroupBy( f => f.Value.Length )
				.OrderBy( g => g.Key );

			builder.Indent( defaultIndent ).AppendLine( "switch (builder.Length)" );
			builder.Indent( defaultIndent ).AppendLine( "{" );

			foreach ( var tagLengthGroup in tagsByLength )
			{
				builder.Indent( defaultIndent + 1 ).AppendLine( $"case {tagLengthGroup.Key}:" );
				AppendTagGroupSelector( builder, defaultIndent + 2, tagLengthGroup.ToArray() );
			}

			builder.Indent( defaultIndent + 1 ).AppendLine( "default:" );
			builder.Indent( defaultIndent + 2 ).AppendLine( "return null;" );
			builder.Indent( defaultIndent ).AppendLine( "}" );

			return builder.ToString();
		}

		public static StringBuilder Indent( this StringBuilder builder, int indentCount ) => builder.Append( ' ', 4 * indentCount );

		private static void AppendTagGroupSelector( StringBuilder builder, int indent, Tag[] tags )
		{
			if ( tags.Length == 1 )
			{
				string tagField = $"{nameof( TagNames )}.{tags[ 0 ].Name}";
				builder.Indent( indent ).AppendLine( $"return CharsAreEqual({_builderVar}, {tagField}) ? {tagField} : null;" );
			}
			else
			{
				int mostDifferentCharIndex = GetMostDifferentCharIndex( tags );

				builder.Indent( indent ).AppendLine( $"switch ({_builderVar}[{mostDifferentCharIndex}])" );
				builder.Indent( indent ).AppendLine( "{" );

				foreach ( var charGroup in tags.GroupBy( t => t.Value[ mostDifferentCharIndex ] ) )
				{
					builder.Indent( indent + 1 ).AppendLine( $"case '{charGroup.Key}':" );

					if ( tags[ 0 ].Value.Length == 1 )
						builder.Indent( indent + 2 ).AppendLine( $"return {charGroup.Single().ToProperty()};" );
					else
						AppendTagGroupSelector( builder, indent + 2, charGroup.ToArray() );
				}

				builder.Indent( indent + 1 ).AppendLine( "default:" );
				builder.Indent( indent + 2 ).AppendLine( "return null;" );
				builder.Indent( indent ).AppendLine( "}" );
			}
		}

		private static int GetMostDifferentCharIndex( Tag[] tags )
			=> Enumerable
			.Range( 0, tags[ 0 ].Value.Length )
			.Select
			(
				index
					=> new
					{
						Index = index,
						DifferrentCharCount = tags.Select( t => t.Value[ index ] ).Distinct().Count()
					}
			)
			.OrderByDescending( p => p.DifferrentCharCount )
			.ThenBy( p => p.Index )
			.Select( p => p.Index )
			.FirstOrDefault();

		private class Tag : IEquatable<Tag>
		{
			public string Name { get; set; }
			public string Value { get; set; }

			public string ToProperty() => $"{nameof( TagNames )}.{Name}";
			public override string ToString() => $"{{{nameof( Name )}: {Name}. {nameof( Value )}: {Value}}}";

			public override int GetHashCode()
			{
				unchecked
				{
					const int prime = -1521134295;
					int hash = nameof( Tag ).GetHashCode();
					hash = hash * prime + EqualityComparer<string>.Default.GetHashCode( Name );
					hash = hash * prime + EqualityComparer<string>.Default.GetHashCode( Value );
					return hash;
				}
			}

			public bool Equals( Tag other ) => !ReferenceEquals( other, null ) && Name == other.Name && Value == other.Value;
			public override bool Equals( object obj ) => Equals( obj as Tag );

			public static bool operator ==( Tag x, Tag y ) => ReferenceEquals( x, y ) || !ReferenceEquals( x, null ) && x.Equals( y );
			public static bool operator !=( Tag x, Tag y ) => !( x == y );
		}
	}
}