Htmx/Htmx.SourceGenerator/HtmxGenerator.cs

using System.Collections.Generic;
using System.Collections.Immutable;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Diagnostics;
using Microsoft.CodeAnalysis.Text;

namespace Htmx.SourceGenerator
{
    [Generator]
    public class HtmxGenerator : IIncrementalGenerator
    {
        public void Initialize(IncrementalGeneratorInitializationContext context)
        {
            var globalOptions = context.AnalyzerConfigOptionsProvider.Select((opt, _) => 
                (RootNamespace: GetOption(opt.GlobalOptions, "build_property.RootNamespace", "HtmxTemplates"),
                 ProjectDir: GetOption(opt.GlobalOptions, "build_property.MSBuildProjectDirectory", "")));

            context.RegisterSourceOutput(globalOptions, (spc, opt) =>
            {
                var infrastructureSource = $@"// <auto-generated/>
using System;
using System.Buffers;
using System.Runtime.CompilerServices;

namespace {opt.RootNamespace};

public readonly struct HtmxRenderContext
{{
    public readonly IBufferWriter<byte> Writer;
    public readonly int Depth;
    private const int MaxDepth = 512;

    public HtmxRenderContext(IBufferWriter<byte> writer, int depth = 0)
    {{
        if (depth > MaxDepth) throw new InvalidOperationException(""Template recursion limit exceeded."");
        Writer = writer;
        Depth = depth;
    }}

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public HtmxRenderContext Next() => new(Writer, Depth + 1);
}}

public interface IHtmxComponent
{{
    void Render(HtmxRenderContext context);
}}

public static class HtmxGeneratedExtensions
{{
    public static byte[] ToUtf8Bytes(this string str) => System.Text.Encoding.UTF8.GetBytes(str);

    public static void WriteHtmx(this IBufferWriter<byte> writer, IHtmxComponent component)
    {{
        component.Render(new HtmxRenderContext(writer));
    }}

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void WriteUtf8(this IBufferWriter<byte> writer, ReadOnlySpan<byte> data)
    {{
        var buffer = writer.GetSpan(data.Length);
        data.CopyTo(buffer);
        writer.Advance(data.Length);
    }}

    public static void WriteHtmxBody(this HttpContext context, IHtmxComponent component)
    {{
        context.Response.ContentType = ""text/html; charset=utf-8"";
        var writerContext = new HtmxRenderContext(context.Response.BodyWriter);
        component.Render(writerContext);
    }}
}}";
                spc.AddSource("HtmxInfrastructure.g.cs", SourceText.From(infrastructureSource, Encoding.UTF8));
            });

            var htmxFiles = context.AdditionalTextsProvider
                .Where(file => file.Path.EndsWith(".htmx"))
                .Combine(globalOptions);

            var templateData = htmxFiles.Select((combined, ct) =>
            {
                var (file, opt) = combined;
                var content = file.GetText(ct)?.ToString() ?? string.Empty;
                
                string relativePath = Path.GetDirectoryName(file.Path) ?? "";
                string subNamespace = "";

                if (!string.IsNullOrEmpty(opt.ProjectDir) && relativePath.StartsWith(opt.ProjectDir))
                {
                    subNamespace = relativePath.Substring(opt.ProjectDir.Length)
                        .Trim(Path.DirectorySeparatorChar)
                        .Replace(Path.DirectorySeparatorChar, '.');
                }

                string fullNamespace = string.IsNullOrEmpty(subNamespace) 
                    ? opt.RootNamespace 
                    : $"{opt.RootNamespace}.{subNamespace}";

                return new TemplateInfo(
                    Path.GetFileNameWithoutExtension(file.Path),
                    fullNamespace,
                    opt.RootNamespace,
                    ParseTemplate(content)
                );
            });

            context.RegisterSourceOutput(templateData, (spc, data) =>
            {
                spc.AddSource($"{data.ClassName}.g.cs", GenerateSource(data));
            });
        }

        private static string GetOption(AnalyzerConfigOptions options, string key, string fallback) 
            => options.TryGetValue(key, out var value) ? value : fallback;

        private static string GenerateSource(TemplateInfo data)
        {
            var sb = new StringBuilder();
            sb.AppendLine("// <auto-generated/>");
            sb.AppendLine("using System;");
            sb.AppendLine("using System.Buffers;");
            sb.AppendLine("using System.Runtime.CompilerServices;");
            
            if (data.Namespace != data.RootNamespace)
            {
                sb.AppendLine($"using {data.RootNamespace};");
            }

            sb.AppendLine($"namespace {data.Namespace};");
            sb.AppendLine();
            sb.AppendLine($"public abstract partial class {data.ClassName}Base : IHtmxComponent");
            sb.AppendLine("{");

            var variables = data.Segments.Where(s => !s.IsStatic).Select(s => s.Value).Distinct();
            foreach (var v in variables)
            {
                sb.AppendLine($"    protected abstract void Render{v}(HtmxRenderContext context);");
            }

            int staticCount = 0;
            foreach (var s in data.Segments.Where(s => s.IsStatic))
            {
                var bytes = Encoding.UTF8.GetBytes(s.Value);
                var byteString = string.Join(", ", bytes.Select(b => $"0x{b:X2}"));
                sb.AppendLine($"    private static ReadOnlySpan<byte> _part{staticCount++} => new byte[] {{ {byteString} }};");
            }

            sb.AppendLine();
            sb.AppendLine("    public void Render(HtmxRenderContext context)");
            sb.AppendLine("    {");

            int sIdx = 0;
            foreach (var segment in data.Segments)
            {
                if (segment.IsStatic)
                    sb.AppendLine($"        context.Writer.WriteUtf8(_part{sIdx++});");
                else
                    sb.AppendLine($"        Render{segment.Value}(context.Next());");
            }

            sb.AppendLine("    }");
            sb.AppendLine("}");

            return sb.ToString();
        }

        private static ImmutableArray<TemplateSegment> ParseTemplate(string content)
        {
            var builder = ImmutableArray.CreateBuilder<TemplateSegment>();
            var variableRegex = new Regex(@"\$\$([a-zA-Z0-9_]+)\$\$", RegexOptions.Compiled);
            
            var matches = variableRegex.Matches(content);
            int lastPos = 0;

            foreach (Match match in matches)
            {
                if (match.Index > lastPos)
                    builder.Add(new(content.Substring(lastPos, match.Index - lastPos), true));
                
                builder.Add(new(match.Groups[1].Value, false));
                lastPos = match.Index + match.Length;
            }

            if (lastPos < content.Length)
                builder.Add(new(content.Substring(lastPos), true));

            return builder.ToImmutable();
        }

        private record TemplateSegment(string Value, bool IsStatic);
        private record TemplateInfo(string ClassName, string Namespace, string RootNamespace, ImmutableArray<TemplateSegment> Segments);
    }
}

namespace System.Runtime.CompilerServices
{
    internal static class IsExternalInit { }
}