servicepoint-tanks/EndiannessSourceGenerator/EndiannessGenerator.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Text;

namespace EndiannessSourceGenerator;

internal class DebugMeException(string message) : Exception(message);

internal class InvalidUsageException(string message) : Exception(message);

[Generator]
public class StructEndiannessSourceGenerator : ISourceGenerator
{
    private const string Namespace = "EndiannessSourceGenerator";
    private const string AttributeName = "StructEndiannessAttribute";
    private const string IsLittleEndianProperty = "IsLittleEndian";

    private const string AttributeSourceCode =
        $$"""
          // <auto-generated/>
          namespace {{Namespace}}
          {
              [System.AttributeUsage(System.AttributeTargets.Struct)]
              public class {{AttributeName}}: System.Attribute
              {
                  public required bool {{IsLittleEndianProperty}} { get; init; }
              }
          }
          """;

    private const string UsingDeclarations =
        """
        using System;
        using System.Buffers.Binary;
        """;

    public void Initialize(GeneratorInitializationContext context)
    {
        // Register the attribute source
        context.RegisterForPostInitialization(i => { i.AddSource($"{AttributeName}.g.cs", AttributeSourceCode); });
        // context.RegisterForSyntaxNotifications(() => new SyntaxCon);
    }

    private readonly SymbolDisplayFormat _namespacedNameFormat =
        new(typeQualificationStyle: SymbolDisplayTypeQualificationStyle.NameAndContainingTypesAndNamespaces);

    // TODO: generate syntax tree with roslyn to get rid of string wrangling and so code is properly formatted
    public void Execute(GeneratorExecutionContext context)
    {
        var treesWithStructsWithAttributes = context.Compilation.SyntaxTrees
            .Where(st => st.GetRoot().DescendantNodes()
                .OfType<StructDeclarationSyntax>()
                .Any(p => p.DescendantNodes()
                    .OfType<AttributeSyntax>()
                    .Any()))
            .ToList();

        foreach (var tree in treesWithStructsWithAttributes)
        {
            var semanticModel = context.Compilation.GetSemanticModel(tree);

            var structsWithAttributes = tree.GetRoot().DescendantNodes()
                .OfType<StructDeclarationSyntax>()
                .Where(cd => cd.DescendantNodes()
                    .OfType<AttributeSyntax>()
                    .Any());

            foreach (var structDeclaration in structsWithAttributes)
            {
                var foundAttribute = GetEndiannessAttribute(structDeclaration, semanticModel);
                // not my type
                if (foundAttribute == null)
                    continue;

                HandleStruct(context, structDeclaration, semanticModel, foundAttribute);
            }
        }
    }

    private static void HandleStruct(GeneratorExecutionContext context, StructDeclarationSyntax structDeclaration,
        SemanticModel semanticModel, AttributeSyntax foundAttribute)
    {
        var isPartial = structDeclaration.Modifiers.Any(m => m.IsKind(SyntaxKind.PartialKeyword));
        if (!isPartial)
            throw new InvalidUsageException("struct is not marked partial");

        var accessibilityModifier = structDeclaration.Modifiers.Any(m => m.IsKind(SyntaxKind.InternalKeyword))
            ? "internal"
            : "public";

        var structType = semanticModel.GetDeclaredSymbol(structDeclaration);
        if (structType == null)
            throw new DebugMeException("struct type info is null");

        var structNamespace = structType.ContainingNamespace?.ToDisplayString();
        if (structNamespace == null)
            throw new InvalidUsageException("struct has to be contained in a namespace");

        var structIsLittleEndian = GetStructIsLittleEndian(foundAttribute);

        var generatedCode = new StringBuilder();
        generatedCode.AppendLine(UsingDeclarations);

        generatedCode.AppendLine($"namespace {structNamespace};");
        generatedCode.AppendLine($$"""{{accessibilityModifier}} partial struct {{structType.Name}} {""");

        var hasProperties = structDeclaration.Members
            .Any(m => m.IsKind(SyntaxKind.PropertyDeclaration));
        if (hasProperties)
            throw new InvalidUsageException("struct cannot have properties");

        var fieldDeclarations = structDeclaration.Members
            .Where(m => m.IsKind(SyntaxKind.FieldDeclaration)).OfType<FieldDeclarationSyntax>();
        GenerateStructProperties(generatedCode, fieldDeclarations, semanticModel, structIsLittleEndian);

        generatedCode.AppendLine("}"); // end of struct

        context.AddSource($"{structNamespace}.{structType.Name}.g.cs",
            SourceText.From(generatedCode.ToString(), Encoding.UTF8));
    }

    private static void GenerateStructProperties(StringBuilder generatedCode,
        IEnumerable<FieldDeclarationSyntax> fieldDeclarations, SemanticModel semanticModel, bool structIsLittleEndian)
    {
        foreach (var field in fieldDeclarations)
        {
            if (!field.Modifiers.Any(m => m.IsKind(SyntaxKind.PrivateKeyword)))
                throw new InvalidUsageException("fields have to be private");

            var variableDeclaration = field.DescendantNodes()
                .OfType<VariableDeclarationSyntax>()
                .FirstOrDefault();
            if (variableDeclaration == null)
                throw new DebugMeException("variable declaration of field declaration null");

            var variableTypeInfo = semanticModel.GetTypeInfo(variableDeclaration.Type).Type;
            if (variableTypeInfo == null)
                throw new DebugMeException("variable type info of field declaration null");

            var typeName = variableTypeInfo.ToDisplayString();
            var fieldName = variableDeclaration.Variables.First().Identifier.ToString();
            var propertyName = GeneratePropertyName(fieldName);

            GenerateProperty(generatedCode, typeName, propertyName, structIsLittleEndian, fieldName);
        }
    }

    private static void GenerateProperty(StringBuilder generatedCode, string typeName, string propertyName,
        bool structIsLittleEndian, string fieldName)
    {
        generatedCode.AppendLine($$"""public {{typeName}} {{propertyName}} {""");

        var maybeNegator = structIsLittleEndian ? string.Empty : "!";
        var sameEndiannessExpression = $"{maybeNegator}BitConverter.IsLittleEndian";

        generatedCode.AppendLine($"get => {sameEndiannessExpression}");
        generatedCode.AppendLine($"    ? {fieldName}");
        generatedCode.AppendLine($"    : BinaryPrimitives.ReverseEndianness({fieldName});");

        generatedCode.AppendLine($"set => {fieldName} = {sameEndiannessExpression}");
        generatedCode.AppendLine("    ? value");
        generatedCode.AppendLine("    : BinaryPrimitives.ReverseEndianness(value);");

        generatedCode.AppendLine("}"); // end of property
    }

    private static string GeneratePropertyName(string fieldName)
    {
        var propertyName = fieldName;
        if (propertyName.StartsWith("_"))
            propertyName = propertyName.Substring(1);
        if (!char.IsLetter(propertyName, 0) || char.IsUpper(propertyName, 0))
            throw new InvalidUsageException("field names have to start with a lower case letter");
        propertyName = propertyName.Substring(0, 1).ToUpperInvariant()
                       + propertyName.Substring(1);
        return propertyName;
    }

    private static AttributeSyntax? GetEndiannessAttribute(StructDeclarationSyntax structDeclaration,
        SemanticModel semanticModel)
    {
        AttributeSyntax? foundAttribute = null;
        foreach (var attributeSyntax in structDeclaration.DescendantNodes().OfType<AttributeSyntax>())
        {
            var attributeTypeInfo = semanticModel.GetTypeInfo(attributeSyntax).Type;
            if (attributeTypeInfo == null)
                throw new DebugMeException("attribute type info is null");

            if (attributeTypeInfo.ContainingNamespace?.Name != Namespace)
                continue;
            if (attributeTypeInfo.Name != AttributeName)
                continue;

            foundAttribute = attributeSyntax;
            break;
        }

        return foundAttribute;
    }

    private static bool GetStructIsLittleEndian(AttributeSyntax foundAttribute)
    {
        var endiannessArguments = foundAttribute.ArgumentList;
        if (endiannessArguments == null)
            throw new InvalidUsageException("endianness attribute has no arguments");

        var isLittleEndianArgumentSyntax = endiannessArguments.Arguments
            .FirstOrDefault(argumentSyntax =>
                argumentSyntax.NameEquals?.Name.Identifier.ToString() == IsLittleEndianProperty);
        if (isLittleEndianArgumentSyntax == null)
            throw new InvalidUsageException("endianness attribute argument not found");

        bool? structIsLittleEndian = isLittleEndianArgumentSyntax.Expression.Kind() switch
        {
            SyntaxKind.FalseLiteralExpression => false,
            SyntaxKind.TrueLiteralExpression => true,
            SyntaxKind.DefaultLiteralExpression => false,
            _ => throw new InvalidUsageException($"{IsLittleEndianProperty} has to be set with a literal")
        };
        return structIsLittleEndian.Value;
    }
}