// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Text;

namespace CommunityToolkit.Mvvm.SourceGenerators;

/// <summary>
/// A source generator for necessary nullability attributes.
/// </summary>
[Generator]
public sealed class NullabilityAttributesGenerator : ISourceGenerator
{
    /// <inheritdoc/>
    public void Initialize(GeneratorInitializationContext context)
    {
    }

    /// <inheritdoc/>
    public void Execute(GeneratorExecutionContext context)
    {
        AddSourceCodeIfTypeIsNotPresent(context, "System.Diagnostics.CodeAnalysis.NotNullAttribute");
        AddSourceCodeIfTypeIsNotPresent(context, "System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute");
    }

    /// <summary>
    /// Adds the source for a given attribute type if it's not present already in the compilation.
    /// </summary>
    private void AddSourceCodeIfTypeIsNotPresent(GeneratorExecutionContext context, string typeFullName)
    {
        // Check that the target attributes are not available in the consuming project. To ensure that
        // this works fine both in .NET (Core) and .NET Standard implementations, we also need to check
        // that the target types are declared as public (we assume that in this case those types are from the BCL).
        // This avoids issues on .NET Standard with Roslyn also seeing internal types from referenced assemblies.
        if (context.Compilation.GetTypeByMetadataName(typeFullName) is { DeclaredAccessibility: Accessibility.Public })
        {
            return;
        }

        string typeName = typeFullName.Split('.').Last();
        string filename = $"CommunityToolkit.Mvvm.SourceGenerators.EmbeddedResources.{typeName}.cs";

        Stream stream = Assembly.GetExecutingAssembly().GetManifestResourceStream(filename);
        StreamReader reader = new(stream);

        string source = reader.ReadToEnd();

        context.AddSource($"{typeFullName}.cs", SourceText.From(source, Encoding.UTF8));
    }
}