// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Linq;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;
 
namespace Microsoft.AspNetCore.SourceGenerators;
 
[Generator]
public class PublicProgramSourceGenerator : IIncrementalGenerator
{
    private const string PublicPartialProgramClassSource = """
// <auto-generated />
/// <summary>
/// Auto-generated public partial Program class for top-level statement apps.
/// </summary>
public partial class Program { }
""";
 
    public void Initialize(IncrementalGeneratorInitializationContext context)
    {
        var internalGeneratedProgramClass = context.CompilationProvider.Select(static (compilation, cancellationToken) =>
            // Get the entry point associated with the compilation, this maps to the Main method definition
            // Get the containing symbol of the entry point, this maps to the Program class
            compilation.GetEntryPoint(cancellationToken)?.ContainingSymbol is INamedTypeSymbol
            {
                // If the program class is already public, we don't need to generate anything
                DeclaredAccessibility: not Accessibility.Public,
                // If the discovered `Program` type is not a class then its not
                // generated and has been defined in source, so we can skip it
                TypeKind: TypeKind.Class,
                // If there are multiple partial declarations, then do nothing since we don't want
                // to trample on visibility explicitly set by the user
                DeclaringSyntaxReferences: { Length: 1 } declaringSyntaxReferences
            } &&
            // If the `Program` class is already declared in user code, we don't need to generate anything
            declaringSyntaxReferences.Single().GetSyntax(cancellationToken) is not ClassDeclarationSyntax
        );
 
        context.RegisterSourceOutput(internalGeneratedProgramClass, (context, result) =>
        {
            if (result)
            {
                context.AddSource("PublicTopLevelProgram.Generated.g.cs", PublicPartialProgramClassSource);
            }
        });
    }
}