// 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;
using System.Collections.Immutable;
using System.Composition;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CodeFixes;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Editing;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.Shared.Extensions;
 
namespace Microsoft.CodeAnalysis.CSharp.CodeFixes.FixIncorrectConstraint;
 
[ExportCodeFixProvider(LanguageNames.CSharp, Name = PredefinedCodeFixProviderNames.FixIncorrectConstraint), Shared]
[method: ImportingConstructor]
[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
internal sealed class CSharpFixIncorrectConstraintCodeFixProvider() : SyntaxEditorBasedCodeFixProvider
{
    private const string CS9010 = nameof(CS9010); // Keyword 'enum' cannot be used as a constraint.Did you mean 'struct, System.Enum'?	Net6 C:\github\repo_find_refs\Net6\Class1.cs 1	Active
    private const string CS9011 = nameof(CS9011); // 'delegate' cannot be used as a constraint.Did you mean 'System.Delegate'?	Net6 C:\github\repo_find_refs\Net6\Class1.cs 1	Active
 
    public override ImmutableArray<string> FixableDiagnosticIds => [CS9010, CS9011];
 
    private static bool TryGetConstraint(
        Diagnostic diagnostic,
        CancellationToken cancellationToken,
        [NotNullWhen(true)] out TypeConstraintSyntax? constraint,
        out SyntaxToken enumOrDelegateKeyword)
    {
        enumOrDelegateKeyword = default;
        constraint = diagnostic.Location.FindNode(cancellationToken) as TypeConstraintSyntax;
        if (constraint == null)
            return false;
 
        if (constraint.Parent is not TypeParameterConstraintClauseSyntax)
            return false;
 
        if (constraint.Type is not IdentifierNameSyntax { Identifier.IsMissing: true } type)
            return false;
 
        var trailingTrivia = type.GetTrailingTrivia();
        if (trailingTrivia.Count == 0)
            return false;
 
        var firstTrivia = trailingTrivia[0];
        if (firstTrivia.Kind() != SyntaxKind.SkippedTokensTrivia)
            return false;
 
        var structure = (SkippedTokensTriviaSyntax)firstTrivia.GetStructure()!;
        if (structure.Tokens.Count != 1)
            return false;
 
        enumOrDelegateKeyword = structure.Tokens[0];
        return enumOrDelegateKeyword.Kind() is SyntaxKind.EnumKeyword or SyntaxKind.DelegateKeyword;
    }
 
    public override Task RegisterCodeFixesAsync(CodeFixContext context)
    {
        var cancellationToken = context.CancellationToken;
 
        var diagnostic = context.Diagnostics.First();
        if (TryGetConstraint(diagnostic, cancellationToken, out _, out _))
        {
            RegisterCodeFix(context, CSharpCodeFixesResources.Fix_constraint, nameof(CSharpFixIncorrectConstraintCodeFixProvider));
        }
 
        return Task.CompletedTask;
    }
 
    protected override async Task FixAllAsync(
        Document document, ImmutableArray<Diagnostic> diagnostics,
        SyntaxEditor editor, CancellationToken cancellationToken)
    {
        var generator = SyntaxGenerator.GetGenerator(document);
        var compilation = await document.Project.GetRequiredCompilationAsync(cancellationToken).ConfigureAwait(false);
 
        foreach (var diagnostic in diagnostics)
        {
            if (TryGetConstraint(diagnostic, cancellationToken, out var constraintSyntax, out var enumOrDelegateKeyword))
            {
                var isEnumConstraint = enumOrDelegateKeyword.Kind() is SyntaxKind.EnumKeyword;
                var newType = generator.TypeExpression(compilation.GetSpecialType(
                    isEnumConstraint ? SpecialType.System_Enum : SpecialType.System_Delegate));
 
                // Skip the first trailing trivia as that's the skipped enum/delegate keyword.
                editor.ReplaceNode(
                    constraintSyntax.Type, newType
                    .WithLeadingTrivia(constraintSyntax.GetLeadingTrivia())
                    .WithTrailingTrivia(constraintSyntax.GetTrailingTrivia().Skip(1)));
 
                // if they added the `enum` constraint, also add `struct` along with `System.Enum` to properly
                // reflect what they meant (and what the diagnostic says).
                if (isEnumConstraint)
                {
                    editor.ReplaceNode(constraintSyntax.GetRequiredParent(), (parent, _) =>
                    {
                        var clause = (TypeParameterConstraintClauseSyntax)parent;
                        return clause.WithConstraints(
                            clause.Constraints.Insert(0, SyntaxFactory.ClassOrStructConstraint(
                                SyntaxKind.StructConstraint)));
                    });
                }
            }
        }
    }
}