// 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.Composition;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.Completion;
using Microsoft.CodeAnalysis.Host.Mef;
 
namespace Microsoft.CodeAnalysis.CSharp.Completion.Providers
{
    [ExportArgumentProvider(nameof(OutVariableArgumentProvider), LanguageNames.CSharp)]
    [ExtensionOrder(After = nameof(ContextVariableArgumentProvider))]
    [Shared]
    internal sealed class OutVariableArgumentProvider : ArgumentProvider
    {
        [ImportingConstructor]
        [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
        public OutVariableArgumentProvider()
        {
        }
 
        public override Task ProvideArgumentAsync(ArgumentContext context)
        {
            if (context.PreviousValue is not null)
            {
                // This argument provider does not attempt to replace arguments already in code.
                return Task.CompletedTask;
            }
 
            if (context.Parameter.RefKind != RefKind.Out)
            {
                // This argument provider only considers 'out' parameters.
                return Task.CompletedTask;
            }
 
            // Since tihs provider runs after ContextVariableArgumentProvider, we know there is no suitable target in
            // the current context. Instead, offer to declare a new variable.
            var name = context.Parameter.Name;
            if (SyntaxFacts.GetKeywordKind(name) != SyntaxKind.None
                || SyntaxFacts.GetContextualKeywordKind(name) != SyntaxKind.None)
            {
                name = "@" + name;
            }
 
            var syntax = SyntaxFactory.Argument(
                nameColon: null,
                refKindKeyword: SyntaxFactory.Token(SyntaxKind.OutKeyword),
                SyntaxFactory.DeclarationExpression(
                    type: SyntaxFactory.IdentifierName("var"),
                    designation: SyntaxFactory.SingleVariableDesignation(SyntaxFactory.Identifier(
                        SyntaxFactory.TriviaList(),
                        contextualKind: SyntaxKind.None,
                        text: name,
                        valueText: context.Parameter.Name,
                        SyntaxFactory.TriviaList()))));
 
            context.DefaultValue = syntax.NormalizeWhitespace().ToFullString();
            return Task.CompletedTask;
        }
    }
}