// 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.Threading;
using Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Shared.Extensions;
 
namespace Microsoft.CodeAnalysis.CSharp.Completion.KeywordRecommenders;
 
internal class ElseKeywordRecommender : AbstractSyntacticSingleKeywordRecommender
{
    public ElseKeywordRecommender()
        : base(SyntaxKind.ElseKeyword, isValidInPreprocessorContext: true)
    {
    }
 
    protected override bool IsValidContext(int position, CSharpSyntaxContext context, CancellationToken cancellationToken)
    {
        if (context.IsPreProcessorKeywordContext)
        {
            return true;
        }
 
        var token = context.TargetToken;
 
        // We have to consider all ancestor if statements of the last token until we find a match for this 'else':
        // while (true)
        //     if (true)
        //         while (true)
        //             if (true)
        //                 Console.WriteLine();
        //             else
        //                 Console.WriteLine();
        //     $$
        foreach (var ifStatement in token.GetAncestors<IfStatementSyntax>())
        {
            // If there's a missing token at the end of the statement, it's incomplete and we do not offer 'else'.
            // context.TargetToken does not include zero width so in that case these will never be equal.
            if (ifStatement.Statement.GetLastToken(includeZeroWidth: true) == token)
            {
                return true;
            }
        }
 
        return false;
    }
}