// 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.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CodeActions;
using Microsoft.CodeAnalysis.CodeFixes;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Diagnostics;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Microsoft.CodeAnalysis.Text;

namespace Microsoft.CodeAnalysis.CSharp.NewLines.ArrowExpressionClausePlacement;

[ExportCodeFixProvider(LanguageNames.CSharp, Name = PredefinedCodeFixProviderNames.ArrowExpressionClausePlacement), Shared]
[method: ImportingConstructor]
[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
internal sealed class ArrowExpressionClausePlacementCodeFixProvider() : CodeFixProvider
{
    public override ImmutableArray<string> FixableDiagnosticIds
        => [IDEDiagnosticIds.ArrowExpressionClausePlacementDiagnosticId];

    public override async Task RegisterCodeFixesAsync(CodeFixContext context)
    {
        var document = context.Document;
        var diagnostic = context.Diagnostics.First();
        context.RegisterCodeFix(
            CodeAction.Create(
                CSharpCodeFixesResources.Place_token_on_following_line,
                c => UpdateDocumentAsync(document, [diagnostic], c),
                nameof(CSharpCodeFixesResources.Place_token_on_following_line)),
            context.Diagnostics);
    }

    private static async Task<Document> UpdateDocumentAsync(
        Document document, ImmutableArray<Diagnostic> diagnostics, CancellationToken cancellationToken)
    {
        var text = await document.GetValueTextAsync(cancellationToken).ConfigureAwait(false);
        var root = await document.GetRequiredSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
        using var _ = ArrayBuilder<TextChange>.GetInstance(out var edits);

        foreach (var diagnostic in diagnostics)
        {
            var arrowToken = root.FindToken(diagnostic.Location.SourceSpan.Start);
            Contract.ThrowIfTrue(arrowToken.Kind() != SyntaxKind.EqualsGreaterThanToken);

            var arrowExpression = (ArrowExpressionClauseSyntax)arrowToken.GetRequiredParent();

            AddEdits(text, arrowExpression.ArrowToken, arrowExpression.Expression, edits);
        }

        var changedText = text.WithChanges(edits);
        return document.WithText(changedText);
    }

    private static void AddEdits(
        SourceText text,
        SyntaxToken token,
        ExpressionSyntax nextExpression,
        ArrayBuilder<TextChange> edits)
    {
        // Cases to consider
        // x =>
        // x => 
        // x => /* comment */
        // x /* comment */ =>
        // x /* comment */ => /* comment */
        //
        // in all these cases, we want to grab the arrow, and any spaces that follow and remove that, but we
        // leave the rest where it is. We then move the arrow right before the start of the next token.

        var start = token.SpanStart;
        var end = token.Span.End;

        while (end < text.Length && text[end] == ' ')
            end++;

        if (end < text.Length && SyntaxFacts.IsNewLine(text[end]))
        {
            while (start > 0 && text[start - 1] == ' ')
                start--;
        }

        edits.Add(new TextChange(TextSpan.FromBounds(start, end), ""));
        edits.Add(new TextChange(new TextSpan(nextExpression.SpanStart, 0), token.Text + " "));
    }

    public override FixAllProvider? GetFixAllProvider()
        => FixAllProvider.Create(async (context, document, diagnostics) => await UpdateDocumentAsync(document, diagnostics, context.CancellationToken).ConfigureAwait(false));
}
