// 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.Diagnostics;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.LanguageService;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis.Shared.Extensions;
 
namespace Microsoft.CodeAnalysis.NewLines.MultipleBlankLines;
 
[ExportCodeFixProvider(LanguageNames.CSharp, LanguageNames.VisualBasic, Name = PredefinedCodeFixProviderNames.RemoveBlankLines), Shared]
[method: ImportingConstructor]
[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
internal sealed class MultipleBlankLinesCodeFixProvider() : CodeFixProvider
{
    public override ImmutableArray<string> FixableDiagnosticIds
        => [IDEDiagnosticIds.MultipleBlankLinesDiagnosticId];
 
    public override Task RegisterCodeFixesAsync(CodeFixContext context)
    {
        var document = context.Document;
        var diagnostic = context.Diagnostics.First();
        context.RegisterCodeFix(CodeAction.Create(
            CodeFixesResources.Remove_extra_blank_lines,
            c => UpdateDocumentAsync(document, diagnostic, c),
            nameof(CodeFixesResources.Remove_extra_blank_lines)),
            context.Diagnostics);
        return Task.CompletedTask;
    }
 
    private static Task<Document> UpdateDocumentAsync(Document document, Diagnostic diagnostic, CancellationToken cancellationToken)
        => FixAllAsync(document, [diagnostic], cancellationToken);
 
    private static async Task<Document> FixAllAsync(
        Document document, ImmutableArray<Diagnostic> diagnostics, CancellationToken cancellationToken)
    {
        var syntaxKinds = document.GetRequiredLanguageService<ISyntaxKindsService>();
        var root = await document.GetRequiredSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
 
        using var _ = PooledDictionary<SyntaxToken, SyntaxToken>.GetInstance(out var replacements);
        foreach (var diagnostic in diagnostics)
        {
            var token = root.FindToken(diagnostic.AdditionalLocations[0].SourceSpan.Start);
            var leadingTrivia = UpdateLeadingTrivia(syntaxKinds, token.LeadingTrivia);
            replacements.Add(token, token.WithLeadingTrivia(leadingTrivia));
        }
 
        var newRoot = root.ReplaceTokens(replacements.Keys, (token, _) => replacements[token]);
 
        return document.WithSyntaxRoot(newRoot);
    }
 
    private static SyntaxTriviaList UpdateLeadingTrivia(ISyntaxKindsService syntaxKinds, SyntaxTriviaList triviaList)
    {
        using var _ = ArrayBuilder<SyntaxTrivia>.GetInstance(out var builder);
 
        var currentStart = 0;
 
        while (currentStart < triviaList.Count)
        {
            var trivia = triviaList[currentStart];
            builder.Add(trivia);
 
            // If it's not an end of line, just keep going.
            if (trivia.RawKind != syntaxKinds.EndOfLineTrivia)
            {
                currentStart++;
                continue;
            }
 
            // We have a newlines.  Walk forward to get to the last newline in this sequence.
            var currentEnd = currentStart + 1;
            while (currentEnd < triviaList.Count &&
                   IsEndOfLine(syntaxKinds, triviaList, currentEnd))
            {
                currentEnd++;
            }
 
            var newLineCount = currentEnd - currentStart;
            if (newLineCount == 1)
            {
                // only a single newline.  keep as is.
                currentStart = currentEnd;
                continue;
            }
 
            // we have two or more newlines.  We have three cases to handle:
            //
            // 1. We're at the start of the token's trivia.  Collapse this down to 1 blank line.
            // 2. We follow structured trivia (i.e. pp-directive or doc comment).  These already end with a newline,
            //    so we only need to add one newline to get a blank line.
            // 3. We follow something else.  We only want to collapse if we have 3 or more newlines.
 
            if (currentStart == 0)
            {
                // case 1.
                // skip the second newline onwards.
                currentStart = currentEnd;
                continue;
            }
 
            if (triviaList[currentStart - 1].HasStructure)
            {
                // case 2.
                // skip the second newline onwards
                currentStart = currentEnd;
                continue;
            }
 
            if (newLineCount >= 3)
            {
                // case 3.  We want to keep the first two newlines to end up with one blank line,
                // and then skip the rest.
                builder.Add(triviaList[currentStart + 1]);
                currentStart = currentEnd;
                continue;
            }
 
            // for anything else just add the trivia and move forward like normal.
            currentStart++;
        }
 
        return new SyntaxTriviaList(builder.ToImmutable());
    }
 
    private static bool IsEndOfLine(ISyntaxKindsService syntaxKinds, SyntaxTriviaList triviaList, int index)
    {
        if (index >= triviaList.Count)
            return false;
 
        var trivia = triviaList[index];
        return trivia.RawKind == syntaxKinds.EndOfLineTrivia;
    }
 
    public override FixAllProvider? GetFixAllProvider()
        => FixAllProvider.Create(async (context, document, diagnostics) => await FixAllAsync(document, diagnostics, context.CancellationToken).ConfigureAwait(false));
}