// 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.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.Completion;
using Microsoft.CodeAnalysis.CSharp.Extensions;
using Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.CSharp.Utilities;
using Microsoft.CodeAnalysis.ErrorReporting;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Microsoft.CodeAnalysis.Text;
 
namespace Microsoft.CodeAnalysis.CSharp.Completion.Providers;
 
[ExportCompletionProvider(nameof(SpeculativeTCompletionProvider), LanguageNames.CSharp)]
[ExtensionOrder(After = nameof(AwaitCompletionProvider))]
[Shared]
internal class SpeculativeTCompletionProvider : LSPCompletionProvider
{
    [ImportingConstructor]
    [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
    public SpeculativeTCompletionProvider()
    {
    }
 
    internal override string Language => LanguageNames.CSharp;
 
    public override bool IsInsertionTrigger(SourceText text, int characterPosition, CompletionOptions options)
        => CompletionUtilities.IsTriggerCharacter(text, characterPosition, options);
 
    public override ImmutableHashSet<char> TriggerCharacters { get; } = CompletionUtilities.CommonTriggerCharacters;
 
    public override async Task ProvideCompletionsAsync(CompletionContext context)
    {
        try
        {
            var document = context.Document;
            var cancellationToken = context.CancellationToken;
 
            var showSpeculativeT = await document.IsValidContextForDocumentOrLinkedDocumentsAsync(
                (doc, ct) => ShouldShowSpeculativeTCompletionItemAsync(doc, context, ct),
                cancellationToken).ConfigureAwait(false);
 
            if (showSpeculativeT)
            {
                const string T = nameof(T);
                context.AddItem(CommonCompletionItem.Create(
                    T, displayTextSuffix: "", CompletionItemRules.Default, glyph: Glyph.TypeParameter));
            }
        }
        catch (Exception e) when (FatalError.ReportAndCatchUnlessCanceled(e, ErrorSeverity.General))
        {
            // nop
        }
    }
 
    private static async Task<bool> ShouldShowSpeculativeTCompletionItemAsync(Document document, CompletionContext completionContext, CancellationToken cancellationToken)
    {
        var position = completionContext.Position;
        var syntaxTree = await document.GetRequiredSyntaxTreeAsync(cancellationToken).ConfigureAwait(false);
        if (syntaxTree.IsInNonUserCode(position, cancellationToken) ||
            syntaxTree.IsPreProcessorDirectiveContext(position, cancellationToken))
        {
            return false;
        }
 
        // We could be in the middle of a ref/generic/tuple type, instead of a simple T case.
        // If we managed to walk out and get a different SpanStart, we treat it as a simple $$T case.
 
        var context = await completionContext.GetSyntaxContextWithExistingSpeculativeModelAsync(document, cancellationToken).ConfigureAwait(false);
 
        if (context.IsTaskLikeTypeContext)
            return false;
 
        var spanStart = position;
        while (true)
        {
            var oldSpanStart = spanStart;
 
            spanStart = WalkOutOfGenericType(syntaxTree, spanStart, context.SemanticModel, cancellationToken);
            spanStart = WalkOutOfTupleType(syntaxTree, spanStart, cancellationToken);
            spanStart = WalkOutOfRefType(syntaxTree, spanStart, cancellationToken);
 
            if (spanStart == oldSpanStart)
            {
                break;
            }
        }
 
        return IsStartOfSpeculativeTContext(syntaxTree, spanStart, cancellationToken);
    }
 
    private static bool IsStartOfSpeculativeTContext(SyntaxTree syntaxTree, int position, CancellationToken cancellationToken)
    {
        var token = syntaxTree.FindTokenOnLeftOfPosition(position, cancellationToken);
 
        return syntaxTree.IsMemberDeclarationContext(position, context: null, SyntaxKindSet.AllMemberModifiers, SyntaxKindSet.ClassInterfaceStructRecordTypeDeclarations, canBePartial: true, cancellationToken) ||
               syntaxTree.IsStatementContext(position, token, cancellationToken) ||
               syntaxTree.IsGlobalMemberDeclarationContext(position, SyntaxKindSet.AllGlobalMemberModifiers, cancellationToken) ||
               syntaxTree.IsGlobalStatementContext(position, cancellationToken) ||
               syntaxTree.IsDelegateReturnTypeContext(position, token);
    }
 
    private static int WalkOutOfGenericType(SyntaxTree syntaxTree, int position, SemanticModel semanticModel, CancellationToken cancellationToken)
    {
        var spanStart = position;
        var token = syntaxTree.FindTokenOnLeftOfPosition(position, cancellationToken);
 
        if (syntaxTree.IsGenericTypeArgumentContext(position, token, cancellationToken, semanticModel))
        {
            if (syntaxTree.IsInPartiallyWrittenGeneric(spanStart, cancellationToken, out var nameToken))
            {
                spanStart = nameToken.SpanStart;
            }
 
            // If the user types Goo<T, automatic brace completion will insert the close brace
            // and the generic won't be "partially written".
            if (spanStart == position)
            {
                spanStart = token.GetAncestor<GenericNameSyntax>()?.SpanStart ?? spanStart;
            }
 
            var tokenLeftOfGenericName = syntaxTree.FindTokenOnLeftOfPosition(spanStart, cancellationToken);
            if (tokenLeftOfGenericName.IsKind(SyntaxKind.DotToken) && tokenLeftOfGenericName.Parent.IsKind(SyntaxKind.QualifiedName))
            {
                spanStart = tokenLeftOfGenericName.Parent.SpanStart;
            }
        }
 
        return spanStart;
    }
 
    private static int WalkOutOfRefType(SyntaxTree syntaxTree, int position, CancellationToken cancellationToken)
    {
        var prevToken = syntaxTree.FindTokenOnLeftOfPosition(position, cancellationToken)
                                  .GetPreviousTokenIfTouchingWord(position);
 
        if (prevToken.Kind() is SyntaxKind.RefKeyword or SyntaxKind.ReadOnlyKeyword && prevToken.Parent.IsKind(SyntaxKind.RefType))
        {
            return prevToken.SpanStart;
        }
 
        return position;
    }
 
    private static int WalkOutOfTupleType(SyntaxTree syntaxTree, int position, CancellationToken cancellationToken)
    {
        var prevToken = syntaxTree.FindTokenOnLeftOfPosition(position, cancellationToken)
                                  .GetPreviousTokenIfTouchingWord(position);
 
        if (prevToken.IsPossibleTupleOpenParenOrComma())
        {
            return prevToken.Parent!.SpanStart;
        }
 
        return position;
    }
}