// 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.Linq;
using System.Threading;
using Microsoft.CodeAnalysis.Classification;
using Microsoft.CodeAnalysis.Classification.Classifiers;
using Microsoft.CodeAnalysis.Collections;
using Microsoft.CodeAnalysis.CSharp.Extensions;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Microsoft.CodeAnalysis.Text;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.CSharp.Classification.Classifiers;

internal sealed class SyntaxTokenClassifier : AbstractSyntaxClassifier
{
    public override ImmutableArray<int> SyntaxTokenKinds { get; } = [(int)SyntaxKind.LessThanToken];

    private static readonly Func<ITypeSymbol, bool> s_shouldInclude = t => t.TypeKind != TypeKind.Error && t.GetArity() > 0;

    public override void AddClassifications(
        SyntaxToken lessThanToken,
        TextSpan textSpan,
        SemanticModel semanticModel,
        ClassificationOptions options,
        SegmentedList<ClassifiedSpan> result,
        CancellationToken cancellationToken)
    {
        var syntaxTree = semanticModel.SyntaxTree;
        if (syntaxTree.IsInPartiallyWrittenGeneric(lessThanToken.Span.End, cancellationToken, out var identifier))
        {
            // IsInPartiallyWrittenGeneric will return true for things that could be 
            // partially generic method calls (as opposed to partially written types).
            //
            // For example: X?.Y<
            //
            // In this case, this could never be a type, and we do not want to try to 
            // resolve it as such as it can lead to inappropriate classifications.
            if (CouldBeGenericType(identifier))
            {
                var types = semanticModel.LookupTypeRegardlessOfArity(identifier, cancellationToken);
                if (types.Any(s_shouldInclude))
                {
#nullable disable // Can 'GetClassificationForType(types.First()' be null here?
                    result.Add(new ClassifiedSpan(identifier.Span, GetClassificationForType(types.First())));
#nullable enable
                }
            }
        }
    }

    private static bool CouldBeGenericType(SyntaxToken identifier)
    {
        // Look for patterns that indicate that this could never be a partially written 
        // generic *Type* (although it could be a partially written generic method).

        if (identifier.Parent is not IdentifierNameSyntax identifierName)
        {
            // Definitely not a generic type if this isn't even an identifier name.
            return false;
        }

        if (identifierName.IsParentKind(SyntaxKind.MemberBindingExpression))
        {
            // anything?.Identifier is never a generic type.
            return false;
        }

        // ?.X.Identifier   or  ?.X.Y.Identifier  is never a generic type.
        if (identifierName.IsSimpleMemberAccessExpressionName() ||
            identifierName.IsMemberBindingExpressionName())
        {
            if (identifier.Parent.IsParentKind(SyntaxKind.ConditionalAccessExpression))
                return false;
        }

        // Add more cases as necessary.
        return true;
    }
}