// 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.

#nullable disable

using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Debugging;
using Microsoft.CodeAnalysis.LanguageService;
using Microsoft.CodeAnalysis.Shared.Extensions;

namespace Microsoft.CodeAnalysis.CSharp.Debugging;

internal static class LocationInfoGetter
{
    internal static async Task<DebugLocationInfo> GetInfoAsync(Document document, int position, CancellationToken cancellationToken)
    {
        // PERF:  This method will be called synchronously on the UI thread for every breakpoint in the solution.
        // Therefore, it is important that we make this call as cheap as possible.  Rather than constructing a
        // containing Symbol and using ToDisplayString (which might be more *correct*), we'll just do the best we
        // can with Syntax.  This approach is capable of providing parity with the pre-Roslyn implementation.
        var tree = await document.GetSyntaxTreeAsync(cancellationToken).ConfigureAwait(false);
        var root = await tree.GetRootAsync(cancellationToken).ConfigureAwait(false);
        var syntaxFactsService = document.GetLanguageService<ISyntaxFactsService>();
        var memberDeclaration = syntaxFactsService.GetContainingMemberDeclaration(root, position, useFullSpan: true);

        // It might be reasonable to return an empty Name and a LineOffset from the beginning of the
        // file for GlobalStatements.  However, the only known caller (Breakpoints Window) doesn't
        // appear to consume this information, so we'll just return the simplest thing (no location).
        if ((memberDeclaration == null) || (memberDeclaration.Kind() == SyntaxKind.GlobalStatement))
        {
            return default;
        }

        // field or event field declarations may contain multiple variable declarators. Try finding the correct one.
        // If the position does not point to one, try using the first one.
        VariableDeclaratorSyntax fieldDeclarator = null;
        if (memberDeclaration.Kind() is SyntaxKind.FieldDeclaration or SyntaxKind.EventFieldDeclaration)
        {
            var variableDeclarators = ((BaseFieldDeclarationSyntax)memberDeclaration).Declaration.Variables;

            foreach (var declarator in variableDeclarators)
            {
                if (declarator.FullSpan.Contains(position))
                {
                    fieldDeclarator = declarator;
                    break;
                }
            }

            fieldDeclarator ??= variableDeclarators.Count > 0 ? variableDeclarators[0] : null;
        }

        var name = syntaxFactsService.GetDisplayName(fieldDeclarator ?? memberDeclaration,
            DisplayNameOptions.IncludeNamespaces |
            DisplayNameOptions.IncludeParameters);

        var text = await document.GetValueTextAsync(cancellationToken).ConfigureAwait(false);
        var lineNumber = text.Lines.GetLineFromPosition(position).LineNumber;
        var accessor = memberDeclaration.GetAncestorOrThis<AccessorDeclarationSyntax>();
        var memberLine = text.Lines.GetLineFromPosition(accessor?.SpanStart ?? memberDeclaration.SpanStart).LineNumber;
        var lineOffset = lineNumber - memberLine;

        return new DebugLocationInfo(name, lineOffset);
    }
}