// 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.Collections.Immutable;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.FindSymbols;
using Microsoft.CodeAnalysis.FindUsages;
using Microsoft.CodeAnalysis.Navigation;
using Microsoft.CodeAnalysis.PooledObjects;
 
namespace Microsoft.CodeAnalysis.GoToDefinition;
 
internal static class GoToDefinitionFeatureHelpers
{
    public static ISymbol? TryGetPreferredSymbol(
        Solution solution, ISymbol? symbol, CancellationToken cancellationToken)
    {
        // VB global import aliases have a synthesized SyntaxTree.
        // We can't go to the definition of the alias, so use the target type.
 
        var alias = symbol as IAliasSymbol;
        if (alias != null)
        {
            if (alias.Target is INamespaceSymbol ns && ns.IsGlobalNamespace)
                return null;
        }
 
        // VB global import aliases have a synthesized SyntaxTree.
        // We can't go to the definition of the alias, so use the target type.
 
        if (alias != null)
        {
            var sourceLocations = NavigableItemFactory.GetPreferredSourceLocations(
                solution, symbol, cancellationToken);
 
            if (sourceLocations.All(l => solution.GetDocument(l.SourceTree) == null))
                symbol = alias.Target;
        }
 
        var definition = SymbolFinder.FindSourceDefinition(symbol, solution, cancellationToken);
        cancellationToken.ThrowIfCancellationRequested();
 
        symbol = definition ?? symbol;
 
        // If symbol has a partial implementation part, prefer to go to it, since that is where the body is.
        symbol = (symbol as IMethodSymbol)?.PartialImplementationPart ?? symbol;
        symbol = (symbol as IPropertySymbol)?.PartialImplementationPart ?? symbol;
 
        return symbol;
    }
 
    public static async Task<ImmutableArray<DefinitionItem>> GetDefinitionsAsync(
        ISymbol? symbol,
        Solution solution,
        bool thirdPartyNavigationAllowed,
        CancellationToken cancellationToken)
    {
        symbol = TryGetPreferredSymbol(solution, symbol, cancellationToken);
        if (symbol is null)
            return [];
 
        using var _ = ArrayBuilder<DefinitionItem>.GetInstance(out var definitions);
 
        // Going to a symbol may end up actually showing the symbol in the Find-Usages window.
        // This happens when there is more than one location for the symbol (i.e. for partial
        // symbols) and we don't know the best place to take you to.
        //
        // The FindUsages window supports showing the classified text for an item.  It does this
        // in two ways.  Either the item can pass along its classified text (and the window will
        // defer to that), or the item will have no classified text, and the window will compute
        // it in the BG.
        //
        // Passing along the classified information is valuable for OOP scenarios where we want
        // all that expensive computation done on the OOP side and not in the VS side.
        //
        // However, Go To Definition is all in-process, and is also synchronous.  So we do not
        // want to fetch the classifications here.  It slows down the command and leads to a
        // measurable delay in our perf tests.
        //
        // So, if we only have a single location to go to, this does no unnecessary work.  And,
        // if we do have multiple locations to show, it will just be done in the BG, unblocking
        // this command thread so it can return the user faster.
        var definitionItem = symbol.ToNonClassifiedDefinitionItem(solution, includeHiddenLocations: true);
 
        if (thirdPartyNavigationAllowed)
        {
            var provider = solution.Services.GetService<IExternalDefinitionItemProvider>();
            if (provider != null)
            {
                var thirdPartyItem = await provider.GetThirdPartyDefinitionItemAsync(solution, definitionItem, cancellationToken).ConfigureAwait(false);
                definitions.AddIfNotNull(thirdPartyItem);
            }
        }
 
        definitions.Add(definitionItem);
        return definitions.ToImmutableAndClear();
    }
}