// 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;
using System.Collections.Generic;
using System.Collections.Immutable;
using Roslyn.Utilities;
 
namespace Microsoft.CodeAnalysis.Host
{
    /// <summary>
    /// A cache for metadata references.
    /// </summary>
    internal class MetadataReferenceCache
    {
        private ImmutableDictionary<string, ReferenceSet> _referenceSets
            = ImmutableDictionary<string, ReferenceSet>.Empty;
 
        private readonly Func<string, MetadataReferenceProperties, MetadataReference> _createReference;
 
        public MetadataReferenceCache(Func<string, MetadataReferenceProperties, MetadataReference> createReference)
            => _createReference = createReference ?? throw new ArgumentNullException(nameof(createReference));
 
        public MetadataReference GetReference(string path, MetadataReferenceProperties properties)
        {
            if (!_referenceSets.TryGetValue(path, out var referenceSet))
            {
                referenceSet = ImmutableInterlocked.GetOrAdd(ref _referenceSets, path, new ReferenceSet(this));
            }
 
            return referenceSet.GetAddOrUpdate(path, properties);
        }
 
        /// <summary>
        /// A collection of references to the same underlying metadata, each with different properties.
        /// </summary>
        private class ReferenceSet
        {
            private readonly MetadataReferenceCache _cache;
 
            private readonly NonReentrantLock _gate = new();
 
            // metadata references are held weakly, so even though this is a cache that enables reuse, it does not control lifetime.
            private readonly Dictionary<MetadataReferenceProperties, WeakReference<MetadataReference>> _references
                = new();
 
            public ReferenceSet(MetadataReferenceCache cache)
                => _cache = cache;
 
            public MetadataReference GetAddOrUpdate(string path, MetadataReferenceProperties properties)
            {
                using (_gate.DisposableWait())
                {
                    MetadataReference mref = null;
                    if (!(_references.TryGetValue(properties, out var weakref) && weakref.TryGetTarget(out mref)))
                    {
                        // try to base this metadata reference off of an existing one, so we don't load the metadata bytes twice.
                        foreach (var wr in _references.Values)
                        {
                            if (wr.TryGetTarget(out mref))
                            {
                                mref = mref.WithProperties(properties);
                                break;
                            }
                        }
 
                        mref ??= _cache._createReference(path, properties);
 
                        _references[properties] = new WeakReference<MetadataReference>(mref);
                    }
 
                    return mref;
                }
            }
        }
    }
}