// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Collections.Concurrent;
using System.IO;
using System.Reflection.Metadata;
using System.Reflection.PortableExecutable;
using System.Reflection.TypeLoading;
using System.Reflection.TypeLoading.Ecma;
 
namespace System.Reflection
{
    public sealed partial class MetadataLoadContext
    {
        // This maintains the canonical list of Assembly instances for a given def name. Each defname can only appear
        // once in the list and its appearance prevents further assemblies with the same identity from loading unless the Mvids's match.
        // Null entries do *not* appear here.
        private readonly ConcurrentDictionary<RoAssemblyName, RoAssembly> _loadedAssemblies = new ConcurrentDictionary<RoAssemblyName, RoAssembly>();
 
        private RoAssembly LoadFromStreamCore(Stream peStream)
        {
            PEReader peReader = new PEReader(peStream);
            PEReader? peReaderToDispose = peReader; // Ensure peReader is disposed immediately if we throw an exception before we're done.
            try
            {
                if (!peReader.HasMetadata)
                    throw new BadImageFormatException(SR.NoMetadataInPeImage);
 
                string location = (peStream is FileStream fs) ? (fs.Name ?? string.Empty) : string.Empty;
                MetadataReader reader = peReader.GetMetadataReader();
                RoAssembly candidate = new EcmaAssembly(this, peReader, reader, location);
                AssemblyNameData defNameData = candidate.GetAssemblyNameDataNoCopy();
                byte[] pkt = defNameData.PublicKeyToken ?? Array.Empty<byte>();
                if (pkt.Length == 0 && defNameData.PublicKey != null && defNameData.PublicKey.Length != 0)
                {
                    pkt = defNameData.PublicKey.ComputePublicKeyToken()!;
                }
                RoAssemblyName defName = new RoAssemblyName(defNameData.Name, defNameData.Version, defNameData.CultureName, pkt, defNameData.Flags);
 
                RoAssembly winner = _loadedAssemblies.GetOrAdd(defName, candidate);
                if (winner == candidate)
                {
                    // We won the race.
                    RegisterForDisposal(peReader);
                    peReaderToDispose = null;
 
                    // We do not add to the _binds list because the binding list is only for assemblies that have been resolved through
                    // the Resolve method. This allows the resolver to have complete control over selecting the appropriate assembly
                    // based on Version, CultureName and PublicKeyToken.
 
                    return winner;
                }
                else
                {
                    // We lost the race but check for a Mvid mismatch.
                    if (candidate.ManifestModule.ModuleVersionId != winner.ManifestModule.ModuleVersionId)
                        throw new FileLoadException(SR.Format(SR.FileLoadDuplicateAssemblies, defName));
                }
 
                return winner;
            }
            finally
            {
                peReaderToDispose?.Dispose();
            }
        }
    }
}