// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Threading;
using Microsoft.Extensions.Logging;
 
namespace Microsoft.Extensions.Caching.Hybrid.Internal;
 
internal partial class DefaultHybridCache
{
    internal sealed class ImmutableCacheItem<T> : CacheItem<T> // used to hold types that do not require defensive copies
    {
        private static ImmutableCacheItem<T>? _sharedDefault;
 
        public ImmutableCacheItem(long creationTimestamp, TagSet tags)
            : base(creationTimestamp, tags)
        {
        }
 
        private T _value = default!; // deferred until SetValue
 
        public long Size { get; private set; } = -1;
 
        public override bool DebugIsImmutable => true;
 
        // get a shared instance that passes as "reserved"; doesn't need to be 100% singleton,
        // but we don't want to break the reservation rules either; if we can't reserve: create new
        public static ImmutableCacheItem<T> GetReservedShared()
        {
            ImmutableCacheItem<T>? obj = Volatile.Read(ref _sharedDefault);
            if (obj is null || !obj.TryReserve())
            {
                obj = new(0, TagSet.Empty); // timestamp doesn't matter - not used in L1/L2
                _ = obj.TryReserve(); // this is reliable on a new instance
                Volatile.Write(ref _sharedDefault, obj);
            }
 
            return obj;
        }
 
        public void SetValue(T value, long size)
        {
            _value = value;
            Size = size;
        }
 
        public override bool TryGetValue(ILogger log, out T value)
        {
            value = _value;
            return true; // always available
        }
 
        public override bool TryGetSize(out long size)
        {
            size = Size;
            return size >= 0;
        }
 
        public override bool TryReserveBuffer(out BufferChunk buffer)
        {
            buffer = default;
            return false; // we don't have one to reserve!
        }
    }
}