// 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;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Diagnostics;
using System.Threading;
 
#pragma warning disable CA1000 // Do not declare static members on generic types
 
namespace Analyzer.Utilities.PooledObjects
{
    // HashSet that can be recycled via an object pool
    internal sealed class PooledHashSet<T> : HashSet<T>, IDisposable
    {
        private readonly ObjectPool<PooledHashSet<T>>? _pool;
 
        private PooledHashSet(ObjectPool<PooledHashSet<T>>? pool, IEqualityComparer<T>? comparer)
            : base(comparer)
        {
            _pool = pool;
        }
 
        public void Dispose() => Free(CancellationToken.None);
 
        public void Free(CancellationToken cancellationToken)
        {
            // Do not free in presence of cancellation.
            // See https://github.com/dotnet/roslyn/issues/46859 for details.
            if (cancellationToken.IsCancellationRequested)
            {
                return;
            }
 
            this.Clear();
            _pool?.Free(this, cancellationToken);
        }
 
        public ImmutableHashSet<T> ToImmutableAndFree()
        {
            ImmutableHashSet<T> result;
            if (Count == 0)
            {
                result = ImmutableHashSet<T>.Empty;
            }
            else
            {
                result = this.ToImmutableHashSet(Comparer);
                this.Clear();
            }
 
            _pool?.Free(this, CancellationToken.None);
            return result;
        }
 
        public ImmutableHashSet<T> ToImmutable()
            => Count == 0 ? ImmutableHashSet<T>.Empty : this.ToImmutableHashSet(Comparer);
 
        // global pool
        private static readonly ObjectPool<PooledHashSet<T>> s_poolInstance = CreatePool();
        private static readonly ConcurrentDictionary<IEqualityComparer<T>, ObjectPool<PooledHashSet<T>>> s_poolInstancesByComparer = new();
 
        // if someone needs to create a pool;
        public static ObjectPool<PooledHashSet<T>> CreatePool(IEqualityComparer<T>? comparer = null)
        {
            ObjectPool<PooledHashSet<T>>? pool = null;
            pool = new ObjectPool<PooledHashSet<T>>(() => new PooledHashSet<T>(pool, comparer), 128);
            return pool;
        }
 
        public static PooledHashSet<T> GetInstance(IEqualityComparer<T>? comparer = null)
        {
            var pool = comparer == null ?
                s_poolInstance :
                s_poolInstancesByComparer.GetOrAdd(comparer, CreatePool);
            var instance = pool.Allocate();
            Debug.Assert(instance.Count == 0);
            return instance;
        }
 
        public static PooledHashSet<T> GetInstance(IEnumerable<T> initializer, IEqualityComparer<T>? comparer = null)
        {
            var instance = GetInstance(comparer);
            foreach (var value in initializer)
            {
                instance.Add(value);
            }
 
            return instance;
        }
    }
}