// 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.Diagnostics.CodeAnalysis;
using System.Threading;
 
namespace Roslyn.Utilities
{
    internal class CancellableLazy<T>
    {
        private NonReentrantLock? _gate;
        private Func<CancellationToken, T>? _valueFactory;
        private T? _value;
 
        public CancellableLazy(Func<CancellationToken, T> valueFactory)
        {
            _gate = new NonReentrantLock();
            _valueFactory = valueFactory;
        }
 
        public CancellableLazy(T value)
            => _value = value;
 
        public bool HasValue
        {
            get
            {
                return this.TryGetValue(out _);
            }
        }
 
        public bool TryGetValue([MaybeNullWhen(false)] out T value)
        {
            if (_valueFactory == null)
            {
                value = _value!;
                return true;
            }
            else
            {
                value = default;
                return false;
            }
        }
 
        public T GetValue(CancellationToken cancellationToken = default)
        {
            var gate = _gate;
            if (gate != null)
            {
                using (gate.DisposableWait(cancellationToken))
                {
                    if (_valueFactory != null)
                    {
                        _value = _valueFactory(cancellationToken);
                        Interlocked.Exchange<Func<CancellationToken, T>?>(ref _valueFactory, null);
                    }
 
                    Interlocked.Exchange(ref _gate, null);
                }
            }
 
            return _value!;
        }
    }
}