// 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.Collections.Generic;
using Roslyn.Utilities;
using Xunit;
 
namespace Microsoft.CodeAnalysis.UnitTests
{
    public class ReferenceHolderTests
    {
        [Fact]
        public void SameStrongObjectsEqual()
        {
            var obj = new object();
            var first = ReferenceHolder<object?>.Strong(obj);
            var second = ReferenceHolder<object?>.Strong(obj);
 
            VerifyEqual(first, second);
        }
 
        [Fact]
        public void SameWeakObjectsEqual()
        {
            var obj = new object();
            var first = ReferenceHolder<object?>.Weak(obj);
            var second = ReferenceHolder<object?>.Weak(obj);
 
            // 📝 There is no need for a GC.KeepAlive(obj) here. 'VerifyEqual' will produce correct results whether
            // or not the object is still alive. When the object is alive, the equality path is the same as
            // SameStrongObjectsEqual. When the object is not alive, the equality path is the same as
            // ExpiredSameValuesEqual.
            VerifyEqual(first, second);
        }
 
        [Fact]
        public void SameMixedObjectsEqual()
        {
            var obj = new object();
            var first = ReferenceHolder<object?>.Strong(obj);
            var second = ReferenceHolder<object?>.Weak(obj);
 
            VerifyEqual(first, second);
        }
 
        [Fact]
        public void NullValuesEqual()
        {
            var first = ReferenceHolder<object?>.Strong(null);
            var second = ReferenceHolder<object?>.Weak(null);
 
            VerifyEqual(first, second);
        }
 
        [Fact]
        public void ExpiredValueNotEqualToNull()
        {
            var strongNull = ReferenceHolder<object?>.Strong(null);
            var weakNull = ReferenceHolder<object?>.Weak(null);
            var expired = ReferenceHolder<object?>.TestAccessor.ReleasedWeak(hashCode: EqualityComparer<object?>.Default.GetHashCode(null!));
 
            Assert.Equal(strongNull.GetHashCode(), expired.GetHashCode());
            VerifyNotEqual(strongNull, expired);
            VerifyNotEqual(weakNull, expired);
        }
 
        [Fact]
        public void ExpiredSameValuesEqual()
        {
            var first = ReferenceHolder<object?>.TestAccessor.ReleasedWeak(hashCode: 1);
            var second = ReferenceHolder<object?>.TestAccessor.ReleasedWeak(hashCode: 1);
 
            Assert.Null(first.TryGetTarget());
            Assert.Null(second.TryGetTarget());
            VerifyEqual(first, second);
        }
 
        [Fact]
        public void ExpiredDifferentValuesNotEqual()
        {
            var first = ReferenceHolder<object?>.TestAccessor.ReleasedWeak(hashCode: 1);
            var second = ReferenceHolder<object?>.TestAccessor.ReleasedWeak(hashCode: 2);
 
            Assert.Null(first.TryGetTarget());
            Assert.Null(second.TryGetTarget());
            VerifyNotEqual(first, second);
        }
 
        private static void VerifyEqual<T>(ReferenceHolder<T> x, ReferenceHolder<T> y)
            where T : class?
        {
            Assert.Equal(x.GetHashCode(), y.GetHashCode());
            Assert.True(x.Equals(y));
            Assert.True(y.Equals(x));
        }
 
        private static void VerifyNotEqual<T>(ReferenceHolder<T> x, ReferenceHolder<T> y)
            where T : class?
        {
            Assert.False(x.Equals(y));
            Assert.False(y.Equals(x));
        }
    }
}