// 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;
using System.Diagnostics.CodeAnalysis;
using System.Diagnostics.Contracts;
using System.Runtime.CompilerServices;

namespace CommunityToolkit.HighPerformance;

/// <summary>
/// A <see langword="class"/> that represents a boxed <typeparamref name="T"/> value on the managed heap.
/// This is a "shadow" type that can be used in place of a non-generic <see cref="object"/> reference to a
/// boxed value type, to make the code more expressive and reduce the chances of errors.
/// Consider this example:
/// <code>
/// object obj = 42;
///
/// // Manual, error prone unboxing
/// int sum = (int)obj + 1;
/// </code>
/// In this example, it is not possible to know in advance what type is actually being boxed in a given
/// <see cref="object"/> instance, making the code less robust at build time. The <see cref="Box{T}"/>
/// type can be used as a drop-in replacement in this case, like so:
/// <code>
/// Box&lt;int> box = 42;
///
/// // Build-time validation, automatic unboxing
/// int sum = box.Value + 1;
/// </code>
/// This type can also be useful when dealing with large custom value types that are also boxed, as
/// it allows to retrieve a mutable reference to the boxing value. This means that a given boxed
/// value can be mutated in-place, instead of having to allocate a new updated boxed instance.
/// </summary>
/// <typeparam name="T">The type of value being boxed.</typeparam>
[DebuggerDisplay("{ToString(),raw}")]
public sealed class Box<T>
    where T : struct
{
    // Boxed value types in the CLR are represented in memory as simple objects that store the method table of
    // the corresponding T value type being boxed, and then the data of the value being boxed:
    // [ sync block || pMethodTable || boxed T value ]
    //                 ^               ^
    //                 |               \-- Unsafe.Unbox<T>(Box<T>)
    //                 \-- Box<T> reference
    // For more info, see: https://mattwarren.org/2017/08/02/A-look-at-the-internals-of-boxing-in-the-CLR/.
    // Note that there might be some padding before the actual data representing the boxed value,
    // which might depend on both the runtime and the exact CPU architecture.
    // This is automatically handled by the unbox !!T instruction in IL, which
    // unboxes a given value type T and returns a reference to its boxed data.

    /// <summary>
    /// Initializes a new instance of the <see cref="Box{T}"/> class.
    /// </summary>
    /// <remarks>
    /// This constructor is never used, it is only declared in order to mark it with
    /// the <see langword="private"/> visibility modifier and prevent direct use.
    /// </remarks>
    /// <exception cref="InvalidOperationException">Always thrown when this constructor is used (eg. from reflection).</exception>
    private Box()
    {
        throw new InvalidOperationException("The CommunityToolkit.HighPerformance.Box<T> constructor should never be used.");
    }

    /// <summary>
    /// Returns a <see cref="Box{T}"/> reference from the input <see cref="object"/> instance.
    /// </summary>
    /// <param name="obj">The input <see cref="object"/> instance, representing a boxed <typeparamref name="T"/> value.</param>
    /// <returns>A <see cref="Box{T}"/> reference pointing to <paramref name="obj"/>.</returns>
    [Pure]
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static Box<T> GetFrom(object obj)
    {
        if (obj.GetType() != typeof(T))
        {
            ThrowInvalidCastExceptionForGetFrom();
        }

        return Unsafe.As<Box<T>>(obj)!;
    }

    /// <summary>
    /// Returns a <see cref="Box{T}"/> reference from the input <see cref="object"/> instance.
    /// </summary>
    /// <param name="obj">The input <see cref="object"/> instance, representing a boxed <typeparamref name="T"/> value.</param>
    /// <returns>A <see cref="Box{T}"/> reference pointing to <paramref name="obj"/>.</returns>
    /// <remarks>
    /// This method doesn't check the actual type of <paramref name="obj"/>, so it is responsibility of the caller
    /// to ensure it actually represents a boxed <typeparamref name="T"/> value and not some other instance.
    /// </remarks>
    [Pure]
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static Box<T> DangerousGetFrom(object obj)
    {
        return Unsafe.As<Box<T>>(obj)!;
    }

    /// <summary>
    /// Tries to get a <see cref="Box{T}"/> reference from an input <see cref="object"/> representing a boxed <typeparamref name="T"/> value.
    /// </summary>
    /// <param name="obj">The input <see cref="object"/> instance to check.</param>
    /// <param name="box">The resulting <see cref="Box{T}"/> reference, if <paramref name="obj"/> was a boxed <typeparamref name="T"/> value.</param>
    /// <returns><see langword="true"/> if a <see cref="Box{T}"/> instance was retrieved correctly, <see langword="false"/> otherwise.</returns>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool TryGetFrom(object obj, [NotNullWhen(true)] out Box<T>? box)
    {
        if (obj.GetType() == typeof(T))
        {
            box = Unsafe.As<Box<T>>(obj)!;

            return true;
        }

        box = null;

        return false;
    }

    /// <summary>
    /// Implicitly gets the <typeparamref name="T"/> value from a given <see cref="Box{T}"/> instance.
    /// </summary>
    /// <param name="box">The input <see cref="Box{T}"/> instance.</param>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static implicit operator T(Box<T> box)
    {
        return (T)(object)box;
    }

    /// <summary>
    /// Implicitly creates a new <see cref="Box{T}"/> instance from a given <typeparamref name="T"/> value.
    /// </summary>
    /// <param name="value">The input <typeparamref name="T"/> value to wrap.</param>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static implicit operator Box<T>(T value)
    {
        // The Box<T> type is never actually instantiated.
        // Here we are just boxing the input T value, and then reinterpreting
        // that object reference as a Box<T> reference. As such, the Box<T>
        // type is really only used as an interface to access the contents
        // of a boxed value type. This also makes it so that additional methods
        // like ToString() or GetHashCode() will automatically be referenced from
        // the method table of the boxed object, meaning that they don't need to
        // manually be implemented in the Box<T> type. For instance, boxing a float
        // and calling ToString() on it directly, on its boxed object or on a Box<T>
        // reference retrieved from it will produce the same result in all cases.
        return Unsafe.As<Box<T>>(value)!;
    }

    /// <inheritdoc/>
    public override string ToString()
    {
        // Here we're overriding the base object virtual methods to ensure
        // calls to those methods have a correct results on all runtimes.
        // For instance, not doing so is causing issue on .NET Core 2.1 Release
        // due to how the runtime handles the Box<T> reference to an actual
        // boxed T value (not a concrete Box<T> instance as it would expect).
        // To fix that, the overrides will simply call the expected methods
        // directly on the boxed T values. These methods will be directly
        // invoked by the JIT compiler when using a Box<T> reference. When
        // an object reference is used instead, the call would be forwarded
        // to those same methods anyway, since the method table for an object
        // representing a T instance is the one of type T anyway.
        return this.GetReference().ToString()!;
    }

    /// <inheritdoc/>
    public override bool Equals(object? obj)
    {
        return Equals(this, obj);
    }

    /// <inheritdoc/>
    public override int GetHashCode()
    {
        return this.GetReference().GetHashCode();
    }

    /// <summary>
    /// Throws an <see cref="InvalidCastException"/> when a cast from an invalid <see cref="object"/> is attempted.
    /// </summary>
    private static void ThrowInvalidCastExceptionForGetFrom()
    {
        throw new InvalidCastException($"Can't cast the input object to the type Box<{typeof(T)}>");
    }
}

/// <summary>
/// Helpers for working with the <see cref="Box{T}"/> type.
/// </summary>
public static class BoxExtensions
{
    /// <summary>
    /// Gets a <typeparamref name="T"/> reference from a <see cref="Box{T}"/> instance.
    /// </summary>
    /// <typeparam name="T">The type of reference to retrieve.</typeparam>
    /// <param name="box">The input <see cref="Box{T}"/> instance.</param>
    /// <returns>A <typeparamref name="T"/> reference to the boxed value within <paramref name="box"/>.</returns>
    [Pure]
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ref T GetReference<T>(this Box<T> box)
        where T : struct
    {
        // The reason why this method is an extension and is not part of
        // the Box<T> type itself is that Box<T> is really just a mask
        // used over object references, but it is never actually instantiated.
        // Because of this, the method table of the objects in the heap will
        // be the one of type T created by the runtime, and not the one of
        // the Box<T> type. To avoid potential issues when invoking this method
        // on different runtimes, which might handle that scenario differently,
        // we use an extension method, which is just syntactic sugar for a static
        // method belonging to another class. This isn't technically necessary,
        // but it's just an extra precaution since the syntax for users remains
        // exactly the same anyway. Here we just call the Unsafe.Unbox<T>(object)
        // API, which is hidden away for users of the type for simplicity.
        // Note that this API will always actually involve a conditional
        // branch, which is introduced by the JIT compiler to validate the
        // object instance being unboxed. But since the alternative of
        // manually tracking the offset to the boxed data would be both
        // more error prone, and it would still introduce some overhead,
        // this doesn't really matter in this case anyway.
        return ref Unsafe.Unbox<T>(box);
    }
}