// 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.Diagnostics;
using System.Runtime;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using EditorBrowsableState = System.ComponentModel.EditorBrowsableState;
using EditorBrowsableAttribute = System.ComponentModel.EditorBrowsableAttribute;

#pragma warning disable 0809  //warning CS0809: Obsolete member 'Span<T>.Equals(object)' overrides non-obsolete member 'object.Equals(object)'

#if BIT64
using nuint = System.UInt64;
#else
using nuint = System.UInt32;
#endif

namespace System
{
    /// <summary>
    /// Span represents a contiguous region of arbitrary memory. Unlike arrays, it can point to either managed
    /// or native memory, or to memory allocated on the stack. It is type- and memory-safe.
    /// </summary>
    public struct Span<T>
    {
        /// <summary>A byref or a native ptr.</summary>
        private readonly ByReference<T> _pointer;
        /// <summary>The number of elements this Span contains.</summary>
        private readonly int _length;

        /// <summary>
        /// Creates a new span over the entirety of the target array.
        /// </summary>
        /// <param name="array">The target array.</param>
        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
        /// reference (Nothing in Visual Basic).</exception>
        /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span(T[] array)
        {
            if (array == null)
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
            if (default(T) == null && array.GetType() != typeof(T[]))
                ThrowHelper.ThrowArrayTypeMismatchException();

            _pointer = new ByReference<T>(ref JitHelpers.GetArrayData(array));
            _length = array.Length;
        }

        /// <summary>
        /// Creates a new span over the portion of the target array beginning
        /// at 'start' index and covering the remainder of the array.
        /// </summary>
        /// <param name="array">The target array.</param>
        /// <param name="start">The index at which to begin the span.</param>
        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
        /// reference (Nothing in Visual Basic).</exception>
        /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// Thrown when the specified <paramref name="start"/> is not in the range (&lt;0 or &gt;=Length).
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span(T[] array, int start)
        {
            if (array == null)
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
            if (default(T) == null && array.GetType() != typeof(T[]))
                ThrowHelper.ThrowArrayTypeMismatchException();
            if ((uint)start > (uint)array.Length)
                ThrowHelper.ThrowArgumentOutOfRangeException();

            _pointer = new ByReference<T>(ref Unsafe.Add(ref JitHelpers.GetArrayData(array), start));
            _length = array.Length - start;
        }

        /// <summary>
        /// Creates a new span over the portion of the target array beginning
        /// at 'start' index and ending at 'end' index (exclusive).
        /// </summary>
        /// <param name="array">The target array.</param>
        /// <param name="start">The index at which to begin the span.</param>
        /// <param name="length">The number of items in the span.</param>
        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
        /// reference (Nothing in Visual Basic).</exception>
        /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// Thrown when the specified <paramref name="start"/> or end index is not in the range (&lt;0 or &gt;=Length).
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span(T[] array, int start, int length)
        {
            if (array == null)
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
            if (default(T) == null && array.GetType() != typeof(T[]))
                ThrowHelper.ThrowArrayTypeMismatchException();
            if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
                ThrowHelper.ThrowArgumentOutOfRangeException();

            _pointer = new ByReference<T>(ref Unsafe.Add(ref JitHelpers.GetArrayData(array), start));
            _length = length;
        }

        /// <summary>
        /// Creates a new span over the target unmanaged buffer.  Clearly this
        /// is quite dangerous, because we are creating arbitrarily typed T's
        /// out of a void*-typed block of memory.  And the length is not checked.
        /// But if this creation is correct, then all subsequent uses are correct.
        /// </summary>
        /// <param name="pointer">An unmanaged pointer to memory.</param>
        /// <param name="length">The number of <typeparamref name="T"/> elements the memory contains.</param>
        /// <exception cref="System.ArgumentException">
        /// Thrown when <typeparamref name="T"/> is reference type or contains pointers and hence cannot be stored in unmanaged memory.
        /// </exception>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// Thrown when the specified <paramref name="length"/> is negative.
        /// </exception>
        [CLSCompliant(false)]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public unsafe Span(void* pointer, int length)
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(T));
            if (length < 0)
                ThrowHelper.ThrowArgumentOutOfRangeException();

            _pointer = new ByReference<T>(ref Unsafe.As<byte, T>(ref *(byte*)pointer));
            _length = length;
        }

        /// <summary>
        /// Create a new span over a portion of a regular managed object. This can be useful
        /// if part of a managed object represents a "fixed array." This is dangerous because neither the
        /// <paramref name="length"/> is checked, nor <paramref name="obj"/> being null, nor the fact that
        /// "rawPointer" actually lies within <paramref name="obj"/>.
        /// </summary>
        /// <param name="obj">The managed object that contains the data to span over.</param>
        /// <param name="objectData">A reference to data within that object.</param>
        /// <param name="length">The number of <typeparamref name="T"/> elements the memory contains.</param>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static Span<T> DangerousCreate(object obj, ref T objectData, int length) => new Span<T>(ref objectData, length);

        // Constructor for internal use only.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal Span(ref T ptr, int length)
        {
            Debug.Assert(length >= 0);

            _pointer = new ByReference<T>(ref ptr);
            _length = length;
        }

        /// <summary>
        /// Returns a reference to the 0th element of the Span. If the Span is empty, returns a reference to the location where the 0th element
        /// would have been stored. Such a reference can be used for pinning but must never be dereferenced.
        /// </summary>
        public ref T DangerousGetPinnableReference()
        {
            return ref _pointer.Value;
        }

        /// <summary>
        /// The number of items in the span.
        /// </summary>
        public int Length => _length;

        /// <summary>
        /// Returns true if Length is 0.
        /// </summary>
        public bool IsEmpty => _length == 0;

        /// <summary>
        /// Returns a reference to specified element of the Span.
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        /// <exception cref="System.IndexOutOfRangeException">
        /// Thrown when index less than 0 or index greater than or equal to Length
        /// </exception>
        public ref T this[int index]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get
            {
                if ((uint)index >= (uint)_length)
                    ThrowHelper.ThrowIndexOutOfRangeException();

                return ref Unsafe.Add(ref _pointer.Value, index);
            }
        }

        /// <summary>
        /// Clears the contents of this span.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Clear()
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
            {
                SpanHelper.ClearWithReferences(ref Unsafe.As<T, IntPtr>(ref _pointer.Value), (nuint)_length * (nuint)(Unsafe.SizeOf<T>() / sizeof(nuint)));
            }
            else
            {
                SpanHelper.ClearWithoutReferences(ref Unsafe.As<T, byte>(ref _pointer.Value), (nuint)_length * (nuint)Unsafe.SizeOf<T>());
            }
        }

        /// <summary>
        /// Fills the contents of this span with the given value.
        /// </summary>
        public void Fill(T value)
        {
            if (Unsafe.SizeOf<T>() == 1)
            {
                uint length = (uint)_length;
                if (length == 0)
                    return;

                T tmp = value; // Avoid taking address of the "value" argument. It would regress performance of the loop below.
                Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _pointer.Value), Unsafe.As<T, byte>(ref tmp), length);
            }
            else
            {
                // Do all math as nuint to avoid unnecessary 64->32->64 bit integer truncations
                nuint length = (uint)_length;
                if (length == 0)
                    return;

                ref T r = ref DangerousGetPinnableReference();

                // TODO: Create block fill for value types of power of two sizes e.g. 2,4,8,16

                nuint elementSize = (uint)Unsafe.SizeOf<T>();
                nuint i = 0;
                for (; i < (length & ~(nuint)7); i += 8)
                {
                    Unsafe.AddByteOffset<T>(ref r, (i + 0) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 1) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 2) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 3) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 4) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 5) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 6) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 7) * elementSize) = value;
                }
                if (i < (length & ~(nuint)3))
                {
                    Unsafe.AddByteOffset<T>(ref r, (i + 0) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 1) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 2) * elementSize) = value;
                    Unsafe.AddByteOffset<T>(ref r, (i + 3) * elementSize) = value;
                    i += 4;
                }
                for (; i < length; i++)
                {
                    Unsafe.AddByteOffset<T>(ref r, i * elementSize) = value;
                }
            }
        }

        /// <summary>
        /// Copies the contents of this span into destination span. If the source
        /// and destinations overlap, this method behaves as if the original values in
        /// a temporary location before the destination is overwritten.
        /// </summary>
        /// <param name="destination">The span to copy items into.</param>
        /// <exception cref="System.ArgumentException">
        /// Thrown when the destination Span is shorter than the source Span.
        /// </exception>
        public void CopyTo(Span<T> destination)
        {
            if (!TryCopyTo(destination))
                ThrowHelper.ThrowArgumentException_DestinationTooShort();
        }

        /// <summary>
        /// Copies the contents of this span into destination span. If the source
        /// and destinations overlap, this method behaves as if the original values in
        /// a temporary location before the destination is overwritten.
        /// </summary>
        /// <param name="destination">The span to copy items into.</param>
        /// <returns>If the destination span is shorter than the source span, this method
        /// return false and no data is written to the destination.</returns>        
        public bool TryCopyTo(Span<T> destination)
        {
            if ((uint)_length > (uint)destination.Length)
                return false;

            SpanHelper.CopyTo<T>(ref destination._pointer.Value, ref _pointer.Value, _length);
            return true;
        }

        /// <summary>
        /// Returns true if left and right point at the same memory and have the same length.  Note that
        /// this does *not* check to see if the *contents* are equal.
        /// </summary>
        public static bool operator ==(Span<T> left, Span<T> right)
        {
            return left._length == right._length && Unsafe.AreSame<T>(ref left._pointer.Value, ref right._pointer.Value);
        }

        /// <summary>
        /// Returns false if left and right point at the same memory and have the same length.  Note that
        /// this does *not* check to see if the *contents* are equal.
        /// </summary>
        public static bool operator !=(Span<T> left, Span<T> right) => !(left == right);

        /// <summary>
        /// This method is not supported as spans cannot be boxed. To compare two spans, use operator==.
        /// <exception cref="System.NotSupportedException">
        /// Always thrown by this method.
        /// </exception>
        /// </summary>
        [Obsolete("Equals() on Span will always throw an exception. Use == instead.")]
        [EditorBrowsable(EditorBrowsableState.Never)]
        public override bool Equals(object obj)
        {
            throw new NotSupportedException(SR.NotSupported_CannotCallEqualsOnSpan);
        }

        /// <summary>
        /// This method is not supported as spans cannot be boxed.
        /// <exception cref="System.NotSupportedException">
        /// Always thrown by this method.
        /// </exception>
        /// </summary>
        [Obsolete("GetHashCode() on Span will always throw an exception.")]
        [EditorBrowsable(EditorBrowsableState.Never)]
        public override int GetHashCode()
        {
            throw new NotSupportedException(SR.NotSupported_CannotCallGetHashCodeOnSpan);
        }

        /// <summary>
        /// Defines an implicit conversion of an array to a <see cref="Span{T}"/>
        /// </summary>
        public static implicit operator Span<T>(T[] array) => new Span<T>(array);

        /// <summary>
        /// Defines an implicit conversion of a <see cref="ArraySegment{T}"/> to a <see cref="Span{T}"/>
        /// </summary>
        public static implicit operator Span<T>(ArraySegment<T> arraySegment) => new Span<T>(arraySegment.Array, arraySegment.Offset, arraySegment.Count);

        /// <summary>
        /// Defines an implicit conversion of a <see cref="Span{T}"/> to a <see cref="ReadOnlySpan{T}"/>
        /// </summary>
        public static implicit operator ReadOnlySpan<T>(Span<T> span) => new ReadOnlySpan<T>(ref span._pointer.Value, span._length);

        /// <summary>
        /// Forms a slice out of the given span, beginning at 'start'.
        /// </summary>
        /// <param name="start">The index at which to begin this slice.</param>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;=Length).
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span<T> Slice(int start)
        {
            if ((uint)start > (uint)_length)
                ThrowHelper.ThrowArgumentOutOfRangeException();

            return new Span<T>(ref Unsafe.Add(ref _pointer.Value, start), _length - start);
        }

        /// <summary>
        /// Forms a slice out of the given span, beginning at 'start', of given length
        /// </summary>
        /// <param name="start">The index at which to begin this slice.</param>
        /// <param name="length">The desired length for the slice (exclusive).</param>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// Thrown when the specified <paramref name="start"/> or end index is not in range (&lt;0 or &gt;=Length).
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span<T> Slice(int start, int length)
        {
            if ((uint)start > (uint)_length || (uint)length > (uint)(_length - start))
                ThrowHelper.ThrowArgumentOutOfRangeException();

            return new Span<T>(ref Unsafe.Add(ref _pointer.Value, start), length);
        }

        /// <summary>
        /// Copies the contents of this span into a new array.  This heap
        /// allocates, so should generally be avoided, however it is sometimes
        /// necessary to bridge the gap with APIs written in terms of arrays.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T[] ToArray()
        {
            if (_length == 0)
                return Array.Empty<T>();

            var destination = new T[_length];
            SpanHelper.CopyTo<T>(ref JitHelpers.GetArrayData(destination), ref _pointer.Value, _length);
            return destination;
        }

        // <summary>
        /// Returns an empty <see cref="Span{T}"/>
        /// </summary>
        public static Span<T> Empty => default(Span<T>);
    }

    public static class Span
    {
        /// <summary>
        /// Casts a Span of one primitive type <typeparamref name="T"/> to Span of bytes.
        /// That type may not contain pointers or references. This is checked at runtime in order to preserve type safety.
        /// </summary>
        /// <param name="source">The source slice, of type <typeparamref name="T"/>.</param>
        /// <exception cref="System.ArgumentException">
        /// Thrown when <typeparamref name="T"/> contains pointers.
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static Span<byte> AsBytes<T>(this Span<T> source)
            where T : struct
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(T));

            return new Span<byte>(
                ref Unsafe.As<T, byte>(ref source.DangerousGetPinnableReference()),
                checked(source.Length * Unsafe.SizeOf<T>()));
        }

        /// <summary>
        /// Casts a ReadOnlySpan of one primitive type <typeparamref name="T"/> to ReadOnlySpan of bytes.
        /// That type may not contain pointers or references. This is checked at runtime in order to preserve type safety.
        /// </summary>
        /// <param name="source">The source slice, of type <typeparamref name="T"/>.</param>
        /// <exception cref="System.ArgumentException">
        /// Thrown when <typeparamref name="T"/> contains pointers.
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ReadOnlySpan<byte> AsBytes<T>(this ReadOnlySpan<T> source)
            where T : struct
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(T));

            return new ReadOnlySpan<byte>(
                ref Unsafe.As<T, byte>(ref source.DangerousGetPinnableReference()),
                checked(source.Length * Unsafe.SizeOf<T>()));
        }

        /// <summary>
        /// Casts a Span of one primitive type <typeparamref name="TFrom"/> to another primitive type <typeparamref name="TTo"/>.
        /// These types may not contain pointers or references. This is checked at runtime in order to preserve type safety.
        /// </summary>
        /// <remarks>
        /// Supported only for platforms that support misaligned memory access.
        /// </remarks>
        /// <param name="source">The source slice, of type <typeparamref name="TFrom"/>.</param>
        /// <exception cref="System.ArgumentException">
        /// Thrown when <typeparamref name="TFrom"/> or <typeparamref name="TTo"/> contains pointers.
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static Span<TTo> NonPortableCast<TFrom, TTo>(this Span<TFrom> source)
            where TFrom : struct
            where TTo : struct
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<TFrom>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(TFrom));
            if (RuntimeHelpers.IsReferenceOrContainsReferences<TTo>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(TTo));

            return new Span<TTo>(
                ref Unsafe.As<TFrom, TTo>(ref source.DangerousGetPinnableReference()),
                checked((int)((long)source.Length * Unsafe.SizeOf<TFrom>() / Unsafe.SizeOf<TTo>())));
        }

        /// <summary>
        /// Casts a ReadOnlySpan of one primitive type <typeparamref name="TFrom"/> to another primitive type <typeparamref name="TTo"/>.
        /// These types may not contain pointers or references. This is checked at runtime in order to preserve type safety.
        /// </summary>
        /// <remarks>
        /// Supported only for platforms that support misaligned memory access.
        /// </remarks>
        /// <param name="source">The source slice, of type <typeparamref name="TFrom"/>.</param>
        /// <exception cref="System.ArgumentException">
        /// Thrown when <typeparamref name="TFrom"/> or <typeparamref name="TTo"/> contains pointers.
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ReadOnlySpan<TTo> NonPortableCast<TFrom, TTo>(this ReadOnlySpan<TFrom> source)
            where TFrom : struct
            where TTo : struct
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<TFrom>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(TFrom));
            if (RuntimeHelpers.IsReferenceOrContainsReferences<TTo>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(TTo));

            return new ReadOnlySpan<TTo>(
                ref Unsafe.As<TFrom, TTo>(ref source.DangerousGetPinnableReference()),
                checked((int)((long)source.Length * Unsafe.SizeOf<TFrom>() / Unsafe.SizeOf<TTo>())));
        }

        /// <summary>
        /// Creates a new readonly span over the portion of the target string.
        /// </summary>
        /// <param name="text">The target string.</param>
        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null
        /// reference (Nothing in Visual Basic).</exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ReadOnlySpan<char> AsSpan(this string text)
        {
            if (text == null)
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);

            return new ReadOnlySpan<char>(ref text.GetFirstCharRef(), text.Length);
        }
    }

    internal static class SpanHelper
    {
        internal static unsafe void CopyTo<T>(ref T destination, ref T source, int elementsCount)
        {
            if (Unsafe.AreSame(ref destination, ref source))
                return;

            if (elementsCount <= 1)
            {
                if (elementsCount == 1)
                {
                    destination = source;
                }
                return;
            }

            nuint byteCount = (nuint)elementsCount * (nuint)Unsafe.SizeOf<T>();
            if (!RuntimeHelpers.IsReferenceOrContainsReferences<T>())
            {
                fixed (byte* pDestination = &Unsafe.As<T, byte>(ref destination))
                {
                    fixed (byte* pSource = &Unsafe.As<T, byte>(ref source))
                    {
                        Buffer.Memmove(pDestination, pSource, byteCount);
                    }
                }
            }
            else
            {
                RuntimeImports.RhBulkMoveWithWriteBarrier(
                    ref Unsafe.As<T, byte>(ref destination),
                    ref Unsafe.As<T, byte>(ref source),
                    byteCount);
            }
        }

        internal static unsafe void ClearWithoutReferences(ref byte b, nuint byteLength)
        {
            if (byteLength == 0)
                return;
            
#if AMD64
            if (byteLength > 4096) goto PInvoke;
            Unsafe.InitBlockUnaligned(ref b, 0, (uint)byteLength);
            return;
#else // AMD64
            // TODO: Optimize this method on X86 machine
            // Note: It's important that this switch handles lengths at least up to 22.
            // See notes below near the main loop for why.

            // The switch will be very fast since it can be implemented using a jump
            // table in assembly. See http://stackoverflow.com/a/449297/4077294 for more info.

            switch (byteLength)
            {
                case 1:
                    b = 0;
                    return;
                case 2:
                    Unsafe.As<byte, short>(ref b) = 0;
                    return;
                case 3:
                    Unsafe.As<byte, short>(ref b) = 0;
                    Unsafe.Add<byte>(ref b, 2) = 0;
                    return;
                case 4:
                    Unsafe.As<byte, int>(ref b) = 0;
                    return;
                case 5:
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.Add<byte>(ref b, 4) = 0;
                    return;
                case 6:
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    return;
                case 7:
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.Add<byte>(ref b, 6) = 0;
                    return;
                case 8:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    return;
                case 9:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.Add<byte>(ref b, 8) = 0;
                    return;
                case 10:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    return;
                case 11:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.Add<byte>(ref b, 10) = 0;
                    return;
                case 12:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    return;
                case 13:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.Add<byte>(ref b, 12) = 0;
                    return;
                case 14:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
                    return;
                case 15:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
                    Unsafe.Add<byte>(ref b, 14) = 0;
                    return;
                case 16:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    return;
                case 17:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    Unsafe.Add<byte>(ref b, 16) = 0;
                    return;
                case 18:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 16)) = 0;
                    return;
                case 19:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 16)) = 0;
                    Unsafe.Add<byte>(ref b, 18) = 0;
                    return;
                case 20:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 16)) = 0;
                    return;
                case 21:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 16)) = 0;
                    Unsafe.Add<byte>(ref b, 20) = 0;
                    return;
                case 22:
#if BIT64
                    Unsafe.As<byte, long>(ref b) = 0;
                    Unsafe.As<byte, long>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
#else
                    Unsafe.As<byte, int>(ref b) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 4)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 8)) = 0;
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 12)) = 0;
#endif
                    Unsafe.As<byte, int>(ref Unsafe.Add<byte>(ref b, 16)) = 0;
                    Unsafe.As<byte, short>(ref Unsafe.Add<byte>(ref b, 20)) = 0;
                    return;
            }

            // P/Invoke into the native version for large lengths
            if (byteLength >= 512) goto PInvoke;

            nuint i = 0; // byte offset at which we're copying

            if ((Unsafe.As<byte, int>(ref b) & 3) != 0)
            {
                if ((Unsafe.As<byte, int>(ref b) & 1) != 0)
                {
                    Unsafe.AddByteOffset<byte>(ref b, i) = 0;
                    i += 1;
                    if ((Unsafe.As<byte, int>(ref b) & 2) != 0)
                        goto IntAligned;
                }
                Unsafe.As<byte, short>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                i += 2;
            }

            IntAligned:

            // On 64-bit IntPtr.Size == 8, so we want to advance to the next 8-aligned address. If
            // (int)b % 8 is 0, 5, 6, or 7, we will already have advanced by 0, 3, 2, or 1
            // bytes to the next aligned address (respectively), so do nothing. On the other hand,
            // if it is 1, 2, 3, or 4 we will want to copy-and-advance another 4 bytes until
            // we're aligned.
            // The thing 1, 2, 3, and 4 have in common that the others don't is that if you
            // subtract one from them, their 3rd lsb will not be set. Hence, the below check.

            if (((Unsafe.As<byte, int>(ref b) - 1) & 4) == 0)
            {
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                i += 4;
            }

            nuint end = byteLength - 16;
            byteLength -= i; // lower 4 bits of byteLength represent how many bytes are left *after* the unrolled loop

            // We know due to the above switch-case that this loop will always run 1 iteration; max
            // bytes we clear before checking is 23 (7 to align the pointers, 16 for 1 iteration) so
            // the switch handles lengths 0-22.
            Debug.Assert(end >= 7 && i <= end);

            // This is separated out into a different variable, so the i + 16 addition can be
            // performed at the start of the pipeline and the loop condition does not have
            // a dependency on the writes.
            nuint counter;

            do
            {
                counter = i + 16;

                // This loop looks very costly since there appear to be a bunch of temporary values
                // being created with the adds, but the jit (for x86 anyways) will convert each of
                // these to use memory addressing operands.

                // So the only cost is a bit of code size, which is made up for by the fact that
                // we save on writes to b.

#if BIT64
                Unsafe.As<byte, long>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                Unsafe.As<byte, long>(ref Unsafe.AddByteOffset<byte>(ref b, i + 8)) = 0;
#else
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i + 4)) = 0;
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i + 8)) = 0;
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i + 12)) = 0;
#endif

                i = counter;

                // See notes above for why this wasn't used instead
                // i += 16;
            }
            while (counter <= end);

            if ((byteLength & 8) != 0)
            {
#if BIT64
                Unsafe.As<byte, long>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
#else
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i + 4)) = 0;
#endif
                i += 8;
            }
            if ((byteLength & 4) != 0)
            {
                Unsafe.As<byte, int>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                i += 4;
            }
            if ((byteLength & 2) != 0)
            {
                Unsafe.As<byte, short>(ref Unsafe.AddByteOffset<byte>(ref b, i)) = 0;
                i += 2;
            }
            if ((byteLength & 1) != 0)
            {
                Unsafe.AddByteOffset<byte>(ref b, i) = 0;
                // We're not using i after this, so not needed
                // i += 1;
            }

            return;
#endif // AMD64
            
            PInvoke:
            RuntimeImports.RhZeroMemory(ref b, byteLength);
        }

        internal static unsafe void ClearWithReferences(ref IntPtr ip, nuint pointerSizeLength)
        {
            if (pointerSizeLength == 0)
                return;

            // TODO: Perhaps do switch casing to improve small size perf

            nuint i = 0;
            nuint n = 0;
            while ((n = i + 8) <= (pointerSizeLength))
            {
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 0) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 1) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 2) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 3) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 4) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 5) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 6) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 7) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                i = n;
            }
            if ((n = i + 4) <= (pointerSizeLength))
            {
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 0) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 1) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 2) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 3) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                i = n;
            }
            if ((n = i + 2) <= (pointerSizeLength))
            {
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 0) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 1) * (nuint)sizeof(IntPtr)) = default(IntPtr);
                i = n;
            }
            if ((i + 1) <= (pointerSizeLength))
            {
                Unsafe.AddByteOffset<IntPtr>(ref ip, (i + 0) * (nuint)sizeof(IntPtr)) = default(IntPtr);
            }
        }
    }
}