ReadOnlySpan.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;
using System.Runtime.Versioning;

using EditorBrowsableAttribute = System.ComponentModel.EditorBrowsableAttribute;
using EditorBrowsableState = System.ComponentModel.EditorBrowsableState;

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

namespace System
{
    /// <summary>
    /// ReadOnlySpan 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>
    [DebuggerTypeProxy(typeof(SpanDebugView<>))]
    [DebuggerDisplay("{ToString(),raw}")]
    [NonVersionable]
#pragma warning disable SYSLIB1056 // Specified native type is invalid
    [NativeMarshalling(typeof(ReadOnlySpanMarshaller<,>))]
#pragma warning restore SYSLIB1056 // Specified native type is invalid
    [Intrinsic]
    public readonly ref struct ReadOnlySpan<T>
    {
        /// <summary>A byref or a native ptr.</summary>
        internal readonly ref T _reference;
        /// <summary>The number of elements this ReadOnlySpan contains.</summary>
        private readonly int _length;

        /// <summary>
        /// Creates a new read-only span over the entirety of the target array.
        /// </summary>
        /// <param name="array">The target array.</param>
        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ReadOnlySpan(T[]? array)
        {
            if (array == null)
            {
                this = default;
                return; // returns default
            }

            _reference = ref MemoryMarshal.GetArrayDataReference(array);
            _length = array.Length;
        }

        /// <summary>
        /// Creates a new read-only 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 read-only span.</param>
        /// <param name="length">The number of items in the read-only span.</param>
        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
        /// <exception cref="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 ReadOnlySpan(T[]? array, int start, int length)
        {
            if (array == null)
            {
                if (start != 0 || length != 0)
                    ThrowHelper.ThrowArgumentOutOfRangeException();
                this = default;
                return; // returns default
            }
#if TARGET_64BIT
            // See comment in Span<T>.Slice for how this works.
            if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)array.Length)
                ThrowHelper.ThrowArgumentOutOfRangeException();
#else
            if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
                ThrowHelper.ThrowArgumentOutOfRangeException();
#endif

            _reference = ref Unsafe.Add(ref MemoryMarshal.GetArrayDataReference(array), (nint)(uint)start /* force zero-extension */);
            _length = length;
        }

        /// <summary>
        /// Creates a new read-only 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="ArgumentException">
        /// Thrown when <typeparamref name="T"/> is reference type or contains pointers and hence cannot be stored in unmanaged memory.
        /// </exception>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown when the specified <paramref name="length"/> is negative.
        /// </exception>
        [CLSCompliant(false)]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public unsafe ReadOnlySpan(void* pointer, int length)
        {
            if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
                ThrowHelper.ThrowInvalidTypeWithPointersNotSupported(typeof(T));
            if (length < 0)
                ThrowHelper.ThrowArgumentOutOfRangeException();

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

        /// <summary>Creates a new <see cref="ReadOnlySpan{T}"/> of length 1 around the specified reference.</summary>
        /// <param name="reference">A reference to data.</param>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ReadOnlySpan(ref readonly T reference)
        {
            _reference = ref Unsafe.AsRef(in reference);
            _length = 1;
        }

#pragma warning disable IDE0060 // https://github.com/dotnet/roslyn-analyzers/issues/6228
        // Constructor for internal use only. It is not safe to expose publicly, and is instead exposed via the unsafe MemoryMarshal.CreateReadOnlySpan.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal ReadOnlySpan(ref T reference, int length)
        {
            Debug.Assert(length >= 0);

            _reference = ref reference;
            _length = length;
        }
#pragma warning restore IDE0060 // https://github.com/dotnet/roslyn-analyzers/issues/6228

        /// <summary>
        /// Returns the specified element of the read-only span.
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        /// <exception cref="IndexOutOfRangeException">
        /// Thrown when index less than 0 or index greater than or equal to Length
        /// </exception>
        public ref readonly T this[int index]
        {
            [Intrinsic]
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            [NonVersionable]
            get
            {
                if ((uint)index >= (uint)_length)
                    ThrowHelper.ThrowIndexOutOfRangeException();
                return ref Unsafe.Add(ref _reference, (nint)(uint)index /* force zero-extension */);
            }
        }

        /// <summary>
        /// The number of items in the read-only span.
        /// </summary>
        public int Length
        {
            [Intrinsic]
            [NonVersionable]
            get => _length;
        }

        /// <summary>
        /// Gets a value indicating whether this <see cref="ReadOnlySpan{T}"/> is empty.
        /// </summary>
        /// <value><see langword="true"/> if this span is empty; otherwise, <see langword="false"/>.</value>
        public bool IsEmpty
        {
            [NonVersionable]
            get => _length == 0;
        }

        /// <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 !=(ReadOnlySpan<T> left, ReadOnlySpan<T> right) => !(left == right);

        /// <summary>
        /// This method is not supported as spans cannot be boxed. To compare two spans, use operator==.
        /// </summary>
        /// <exception cref="NotSupportedException">
        /// Always thrown by this method.
        /// </exception>
        [Obsolete("Equals() on ReadOnlySpan will always throw an exception. Use the equality operator 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.
        /// </summary>
        /// <exception cref="NotSupportedException">
        /// Always thrown by this method.
        /// </exception>
        [Obsolete("GetHashCode() on ReadOnlySpan 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="ReadOnlySpan{T}"/>
        /// </summary>
        public static implicit operator ReadOnlySpan<T>(T[]? array) => new ReadOnlySpan<T>(array);

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

        /// <summary>
        /// Returns a 0-length read-only span whose base is the null pointer.
        /// </summary>
        public static ReadOnlySpan<T> Empty => default;

        /// <summary>Gets an enumerator for this span.</summary>
        public Enumerator GetEnumerator() => new Enumerator(this);

        /// <summary>Enumerates the elements of a <see cref="ReadOnlySpan{T}"/>.</summary>
        public ref struct Enumerator
        {
            /// <summary>The span being enumerated.</summary>
            private readonly ReadOnlySpan<T> _span;
            /// <summary>The next index to yield.</summary>
            private int _index;

            /// <summary>Initialize the enumerator.</summary>
            /// <param name="span">The span to enumerate.</param>
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            internal Enumerator(ReadOnlySpan<T> span)
            {
                _span = span;
                _index = -1;
            }

            /// <summary>Advances the enumerator to the next element of the span.</summary>
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            public bool MoveNext()
            {
                int index = _index + 1;
                if (index < _span.Length)
                {
                    _index = index;
                    return true;
                }

                return false;
            }

            /// <summary>Gets the element at the current position of the enumerator.</summary>
            public ref readonly T Current
            {
                [MethodImpl(MethodImplOptions.AggressiveInlining)]
                get => ref _span[_index];
            }
        }

        /// <summary>
        /// Returns a reference to the 0th element of the Span. If the Span is empty, returns null reference.
        /// It can be used for pinning and is required to support the use of span within a fixed statement.
        /// </summary>
        [EditorBrowsable(EditorBrowsableState.Never)]
        public ref readonly T GetPinnableReference()
        {
            // Ensure that the native code has just one forward branch that is predicted-not-taken.
            ref T ret = ref Unsafe.NullRef<T>();
            if (_length != 0) ret = ref _reference;
            return ref ret;
        }

        /// <summary>
        /// Copies the contents of this read-only 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="ArgumentException">
        /// Thrown when the destination Span is shorter than the source Span.
        /// </exception>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void CopyTo(Span<T> destination)
        {
            // Using "if (!TryCopyTo(...))" results in two branches: one for the length
            // check, and one for the result of TryCopyTo. Since these checks are equivalent,
            // we can optimize by performing the check once ourselves then calling Memmove directly.

            if ((uint)_length <= (uint)destination.Length)
            {
                Buffer.Memmove(ref destination._reference, ref _reference, (uint)_length);
            }
            else
            {
                ThrowHelper.ThrowArgumentException_DestinationTooShort();
            }
        }

        /// <summary>
        /// Copies the contents of this read-only 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>
        /// <returns>If the destination span is shorter than the source span, this method
        /// return false and no data is written to the destination.</returns>
        /// <param name="destination">The span to copy items into.</param>
        public bool TryCopyTo(Span<T> destination)
        {
            bool retVal = false;
            if ((uint)_length <= (uint)destination.Length)
            {
                Buffer.Memmove(ref destination._reference, ref _reference, (uint)_length);
                retVal = true;
            }
            return retVal;
        }

        /// <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 ==(ReadOnlySpan<T> left, ReadOnlySpan<T> right) =>
            left._length == right._length &&
            Unsafe.AreSame(ref left._reference, ref right._reference);

        /// <summary>
        /// For <see cref="ReadOnlySpan{Char}"/>, returns a new instance of string that represents the characters pointed to by the span.
        /// Otherwise, returns a <see cref="string"/> with the name of the type and the number of elements.
        /// </summary>
        public override string ToString()
        {
            if (typeof(T) == typeof(char))
            {
                return new string(new ReadOnlySpan<char>(ref Unsafe.As<T, char>(ref _reference), _length));
            }
            return $"System.ReadOnlySpan<{typeof(T).Name}>[{_length}]";
        }

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

            return new ReadOnlySpan<T>(ref Unsafe.Add(ref _reference, (nint)(uint)start /* force zero-extension */), _length - start);
        }

        /// <summary>
        /// Forms a slice out of the given read-only 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="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 ReadOnlySpan<T> Slice(int start, int length)
        {
#if TARGET_64BIT
            // See comment in Span<T>.Slice for how this works.
            if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)_length)
                ThrowHelper.ThrowArgumentOutOfRangeException();
#else
            if ((uint)start > (uint)_length || (uint)length > (uint)(_length - start))
                ThrowHelper.ThrowArgumentOutOfRangeException();
#endif

            return new ReadOnlySpan<T>(ref Unsafe.Add(ref _reference, (nint)(uint)start /* force zero-extension */), length);
        }

        /// <summary>
        /// Copies the contents of this read-only 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>
        public T[] ToArray()
        {
            if (_length == 0)
                return Array.Empty<T>();

            var destination = new T[_length];
            Buffer.Memmove(ref MemoryMarshal.GetArrayDataReference(destination), ref _reference, (uint)_length);
            return destination;
        }
    }
}