// 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.ComponentModel;
using System.Diagnostics;
using System.Runtime.CompilerServices;

using System.Runtime.Versioning;

#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}")]
    public readonly ref partial struct ReadOnlySpan<T>
    {
        /// <summary>
        /// The number of items in the read-only span.
        /// </summary>
        public int Length
        {
            [NonVersionable]
            get
            {
                return _length;
            }
        }

        /// <summary>
        /// Returns true if Length is 0.
        /// </summary>
        public bool IsEmpty
        {
            [NonVersionable]
            get
            {
                // Workaround for https://github.com/dotnet/coreclr/issues/19620
                return 0 >= (uint)_length;
            }
        }
        /// <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==.
        /// <exception cref="System.NotSupportedException">
        /// Always thrown by this method.
        /// </exception>
        /// </summary>
        [Obsolete("Equals() on ReadOnlySpan 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 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];
            }
        }
    }
}