// 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.
 
#if NET
 
#pragma warning disable RS0016 // Add public types and members to the declared API (this is a supporting forwarder for an internal polyfill API)
[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Index))]
#pragma warning restore RS0016 // Add public types and members to the declared API
 
#else

using System.Runtime.CompilerServices;
 
namespace System
{
    /// <summary>Represent a type can be used to index a collection either from the start or the end.</summary>
    /// <remarks>
    /// Index is used by the C# compiler to support the new index syntax
    /// <code>
    /// int[] someArray = new int[5] { 1, 2, 3, 4, 5 } ;
    /// int lastElement = someArray[^1]; // lastElement = 5
    /// </code>
    /// </remarks>
    internal readonly struct Index : IEquatable<Index>
    {
        private readonly int _value;
 
        /// <summary>Construct an Index using a value and indicating if the index is from the start or from the end.</summary>
        /// <param name="value">The index value. it has to be zero or positive number.</param>
        /// <param name="fromEnd">Indicating if the index is from the start or from the end.</param>
        /// <remarks>
        /// If the Index constructed from the end, index value 1 means pointing at the last element and index value 0 means pointing at beyond last element.
        /// </remarks>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Index(int value, bool fromEnd = false)
        {
            if (value < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(value), value, "Non-negative number required.");
            }
 
            if (fromEnd)
                _value = ~value;
            else
                _value = value;
        }
 
        // The following private constructors mainly created for perf reason to avoid the checks
        private Index(int value)
        {
            _value = value;
        }
 
        /// <summary>Create an Index pointing at first element.</summary>
        public static Index Start => new Index(0);
 
        /// <summary>Create an Index pointing at beyond last element.</summary>
        public static Index End => new Index(~0);
 
        /// <summary>Create an Index from the start at the position indicated by the value.</summary>
        /// <param name="value">The index value from the start.</param>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static Index FromStart(int value)
        {
            if (value < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(value), value, "Non-negative number required.");
            }
 
            return new Index(value);
        }
 
        /// <summary>Create an Index from the end at the position indicated by the value.</summary>
        /// <param name="value">The index value from the end.</param>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static Index FromEnd(int value)
        {
            if (value < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(value), value, "Non-negative number required.");
            }
 
            return new Index(~value);
        }
 
        /// <summary>Returns the index value.</summary>
        public int Value
        {
            get
            {
                if (_value < 0)
                    return ~_value;
                else
                    return _value;
            }
        }
 
        /// <summary>Indicates whether the index is from the start or the end.</summary>
        public bool IsFromEnd => _value < 0;
 
        /// <summary>Calculate the offset from the start using the giving collection length.</summary>
        /// <param name="length">The length of the collection that the Index will be used with. length has to be a positive value</param>
        /// <remarks>
        /// For performance reason, we don't validate the input length parameter and the returned offset value against negative values.
        /// we don't validate either the returned offset is greater than the input length.
        /// It is expected Index will be used with collections which always have non negative length/count. If the returned offset is negative and
        /// then used to index a collection will get out of range exception which will be same affect as the validation.
        /// </remarks>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public int GetOffset(int length)
        {
            int offset = _value;
            if (IsFromEnd)
            {
                // offset = length - (~value)
                // offset = length + (~(~value) + 1)
                // offset = length + value + 1
 
                offset += length + 1;
            }
            return offset;
        }
 
        /// <summary>Indicates whether the current Index object is equal to another object of the same type.</summary>
        /// <param name="value">An object to compare with this object</param>
        public override bool Equals(object? value) => value is Index && _value == ((Index)value)._value;
 
        /// <summary>Indicates whether the current Index object is equal to another Index object.</summary>
        /// <param name="other">An object to compare with this object</param>
        public bool Equals(Index other) => _value == other._value;
 
        /// <summary>Returns the hash code for this instance.</summary>
        public override int GetHashCode() => _value;
 
        /// <summary>Converts integer number to an Index.</summary>
        public static implicit operator Index(int value) => FromStart(value);
 
        /// <summary>Converts the value of the current Index object to its equivalent string representation.</summary>
        public override string ToString()
        {
            if (IsFromEnd)
                return $"^{((uint)Value).ToString()}";
 
            return ((uint)Value).ToString();
        }
    }
}
#endif