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

#nullable enable

using System.Diagnostics.CodeAnalysis;
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>
///  <para>Index is used by the C# compiler to support the new index syntax</para>
///  <code>
///   int[] someArray = new int[5] { 1, 2, 3, 4, 5 } ;
///   int lastElement = someArray[^1]; // lastElement = 5
///  </code>
/// </remarks>
public 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>
    ///  <para>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.</para>
    /// </remarks>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public Index(int value, bool fromEnd = false)
    {
        if (value < 0)
        {
            ThrowValueArgumentOutOfRange_NeedNonNegNumException();
        }

        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(0);

    /// <summary>Create an Index pointing at beyond last element.</summary>
    public static Index End => new(~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)
        {
            ThrowValueArgumentOutOfRange_NeedNonNegNumException();
        }

        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)
        {
            ThrowValueArgumentOutOfRange_NeedNonNegNumException();
        }

        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>
    ///  <para>
    ///   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 the same effect as the validation.
    ///  </para>
    /// </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>
#pragma warning disable CA1725 // Parameter names should match base declaration
    public override bool Equals([NotNullWhen(true)] object? value) => value is Index index && _value == index._value;
#pragma warning restore CA1725

    /// <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()
    {
        return IsFromEnd ? ToStringFromEnd() : ((uint)Value).ToString();
    }

    private static void ThrowValueArgumentOutOfRange_NeedNonNegNumException()
    {
#if SYSTEM_PRIVATE_CORELIB
        throw new ArgumentOutOfRangeException("value", SR.ArgumentOutOfRange_NeedNonNegNum);
#else
        throw new ArgumentOutOfRangeException("value", "value must be non-negative");
#endif
    }

    private string ToStringFromEnd()
    {
#if (!NETSTANDARD2_0 && !NETFRAMEWORK)
        Span<char> span = stackalloc char[11]; // 1 for ^ and 10 for longest possible uint value
        bool formatted = ((uint)Value).TryFormat(span[1..], out int charsWritten);
        Debug.Assert(formatted);
        span[0] = '^';
        return new string(span[..(charsWritten + 1)]);
#else
        return '^' + Value.ToString();
#endif
    }
}
