Index.cs

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using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
 
namespace System;
 
public readonly struct Index : IEquatable<Index>
{
    private readonly int _value;
 
    public static Index Start => new Index(0);
 
    public static Index End => new Index(-1);
 
    public int Value
    {
        get
        {
            if (_value < 0)
            {
                return ~_value;
            }
            return _value;
        }
    }
 
    public bool IsFromEnd => _value < 0;
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public Index(int value, bool fromEnd = false)
    {
        if (value < 0)
        {
            ThrowHelper.ThrowValueArgumentOutOfRange_NeedNonNegNumException();
        }
        if (fromEnd)
        {
            _value = ~value;
        }
        else
        {
            _value = value;
        }
    }
 
    private Index(int value)
    {
        _value = value;
    }
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static Index FromStart(int value)
    {
        if (value < 0)
        {
            ThrowHelper.ThrowValueArgumentOutOfRange_NeedNonNegNumException();
        }
        return new Index(value);
    }
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static Index FromEnd(int value)
    {
        if (value < 0)
        {
            ThrowHelper.ThrowValueArgumentOutOfRange_NeedNonNegNumException();
        }
        return new Index(~value);
    }
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int GetOffset(int length)
    {
        int num = _value;
        if (IsFromEnd)
        {
            num += length + 1;
        }
        return num;
    }
 
    public override bool Equals([NotNullWhen(true)] object? value)
    {
        if (value is Index)
        {
            return _value == ((Index)value)._value;
        }
        return false;
    }
 
    public bool Equals(Index other)
    {
        return _value == other._value;
    }
 
    public override int GetHashCode()
    {
        return _value;
    }
 
    public static implicit operator Index(int value)
    {
        return FromStart(value);
    }
 
    public override string ToString()
    {
        if (IsFromEnd)
        {
            return ToStringFromEnd();
        }
        return ((uint)Value).ToString();
    }
 
    private string ToStringFromEnd()
    {
        Span<char> span = stackalloc char[11];
        int charsWritten;
        bool flag = ((uint)Value).TryFormat(span.Slice(1), out charsWritten);
        span[0] = '^';
        return new string(span.Slice(0, charsWritten + 1));
    }
}