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