// 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; 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> /// 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> #if SYSTEM_PRIVATE_CORELIB public #else internal #endif 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) { 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) { 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> /// 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([NotNullWhen(true)] 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 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.Slice(1), out int charsWritten); Debug.Assert(formatted); span[0] = '^'; return new string(span.Slice(0, charsWritten + 1)); #else return '^' + Value.ToString(); #endif } } }