Index and Range updates

src/System.Private.CoreLib/shared/System/Index.cs

@@ -3,34 +3,138 @@
 // 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;
 
-        public Index(int value, bool fromEnd)
+        /// <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();
             }
 
-            _value = fromEnd ? ~value : value;
+            if (fromEnd)
+                _value = ~value;
+            else
+                _value = value;
         }
 
-        public int Value => _value < 0 ? ~_value : _value;
-        public bool FromEnd => _value < 0;
+        // 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;
+
+            if (IsFromEnd)
+                offset = length - (~_value);
+            else
+                offset = _value;
+
+            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;
 
-        public override int GetHashCode()
+        /// <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 _value;
-        }
+            if (IsFromEnd)
+                return ToStringFromEnd();
 
-        public override string ToString() => FromEnd ? ToStringFromEnd() : ((uint)Value).ToString();
+            return ((uint)Value).ToString();
+        }
 
         private string ToStringFromEnd()
         {
@@ -41,7 +145,5 @@ private string ToStringFromEnd()
             return new string(span.Slice(0, charsWritten + 1));
         }
 
-        public static implicit operator Index(int value)
-            => new Index(value, fromEnd: false);
     }
 }

src/System.Private.CoreLib/shared/System/Memory.cs

@@ -30,7 +30,7 @@ public readonly struct Memory<T>
         private readonly object _object;
         private readonly int _index;
         private readonly int _length;
-        
+
         /// <summary>
         /// Creates a new memory over the entirety of the target array.
         /// </summary>
@@ -258,6 +258,35 @@ public Memory<T> Slice(int start, int length)
             return new Memory<T>(_object, _index + start, length);
         }
 
+        /// <summary>
+        /// Forms a slice out of the given memory, beginning at 'startIndex'
+        /// </summary>
+        /// <param name="startIndex">The index at which to begin this slice.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Memory<T> Slice(Index startIndex)
+        {
+            int actualIndex = startIndex.GetOffset(_length);
+            return Slice(actualIndex);
+        }
+
+        /// <summary>
+        /// Forms a slice out of the given memory using the range start and end indexes.
+        /// </summary>
+        /// <param name="range">The range used to slice the memory using its start and end indexes.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Memory<T> Slice(Range range)
+        {
+            (int start, int length) = range.GetOffsetAndLength(_length);
+            // It is expected for _index + start to be negative if the memory is already pre-pinned.
+            return new Memory<T>(_object, _index + start, length);
+        }
+
+        /// <summary>
+        /// Forms a slice out of the given memory using the range start and end indexes.
+        /// </summary>
+        /// <param name="range">The range used to slice the memory using its start and end indexes.</param>
+        public Memory<T> this[Range range] => Slice(range);
+
         /// <summary>
         /// Returns a span from the memory.
         /// </summary>
@@ -336,7 +365,7 @@ public unsafe Span<T> Span
                         ThrowHelper.ThrowArgumentOutOfRangeException();
                     }
 #endif
-                    
+
                     refToReturn = ref Unsafe.Add(ref refToReturn, desiredStartIndex);
                     lengthOfUnderlyingSpan = desiredLength;
                 }

src/System.Private.CoreLib/shared/System/MemoryExtensions.Fast.cs

@@ -400,6 +400,54 @@ public static Span<T> AsSpan<T>(this T[] array, int start)
             return new Span<T>(ref Unsafe.Add(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()), start), array.Length - start);
         }
 
+        /// <summary>
+        /// Creates a new span over the portion of the target array.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Span<T> AsSpan<T>(this T[] array, Index startIndex)
+        {
+            if (array == null)
+            {
+                if (!startIndex.Equals(Index.Start))
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+
+                return default;
+            }
+
+            if (default(T) == null && array.GetType() != typeof(T[]))
+                ThrowHelper.ThrowArrayTypeMismatchException();
+
+            int actualIndex = startIndex.GetOffset(array.Length);
+            if ((uint)actualIndex > (uint)array.Length)
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new Span<T>(ref Unsafe.Add(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()), actualIndex), array.Length - actualIndex);
+        }
+
+        /// <summary>
+        /// Creates a new span over the portion of the target array.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Span<T> AsSpan<T>(this T[] array, Range range)
+        {
+            if (array == null)
+            {
+                Index startIndex = range.Start;
+                Index endIndex = range.End;
+
+                if (!startIndex.Equals(Index.Start) || !endIndex.Equals(Index.Start))
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+
+                return default;
+            }
+
+            if (default(T) == null && array.GetType() != typeof(T[]))
+                ThrowHelper.ThrowArrayTypeMismatchException();
+
+            (int start, int length) = range.GetOffsetAndLength(array.Length);
+            return new Span<T>(ref Unsafe.Add(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()), start), length);
+        }
+
         /// <summary>
         /// Creates a new readonly span over the portion of the target string.
         /// </summary>
@@ -504,6 +552,26 @@ public static ReadOnlyMemory<char> AsMemory(this string text, int start)
             return new ReadOnlyMemory<char>(text, start, text.Length - start);
         }
 
+        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target string.</summary>
+        /// <param name="text">The target string.</param>
+        /// <param name="startIndex">The index at which to begin this slice.</param>
+        public static ReadOnlyMemory<char> AsMemory(this string text, Index startIndex)
+        {
+            if (text == null)
+            {
+                if (!startIndex.Equals(Index.Start))
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+
+                return default;
+            }
+
+            int actualIndex = startIndex.GetOffset(text.Length);
+            if ((uint)actualIndex > (uint)text.Length)
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new ReadOnlyMemory<char>(text, actualIndex, text.Length - actualIndex);
+        }
+
         /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target string.</summary>
         /// <param name="text">The target string.</param>
         /// <param name="start">The index at which to begin this slice.</param>
@@ -532,5 +600,25 @@ public static ReadOnlyMemory<char> AsMemory(this string text, int start, int len
 
             return new ReadOnlyMemory<char>(text, start, length);
         }
+
+        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target string.</summary>
+        /// <param name="text">The target string.</param>
+        /// <param name="range">The range used to indicate the start and length of the sliced string.</param>
+        public static ReadOnlyMemory<char> AsMemory(this string text, Range range)
+        {
+            if (text == null)
+            {
+                Index startIndex = range.Start;
+                Index endIndex = range.End;
+
+                if (!startIndex.Equals(Index.Start) || !endIndex.Equals(Index.Start))
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+
+                return default;
+            }
+
+            (int start, int length) = range.GetOffsetAndLength(text.Length);
+            return new ReadOnlyMemory<char>(text, start, length);
+        }
     }
 }