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

using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;

#if NETSTANDARD2_0 || NETFRAMEWORK
using System.Numerics.Hashing;
#endif

namespace System
{
    /// <summary>Represent a range has start and end indexes.</summary>
    /// <remarks>
    /// Range is used by the C# compiler to support the range syntax.
    /// <code>
    /// int[] someArray = new int[5] { 1, 2, 3, 4, 5 };
    /// int[] subArray1 = someArray[0..2]; // { 1, 2 }
    /// int[] subArray2 = someArray[1..^0]; // { 2, 3, 4, 5 }
    /// </code>
    /// </remarks>
#if SYSTEM_PRIVATE_CORELIB || MICROSOFT_BCL_MEMORY
    public
#else
    internal
#endif
    readonly struct Range : IEquatable<Range>
    {
        /// <summary>Represent the inclusive start index of the Range.</summary>
        public Index Start { get; }

        /// <summary>Represent the exclusive end index of the Range.</summary>
        public Index End { get; }

        /// <summary>Construct a Range object using the start and end indexes.</summary>
        /// <param name="start">Represent the inclusive start index of the range.</param>
        /// <param name="end">Represent the exclusive end index of the range.</param>
        public Range(Index start, Index end)
        {
            Start = start;
            End = end;
        }

        /// <summary>Indicates whether the current Range 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 Range r &&
            r.Start.Equals(Start) &&
            r.End.Equals(End);

        /// <summary>Indicates whether the current Range object is equal to another Range object.</summary>
        /// <param name="other">An object to compare with this object</param>
        public bool Equals(Range other) => other.Start.Equals(Start) && other.End.Equals(End);

        /// <summary>Returns the hash code for this instance.</summary>
        public override int GetHashCode()
        {
#if (!NETSTANDARD2_0 && !NETFRAMEWORK)
            return HashCode.Combine(Start.GetHashCode(), End.GetHashCode());
#else
            return HashHelpers.Combine(Start.GetHashCode(), End.GetHashCode());
#endif
        }

        /// <summary>Converts the value of the current Range object to its equivalent string representation.</summary>
        public override unsafe string ToString()
        {
#if (!NETSTANDARD2_0 && !NETFRAMEWORK)
            Span<char> span = stackalloc char[2 + (2 * 11)]; // 2 for "..", then for each index 1 for '^' and 10 for longest possible uint
            int pos = 0;

            if (Start.IsFromEnd)
            {
                span[0] = '^';
                pos = 1;
            }
            bool formatted = ((uint)Start.Value).TryFormat(span.Slice(pos), out int charsWritten);
            Debug.Assert(formatted);
            pos += charsWritten;

            span[pos++] = '.';
            span[pos++] = '.';

            if (End.IsFromEnd)
            {
                span[pos++] = '^';
            }
            formatted = ((uint)End.Value).TryFormat(span.Slice(pos), out charsWritten);
            Debug.Assert(formatted);
            pos += charsWritten;

            return new string(span.Slice(0, pos));
#else
            return Start.ToString() + ".." + End.ToString();
#endif
        }

        /// <summary>Create a Range object starting from start index to the end of the collection.</summary>
        public static Range StartAt(Index start) => new Range(start, Index.End);

        /// <summary>Create a Range object starting from first element in the collection to the end Index.</summary>
        public static Range EndAt(Index end) => new Range(Index.Start, end);

        /// <summary>Create a Range object starting from first element to the end.</summary>
        public static Range All => new Range(Index.Start, Index.End);

        /// <summary>Calculate the start offset and length of range object using a collection length.</summary>
        /// <param name="length">The length of the collection that the range will be used with. length has to be a positive value.</param>
        /// <remarks>
        /// For performance reason, we don't validate the input length parameter against negative values.
        /// It is expected Range will be used with collections which always have non negative length/count.
        /// We validate the range is inside the length scope though.
        /// </remarks>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public (int Offset, int Length) GetOffsetAndLength(int length)
        {
            int start = Start.GetOffset(length);
            int end = End.GetOffset(length);

            if ((uint)end > (uint)length || (uint)start > (uint)end)
            {
                ThrowArgumentOutOfRangeException();
            }

            return (start, end - start);
        }

        private static void ThrowArgumentOutOfRangeException()
        {
            throw new ArgumentOutOfRangeException("length");
        }
    }
}