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

// ReSharper disable RedundantCast

#nullable enable
using System.Collections;

namespace System.Net
{
    internal class CaseInsensitiveAscii : IEqualityComparer, IComparer
    {
        // ASCII char ToLower table
        internal static readonly CaseInsensitiveAscii StaticInstance = new CaseInsensitiveAscii();
        internal static ReadOnlySpan<byte> AsciiToLower => new byte[] {
              0,   1,   2,   3,   4,   5,   6,   7,   8,   9,
             10,  11,  12,  13,  14,  15,  16,  17,  18,  19,
             20,  21,  22,  23,  24,  25,  26,  27,  28,  29,
             30,  31,  32,  33,  34,  35,  36,  37,  38,  39,
             40,  41,  42,  43,  44,  45,  46,  47,  48,  49,
             50,  51,  52,  53,  54,  55,  56,  57,  58,  59,
             60,  61,  62,  63,  64,  97,  98,  99, 100, 101,
            102, 103, 104, 105, 106, 107, 108, 109, 110, 111,
            112, 113, 114, 115, 116, 117, 118, 119, 120, 121,
            122,  91,  92,  93,  94,  95,  96,  97,  98,  99,
            100, 101, 102, 103, 104, 105, 106, 107, 108, 109,
            110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
            120, 121, 122, 123, 124, 125, 126, 127, 128, 129,
            130, 131, 132, 133, 134, 135, 136, 137, 138, 139,
            140, 141, 142, 143, 144, 145, 146, 147, 148, 149,
            150, 151, 152, 153, 154, 155, 156, 157, 158, 159,
            160, 161, 162, 163, 164, 165, 166, 167, 168, 169,
            170, 171, 172, 173, 174, 175, 176, 177, 178, 179,
            180, 181, 182, 183, 184, 185, 186, 187, 188, 189,
            190, 191, 192, 193, 194, 195, 196, 197, 198, 199,
            200, 201, 202, 203, 204, 205, 206, 207, 208, 209,
            210, 211, 212, 213, 214, 215, 216, 217, 218, 219,
            220, 221, 222, 223, 224, 225, 226, 227, 228, 229,
            230, 231, 232, 233, 234, 235, 236, 237, 238, 239,
            240, 241, 242, 243, 244, 245, 246, 247, 248, 249,
            250, 251, 252, 253, 254, 255
        };

        // ASCII string case insensitive hash function
        public int GetHashCode(object myObject)
        {
            string? myString = myObject as string;
            if (myString == null)
            {
                return 0;
            }
            int myHashCode = myString.Length;
            if (myHashCode == 0)
            {
                return 0;
            }
            myHashCode ^= AsciiToLower[(byte)myString[0]] << 24 ^ AsciiToLower[(byte)myString[myHashCode - 1]] << 16;
            return myHashCode;
        }

        // ASCII string case insensitive comparer
        public int Compare(object? firstObject, object? secondObject)
        {
            string? firstString = firstObject as string;
            string? secondString = secondObject as string;
            if (firstString == null)
            {
                return secondString == null ? 0 : -1;
            }
            if (secondString == null)
            {
                return 1;
            }
            int result = firstString.Length - secondString.Length;
            int comparisons = result > 0 ? secondString.Length : firstString.Length;
            int difference, index = 0;
            while (index < comparisons)
            {
                difference = (int)(AsciiToLower[firstString[index]] - AsciiToLower[secondString[index]]);
                if (difference != 0)
                {
                    result = difference;
                    break;
                }
                index++;
            }
            return result;
        }

        // ASCII string case insensitive hash function
        private int FastGetHashCode(string myString)
        {
            int myHashCode = myString.Length;
            if (myHashCode != 0)
            {
                myHashCode ^= AsciiToLower[(byte)myString[0]] << 24 ^ AsciiToLower[(byte)myString[myHashCode - 1]] << 16;
            }
            return myHashCode;
        }

        // ASCII string case insensitive comparer
        public new bool Equals(object? firstObject, object? secondObject)
        {
            string? firstString = firstObject as string;
            string? secondString = secondObject as string;
            if (firstString == null)
            {
                return secondString == null;
            }
            if (secondString != null)
            {
                int index = firstString.Length;
                if (index == secondString.Length)
                {
                    if (FastGetHashCode(firstString) == FastGetHashCode(secondString))
                    {
                        while (index > 0)
                        {
                            index--;
                            if (AsciiToLower[firstString[index]] != AsciiToLower[secondString[index]])
                            {
                                return false;
                            }
                        }
                        return true;
                    }
                }
            }
            return false;
        }
    }
}