// 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.Collections;

namespace System.Net
{
    internal class CaseInsensitiveAscii : IEqualityComparer, IComparer
    {
        // ASCII char ToLower table
        internal static readonly CaseInsensitiveAscii StaticInstance = new CaseInsensitiveAscii();
        internal static readonly 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))
                    {
                        int comparisons = firstString.Length;
                        while (index > 0)
                        {
                            index--;
                            if (AsciiToLower[firstString[index]] != AsciiToLower[secondString[index]])
                            {
                                return false;
                            }
                        }
                        return true;
                    }
                }
            }
            return false;
        }
    }
}
