slots.cs

using System;
using System.Diagnostics;
using System.Text;
using System.Threading;

static class P {
    static int remainingKeys = Crc16.RedisClusterSlotCount;
    static readonly string[] keys = new string[remainingKeys];
    const byte CharsStart = 33, // ignore space
        CharsEnd = 126; // ignore DEL

    static void Main()
    {
        Span<byte> three = stackalloc byte[3], two = three.Slice(0, 2), one = three.Slice(0, 1);
        var watch = Stopwatch.StartNew();
        try
        {
            for (byte i = CharsStart; i <= CharsEnd; i++)
            {
                one[0] = i;
                SetSlot(one);
            }
            // repeat the loops because we want to prefer shorter keys
            for (byte i = CharsStart; i <= CharsEnd; i++)
            {
                two[0] = i;
                for (byte j = CharsStart; j <= CharsEnd; j++)
                {
                    two[1] = j;
                    SetSlot(two);
                }
            }
            for (byte i = CharsStart; i <= CharsEnd; i++)
            {
                three[0] = i;
                for (byte j = CharsStart; j <= CharsEnd; j++)
                {
                    three[1] = j;
                    for (byte k = CharsStart; k <= CharsEnd; k++)
                    {
                        three[2] = k;
                        SetSlot(three);
                    }
                }
            }
        } catch (AllDoneException)
        {
        }
        watch.Stop();
        Console.WriteLine($"{Volatile.Read(ref remainingKeys)} remaining after {watch.Elapsed.TotalSeconds} seconds");
        Console.WriteLine("Press any key to show results...");
        Console.ReadKey();
        for (int i = 0; i < keys.Length; i++)
        {
            Console.WriteLine($"{i}:\t{keys[i]}");
        }


        static void SetSlot(ReadOnlySpan<byte> key)
        {
            var slot = Crc16.GetClusterSlot(key);
            // written with parallelism in mind, so: handles thread safety; for single-thread,
            // this can be simpler and probably faster as just:
            // if (keys[slot] is null)
            // {
            //    keys[slot] = Encoding.ASCII.GetString(key);
            //    if (--remainingKeys <= 0) ThrowAllDone();
            // }
            if (Volatile.Read(ref keys[slot]) is null
                && Interlocked.CompareExchange(ref keys[slot], Encoding.ASCII.GetString(key), null) is null)
            {
                var rem = Interlocked.Decrement(ref remainingKeys);
                if (rem <= 0) ThrowAllDone();
            }
            static void ThrowAllDone() => throw new AllDoneException();
        }
    }

    sealed class AllDoneException : Exception { }// bite me




    static class Crc16
    {
        public const int RedisClusterSlotCount = 16384;

        public static unsafe int GetClusterSlot(ReadOnlySpan<byte> blob)
        {
            unchecked
            {
                fixed (byte* ptr = blob)
                {
                    fixed (ushort* crc16tab = s_crc16tab)
                    {
                        int offset = 0, count = blob.Length, start, end;
                        if ((start = IndexOf(ptr, (byte)'{', 0, count - 1)) >= 0
                            && (end = IndexOf(ptr, (byte)'}', start + 1, count)) >= 0
                            && --end != start)
                        {
                            offset = start + 1;
                            count = end - start; // note we already subtracted one via --end
                        }

                        uint crc = 0;
                        for (int i = 0; i < count; i++)
                            crc = ((crc << 8) ^ crc16tab[((crc >> 8) ^ ptr[offset++]) & 0x00FF]) & 0x0000FFFF;
                        return (int)(crc % RedisClusterSlotCount);
                    }
                }
            }
        }
        private static unsafe int IndexOf(byte* ptr, byte value, int start, int end)
        {
            for (int offset = start; offset < end; offset++)
                if (ptr[offset] == value) return offset;
            return -1;
        }
        private static readonly ushort[] s_crc16tab = new ushort[]
        {
            0x0000,0x1021,0x2042,0x3063,0x4084,0x50a5,0x60c6,0x70e7,
            0x8108,0x9129,0xa14a,0xb16b,0xc18c,0xd1ad,0xe1ce,0xf1ef,
            0x1231,0x0210,0x3273,0x2252,0x52b5,0x4294,0x72f7,0x62d6,
            0x9339,0x8318,0xb37b,0xa35a,0xd3bd,0xc39c,0xf3ff,0xe3de,
            0x2462,0x3443,0x0420,0x1401,0x64e6,0x74c7,0x44a4,0x5485,
            0xa56a,0xb54b,0x8528,0x9509,0xe5ee,0xf5cf,0xc5ac,0xd58d,
            0x3653,0x2672,0x1611,0x0630,0x76d7,0x66f6,0x5695,0x46b4,
            0xb75b,0xa77a,0x9719,0x8738,0xf7df,0xe7fe,0xd79d,0xc7bc,
            0x48c4,0x58e5,0x6886,0x78a7,0x0840,0x1861,0x2802,0x3823,
            0xc9cc,0xd9ed,0xe98e,0xf9af,0x8948,0x9969,0xa90a,0xb92b,
            0x5af5,0x4ad4,0x7ab7,0x6a96,0x1a71,0x0a50,0x3a33,0x2a12,
            0xdbfd,0xcbdc,0xfbbf,0xeb9e,0x9b79,0x8b58,0xbb3b,0xab1a,
            0x6ca6,0x7c87,0x4ce4,0x5cc5,0x2c22,0x3c03,0x0c60,0x1c41,
            0xedae,0xfd8f,0xcdec,0xddcd,0xad2a,0xbd0b,0x8d68,0x9d49,
            0x7e97,0x6eb6,0x5ed5,0x4ef4,0x3e13,0x2e32,0x1e51,0x0e70,
            0xff9f,0xefbe,0xdfdd,0xcffc,0xbf1b,0xaf3a,0x9f59,0x8f78,
            0x9188,0x81a9,0xb1ca,0xa1eb,0xd10c,0xc12d,0xf14e,0xe16f,
            0x1080,0x00a1,0x30c2,0x20e3,0x5004,0x4025,0x7046,0x6067,
            0x83b9,0x9398,0xa3fb,0xb3da,0xc33d,0xd31c,0xe37f,0xf35e,
            0x02b1,0x1290,0x22f3,0x32d2,0x4235,0x5214,0x6277,0x7256,
            0xb5ea,0xa5cb,0x95a8,0x8589,0xf56e,0xe54f,0xd52c,0xc50d,
            0x34e2,0x24c3,0x14a0,0x0481,0x7466,0x6447,0x5424,0x4405,
            0xa7db,0xb7fa,0x8799,0x97b8,0xe75f,0xf77e,0xc71d,0xd73c,
            0x26d3,0x36f2,0x0691,0x16b0,0x6657,0x7676,0x4615,0x5634,
            0xd94c,0xc96d,0xf90e,0xe92f,0x99c8,0x89e9,0xb98a,0xa9ab,
            0x5844,0x4865,0x7806,0x6827,0x18c0,0x08e1,0x3882,0x28a3,
            0xcb7d,0xdb5c,0xeb3f,0xfb1e,0x8bf9,0x9bd8,0xabbb,0xbb9a,
            0x4a75,0x5a54,0x6a37,0x7a16,0x0af1,0x1ad0,0x2ab3,0x3a92,
            0xfd2e,0xed0f,0xdd6c,0xcd4d,0xbdaa,0xad8b,0x9de8,0x8dc9,
            0x7c26,0x6c07,0x5c64,0x4c45,0x3ca2,0x2c83,0x1ce0,0x0cc1,
            0xef1f,0xff3e,0xcf5d,0xdf7c,0xaf9b,0xbfba,0x8fd9,0x9ff8,
            0x6e17,0x7e36,0x4e55,0x5e74,0x2e93,0x3eb2,0x0ed1,0x1ef0
        };
    }
}