ModernUO/Projects/Server/Network/NetworkCompression.cs

using System;
using System.Buffers.Binary;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Server.Network;

/// <summary>
///     Handles outgoing packet compression for the network.
/// </summary>
public static class NetworkCompression
{
    // UO packets may not exceed 64kb in length
    public const int BufferSize = 0x10000;

    // Optimal compression ratio is 2 / 8;  worst compression ratio is 11 / 8
    private const int MinimalCodeLength = 2;

    // Fixed overhead, in bits, per compression call
    private const int TerminalCodeLength = 4;

    // Compress returns 0 for longer input regardless of the output span
    internal const int DefiniteOverflow = (BufferSize * 8 - TerminalCodeLength) / MinimalCodeLength;

    // Packed Table: High 16 bits = Length, Low 16 bits = Value
    private static readonly uint[] _packedHuffmanTable = new uint[257];

    static NetworkCompression()
    {
        ReadOnlySpan<int> rawTable = [
            0x2, 0x000, 0x5, 0x01F, 0x6, 0x022, 0x7, 0x034, 0x7, 0x075, 0x6, 0x028, 0x6, 0x03B, 0x7, 0x032,
            0x8, 0x0E0, 0x8, 0x062, 0x7, 0x056, 0x8, 0x079, 0x9, 0x19D, 0x8, 0x097, 0x6, 0x02A, 0x7, 0x057,
            0x8, 0x071, 0x8, 0x05B, 0x9, 0x1CC, 0x8, 0x0A7, 0x7, 0x025, 0x7, 0x04F, 0x8, 0x066, 0x8, 0x07D,
            0x9, 0x191, 0x9, 0x1CE, 0x7, 0x03F, 0x9, 0x090, 0x8, 0x059, 0x8, 0x07B, 0x8, 0x091, 0x8, 0x0C6,
            0x6, 0x02D, 0x9, 0x186, 0x8, 0x06F, 0x9, 0x093, 0xA, 0x1CC, 0x8, 0x05A, 0xA, 0x1AE, 0xA, 0x1C0,
            0x9, 0x148, 0x9, 0x14A, 0x9, 0x082, 0xA, 0x19F, 0x9, 0x171, 0x9, 0x120, 0x9, 0x0E7, 0xA, 0x1F3,
            0x9, 0x14B, 0x9, 0x100, 0x9, 0x190, 0x6, 0x013, 0x9, 0x161, 0x9, 0x125, 0x9, 0x133, 0x9, 0x195,
            0x9, 0x173, 0x9, 0x1CA, 0x9, 0x086, 0x9, 0x1E9, 0x9, 0x0DB, 0x9, 0x1EC, 0x9, 0x08B, 0x9, 0x085,
            0x5, 0x00A, 0x8, 0x096, 0x8, 0x09C, 0x9, 0x1C3, 0x9, 0x19C, 0x9, 0x08F, 0x9, 0x18F, 0x9, 0x091,
            0x9, 0x087, 0x9, 0x0C6, 0x9, 0x177, 0x9, 0x089, 0x9, 0x0D6, 0x9, 0x08C, 0x9, 0x1EE, 0x9, 0x1EB,
            0x9, 0x084, 0x9, 0x164, 0x9, 0x175, 0x9, 0x1CD, 0x8, 0x05E, 0x9, 0x088, 0x9, 0x12B, 0x9, 0x172,
            0x9, 0x10A, 0x9, 0x08D, 0x9, 0x13A, 0x9, 0x11C, 0xA, 0x1E1, 0xA, 0x1E0, 0x9, 0x187, 0xA, 0x1DC,
            0xA, 0x1DF, 0x7, 0x074, 0x9, 0x19F, 0x8, 0x08D, 0x8, 0x0E4, 0x7, 0x079, 0x9, 0x0EA, 0x9, 0x0E1,
            0x8, 0x040, 0x7, 0x041, 0x9, 0x10B, 0x9, 0x0B0, 0x8, 0x06A, 0x8, 0x0C1, 0x7, 0x071, 0x7, 0x078,
            0x8, 0x0B1, 0x9, 0x14C, 0x7, 0x043, 0x8, 0x076, 0x7, 0x066, 0x7, 0x04D, 0x9, 0x08A, 0x6, 0x02F,
            0x8, 0x0C9, 0x9, 0x0CE, 0x9, 0x149, 0x9, 0x160, 0xA, 0x1BA, 0xA, 0x19E, 0xA, 0x39F, 0x9, 0x0E5,
            0x9, 0x194, 0x9, 0x184, 0x9, 0x126, 0x7, 0x030, 0x8, 0x06C, 0x9, 0x121, 0x9, 0x1E8, 0xA, 0x1C1,
            0xA, 0x11D, 0xA, 0x163, 0xA, 0x385, 0xA, 0x3DB, 0xA, 0x17D, 0xA, 0x106, 0xA, 0x397, 0xA, 0x24E,
            0x7, 0x02E, 0x8, 0x098, 0xA, 0x33C, 0xA, 0x32E, 0xA, 0x1E9, 0x9, 0x0BF, 0xA, 0x3DF, 0xA, 0x1DD,
            0xA, 0x32D, 0xA, 0x2ED, 0xA, 0x30B, 0xA, 0x107, 0xA, 0x2E8, 0xA, 0x3DE, 0xA, 0x125, 0xA, 0x1E8,
            0x9, 0x0E9, 0xA, 0x1CD, 0xA, 0x1B5, 0x9, 0x165, 0xA, 0x232, 0xA, 0x2E1, 0xB, 0x3AE, 0xB, 0x3C6,
            0xB, 0x3E2, 0xA, 0x205, 0xA, 0x29A, 0xA, 0x248, 0xA, 0x2CD, 0xA, 0x23B, 0xB, 0x3C5, 0xA, 0x251,
            0xA, 0x2E9, 0xA, 0x252, 0x9, 0x1EA, 0xB, 0x3A0, 0xB, 0x391, 0xA, 0x23C, 0xB, 0x392, 0xB, 0x3D5,
            0xA, 0x233, 0xA, 0x2CC, 0xB, 0x390, 0xA, 0x1BB, 0xB, 0x3A1, 0xB, 0x3C4, 0xA, 0x211, 0xA, 0x203,
            0x9, 0x12A, 0xA, 0x231, 0xB, 0x3E0, 0xA, 0x29B, 0xB, 0x3D7, 0xA, 0x202, 0xB, 0x3AD, 0xA, 0x213,
            0xA, 0x253, 0xA, 0x32C, 0xA, 0x23D, 0xA, 0x23F, 0xA, 0x32F, 0xA, 0x11C, 0xA, 0x384, 0xA, 0x31C,
            0xA, 0x17C, 0xA, 0x30A, 0xA, 0x2E0, 0xA, 0x276, 0xA, 0x250, 0xB, 0x3E3, 0xA, 0x396, 0xA, 0x18F,
            0xA, 0x204, 0xA, 0x206, 0xA, 0x230, 0xA, 0x265, 0xA, 0x212, 0xA, 0x23E, 0xB, 0x3AC, 0xB, 0x393,
            0xB, 0x3E1, 0xA, 0x1DE, 0xB, 0x3D6, 0xA, 0x31D, 0xB, 0x3E5, 0xB, 0x3E4, 0xA, 0x207, 0xB, 0x3C7,
            0xA, 0x277, 0xB, 0x3D4, 0x8, 0x0C0, 0xA, 0x162, 0xA, 0x3DA, 0xA, 0x124, 0xA, 0x1B4, 0xA, 0x264,
            0xA, 0x33D, 0xA, 0x1D1, 0xA, 0x1AF, 0xA, 0x39E, 0xA, 0x24F, 0xB, 0x373, 0xA, 0x249, 0xB, 0x372,
            0x9, 0x167, 0xA, 0x210, 0xA, 0x23A, 0xA, 0x1B8, 0xB, 0x3AF, 0xA, 0x18E, 0xA, 0x2EC, 0x7, 0x062,
            0x4, 0x00D
        ];

        for (var i = 0; i < 257; i++)
        {
            var len = (uint)rawTable[i * 2];
            var val = (uint)rawTable[i * 2 + 1];
            _packedHuffmanTable[i] = (len << 16) | val;
        }
    }

    public static int Compress(ReadOnlySpan<byte> input, Span<byte> output)
    {
        // output.Length < 4 underflows safeOutputLength below (nuint), defeating the hot loop's
        // bounds check. Reachable whenever the send buffer is nearly full.
        if (input.Length > DefiniteOverflow || output.Length < 4)
        {
            return 0;
        }

        ref var inputRef = ref MemoryMarshal.GetReference(input);
        ref var outputRef = ref MemoryMarshal.GetReference(output);
        ref var tableRef = ref MemoryMarshal.GetReference(_packedHuffmanTable.AsSpan());

        nuint i = 0;
        ulong bitValue = 0;
        var bitCount = 0;
        nuint outputIdx = 0;
        var safeOutputLength = (nuint)output.Length - 4;
        var unrolledLimit = (nuint)input.Length - 1;

        // Unrolled 2x hot loop using native integer indices
        while (i < unrolledLimit)
        {
            var b1 = Unsafe.Add(ref inputRef, i);
            var entry1 = Unsafe.Add(ref tableRef, b1);
            var len1 = (int)(entry1 >> 16);
            ulong val1 = entry1 & 0xFFFF;

            bitValue = (bitValue << len1) | val1;
            bitCount += len1;

            var b2 = Unsafe.Add(ref inputRef, i + 1);
            var entry2 = Unsafe.Add(ref tableRef, b2);
            var len2 = (int)(entry2 >> 16);
            ulong val2 = entry2 & 0xFFFF;

            bitValue = (bitValue << len2) | val2;
            bitCount += len2;

            if (bitCount >= 32)
            {
                if (outputIdx > safeOutputLength)
                {
                    return 0;
                }

                bitCount -= 32;
                var word = (uint)(bitValue >> bitCount);
                Unsafe.WriteUnaligned(ref Unsafe.Add(ref outputRef, outputIdx), BinaryPrimitives.ReverseEndianness(word));
                outputIdx += 4;
            }

            i += 2;
        }

        if (i < (nuint)input.Length)
        {
            var b = Unsafe.Add(ref inputRef, i);
            var entry = Unsafe.Add(ref tableRef, b);
            var len = (int)(entry >> 16);
            ulong val = entry & 0xFFFF;

            bitValue = (bitValue << len) | val;
            bitCount += len;

            if (bitCount >= 32)
            {
                if (outputIdx > safeOutputLength)
                {
                    return 0;
                }

                bitCount -= 32;
                var word = (uint)(bitValue >> bitCount);
                Unsafe.WriteUnaligned(ref Unsafe.Add(ref outputRef, outputIdx), BinaryPrimitives.ReverseEndianness(word));
                outputIdx += 4;
            }
        }

        // Terminal code logic
        var termEntry = Unsafe.Add(ref tableRef, 256);
        var termLen = (int)(termEntry >> 16);
        ulong termVal = termEntry & 0xFFFF;

        bitValue = (bitValue << termLen) | termVal;
        bitCount += termLen;

        var remainder = bitCount & 7;
        if (remainder != 0)
        {
            var padding = 8 - remainder;
            bitValue <<= padding;
            bitCount += padding;
        }

        bitValue <<= 64 - bitCount;

        while (bitCount > 0)
        {
            if (outputIdx >= (nuint)output.Length)
            {
                return 0;
            }

            Unsafe.Add(ref outputRef, outputIdx++) = (byte)(bitValue >> 56);
            bitValue <<= 8;
            bitCount -= 8;
        }

        return (int)outputIdx;
    }
}