/*
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 *    1. Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *    2. Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
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 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 *    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
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 */

/**
 * @file
 *   This file implements an HDLC-lite encoder and decoder.
 */

#include "pch.hpp"
#include "hdlc.tmh"

/**
 * FCS lookup table
 */
enum
{
    kInitFcs = 0xffff,  ///< Initial FCS value.
    kGoodFcs = 0xf0b8,  ///< Good FCS value.
};

static bool
hdlc_byte_needs_escape(UCHAR byte)
{
    switch (byte) 
    {
    case HdlcXOn:
    case HdlcXOff:
    case HdlcEscapeSequence:
    case HdlcFlagSequence:
    case HdlcSpecial:
        return true;

    default:
        return false;
    }
}

/**
 * This method updates an FCS.
 *
 * @param[in]  aFcs   The FCS to update.
 * @param[in]  aByte  The input byte value.
 *
 * @returns The updated FCS.
 */
USHORT UpdateFcs(USHORT aFcs, UCHAR aByte)
{
    const USHORT sFcsTable[256] =
    {
        0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
        0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
        0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
        0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
        0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
        0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
        0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
        0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
        0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
        0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
        0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
        0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
        0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
        0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
        0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
        0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
        0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
        0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
        0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
        0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
        0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
        0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
        0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
        0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
        0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
        0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
        0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
        0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
        0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
        0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
        0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
        0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
    };
    return (aFcs >> 8) ^ sFcsTable[(aFcs ^ aByte) & 0xff];
}

ULONG
HdlcComputeEncodedByteLength(
    _In_ UCHAR          aByte
    )
{
    return hdlc_byte_needs_escape(aByte) ? 2 : 1;
}

ULONG
HdlcComputeEncodedLength(
    _In_reads_bytes_(DecodedBufferLength) 
         PUCHAR         DecodedBuffer,
    _In_ ULONG          DecodedBufferLength
    )
{
    USHORT Fcs = kInitFcs;
    ULONG Length = 2; // Start and end sequence
    
    for (ULONG i = 0; i < DecodedBufferLength; i++)
    {
        Length += HdlcComputeEncodedByteLength(DecodedBuffer[i]);
        Fcs = UpdateFcs(Fcs, DecodedBuffer[i]);
    }
    
    Fcs ^= 0xffff;
    
    Length += HdlcComputeEncodedByteLength(Fcs & 0xff);
    Length += HdlcComputeEncodedByteLength(Fcs >> 8);

    return Length;
}

bool
HdlcEncodeByte(
    _In_    UCHAR       aByte,
    _Inout_ ULONG&      Offset,
    _In_    ULONG       EncodedBufferLength,
    _Out_writes_bytes_(EncodedBufferLength) 
            PUCHAR      EncodedBuffer
    )
{
    if (hdlc_byte_needs_escape(aByte))
    {
        if (Offset + 2 > EncodedBufferLength) return false;

        EncodedBuffer[Offset++] = HdlcEscapeSequence;
        EncodedBuffer[Offset++] = (aByte ^ 0x20);
    }
    else
    {
        if (Offset + 1 > EncodedBufferLength) return false;

        EncodedBuffer[Offset++] = aByte;
    }

    return true;
}

_Success_(return == true)
bool
HdlcEncodeBuffer(
    _In_reads_bytes_(DecodedBufferLength) 
         PUCHAR         DecodedBuffer,
    _In_ ULONG          DecodedBufferLength,
    _Out_writes_bytes_(EncodedBufferLength) 
         PUCHAR         EncodedBuffer,
    _In_ ULONG          EncodedBufferLength
    )
{
    USHORT Fcs = kInitFcs;
    ULONG Offset = 0;
    bool Complete = false;

    if (Offset + 1 > EncodedBufferLength) goto error;
    EncodedBuffer[Offset++] = HdlcFlagSequence;

    for (ULONG i = 0; i < DecodedBufferLength; i++)
    {
        UCHAR aByte = DecodedBuffer[i];

        if (!HdlcEncodeByte(aByte, Offset, EncodedBufferLength, EncodedBuffer)) {
            goto error;
        }

        Fcs = UpdateFcs(Fcs, aByte);
    }
    
    Fcs ^= 0xffff;
    
    if (!HdlcEncodeByte(Fcs & 0xff, Offset, EncodedBufferLength, EncodedBuffer) ||
        !HdlcEncodeByte(Fcs >> 8,   Offset, EncodedBufferLength, EncodedBuffer)) {
        goto error;
    }
    
    if (Offset + 1 > EncodedBufferLength) goto error;
    EncodedBuffer[Offset++] = HdlcFlagSequence;

    NT_ASSERT(Offset == EncodedBufferLength);
    Complete = true;

error:

    return Complete;
}

enum HdlcDecodeState
{
    kStateNoSync = 0,
    kStateSync,
    kStateEscaped,
};

_Success_(return == true)
bool
HdlcDecodeBuffer(
    _In_reads_bytes_(*EncodedBufferLength) 
            PUCHAR      EncodedBuffer,
    _Inout_ PULONG      EncodedBufferLength,
    _Inout_ PULONG      DecodedBufferLength,
    _Out_writes_bytes_opt_(*DecodedBufferLength) 
            PUCHAR      DecodedBuffer,
    _Out_   bool*       IsGood
    )
{
    UCHAR byte;
    HdlcDecodeState state = kStateNoSync;
    ULONG DecodedLength = 0;
    USHORT Fcs = kInitFcs;

    for (ULONG i = 0; i < *EncodedBufferLength; i++)
    {
        byte = EncodedBuffer[i];

        switch (state)
        {
            case kStateNoSync:
            {
                if (byte == HdlcFlagSequence)
                {
                    if (i != 0)
                    {
                        // Set the output for junk length
                        *EncodedBufferLength = i;
                        
                        // Set the output 'IsGood' flag based on FCS
                        *IsGood = false;

                        return true;
                    }
                    else
                    {
                        state = kStateSync;
                    }
                }
                break;
            }
            case kStateSync:
            {
                switch (byte)
                {
                    case HdlcEscapeSequence:
                    {
                        state = kStateEscaped;
                        break;
                    }
                    case HdlcFlagSequence:
                    {
                        // If it wasn't enough buffer to be a complete sequence, maybe we are
                        // actually between sequences. Leave the trailing escape character.
                        if (i < sizeof(USHORT) + 2)
                        {
                            // Set the output encoded buffer length
                            *EncodedBufferLength = i;

                            *IsGood = false;
                        }
                        else
                        {
                            // Set the output encoded buffer length
                            *EncodedBufferLength = i + 1;

                            // Set the decoded buffer length (subtract 2 for the FCS)
                            *DecodedBufferLength = DecodedLength - 2;

                            // Set the output 'IsGood' flag based on FCS
                            *IsGood = (Fcs == kGoodFcs);
                        }

                        return true;
                    }
                    default:
                    {
                        Fcs = UpdateFcs(Fcs, byte);
                        DecodedLength++;
                        if (DecodedBuffer)
                        {
                            if (*DecodedBufferLength >= DecodedLength)
                            {
                                DecodedBuffer[DecodedLength - 1] = byte;
                            }
                        }
                        break;
                    }
                }
                break;
            }
            case kStateEscaped:
            {
                if (i + 1 < *EncodedBufferLength)
                {
                    byte ^= 0x20;
                    Fcs = UpdateFcs(Fcs, byte);
                    DecodedLength++;
                    if (DecodedBuffer)
                    {
                        if (*DecodedBufferLength >= DecodedLength)
                        {
                            DecodedBuffer[DecodedLength - 1] = byte;
                        }
                    }

                    state = kStateSync;
                }
                else
                {
                    // Not enough buffer
                }
                break;
            }
        }
    }

    return false;
}