/*********************************************
** modbus_req_canEstimateLength()
** modbus_rsp_canEstimateLength()
**
**   DESC: Given an arriving Modbus message, see if can estimate it's length
**
**  INPUT: pBuf = ptr to modbus/RTU message (starting at slave address)
**         len  = how many bytes are in pBuf
**
** OUTPUT: rtn > 0, then this is the estimated length with CRC16
**         rtn == EXPECT_NOT_YET, then we need to receive more bytes
**         rtn == EXPECT_CANNOT, then unsupported function code
**
**********************************************/

/*
Example psuedo-code to use

#define EXPECT_NOT_YET  0xFFFE
#define EXPECT_CANNOT   0xFFFF

unit16 len = 0;
uint16 expected = EXPECT_NOT_YET;
uint8  pBuf[256];

while( len < expected)
{
    get_next_byte() - non-blocking or wait for char_timeout
    handle_timeout() - break from while & check CRC

    if( expected == EXPECT_NOT_YET)
    {
        if( is_request)
            expected = modbus_req_canEstimateLength( pBuf, len);
        else
            expected = modbus_rsp_canEstimateLength( pBuf, len);
    }
}
*/

// estimate requests
uint16 modbus_req_canEstimateLength( uint8 *pBuf, uint16 len)
{
    uint16 expected = EXPECT_NOT_YET;

    if( len >= 2)
    { // then we have at least the slave address and function code

        switch( pBuf[1])
        {
        case 1:   // read N coils; func 1 for 0x00000 area
        case 2:   // read N status; func 2 for 1x00000 area
        case 3:   // read N regs; func 3 for 4x00000 area
        case 4:   // read N inputs; func 4 for 3x00000 area
        case 5:   // write 1 coil; func 5 for 0x00000 area
        case 6:   // write 1 reg; func 6 for 4x00000 area
            // these are: SS FF AA AA LL LL CR CR
            expected = 8;
            break;

        case 15:  // write N coil; func 15 for 0x00000 area
        case 16:  // write N regs; func 16 for 4x00000 area
            if( len >= 7)
            { // then have byte count
                // these are: SS FF AA AA LL LL BC ?? CR CR
                expected = 9 + pBuf[6];
            }
            break;

        default:
            // else we have no idea which function this is
            // you can add any additional function codes yourself
            expected = EXPECT_CANNOT;
            break;
        }
    }
    return( expected);
}

// estimate responses
uint16 modbus_rsp_canEstimateLength( uint8 *pBuf, uint16 len)
{
    uint16 expected = EXPECT_NOT_YET;

    if( len >= 2)
    { // then we have at least the slave address and function code

        if( pBuf[1] & 0x80)
        { // then is expection response
            // these are: SS FF CD CR CR
            expected = 5;
        }
        else switch( pBuf[1])
        {
        case 1:   // read N coils; func 1 for 0x00000 area
        case 2:   // read N status; func 2 for 1x00000 area
        case 3:   // read N regs; func 3 for 4x00000 area
        case 4:   // read N inputs; func 4 for 3x00000 area
        case 5:   // write 1 coil; func 5 for 0x00000 area
        case 6:   // write 1 reg; func 6 for 4x00000 area
            if( len >= 3)
            { // then have byte count
                // these are: SS FF BC ?? CR CR
                expected = 5 + pBuf[2];
            }
            break;

        case 15:  // write N coil; func 15 for 0x00000 area
        case 16:  // write N regs; func 16 for 4x00000 area
            // these are: SS FF AA AA LL LL CR CR
            expected = 8;
            break;

        default:
            // else we have no idea which function this is
            // you can add any additional function codes yourself
            expected = EXPECT_CANNOT;
            break;
        }
    }
    return( expected);
}