//-------------------------------------------------------------------
//	usbcrc16.cpp
//
//	This program illustrates the steps needed in order compute
//	the 16-bit Cyclic Redundancy Check for a USB data-packet.
//
//	programmer: ALLAN CRUSE
//	written on: 13 JUN 2011
//-------------------------------------------------------------------

#include <stdio.h>			// for printf() 

#define USB_POLY16	0x18005		// USB CRC-16 polynomial

unsigned int mod2( unsigned int arg32 )
{
	unsigned int	temp = arg32;
	unsigned int	parm = USB_POLY16 << 15;
	unsigned int	pop;
	
	int	bits_to_do = 8;
	while ( bits_to_do > 0 )
		{
		pop = ( temp >> 31 ) & 1;
		if ( pop ) temp = (temp ^ parm);
		
		temp <<= 1;

		--bits_to_do;
		}

	return	temp;
}

unsigned int bit_reflect08( unsigned char arg08 )
{
	unsigned int	temp = 0;
	for (int i = 0; i < 8; i++)
		{
		temp <<= 1;
		if ( (arg08 >> i) & 1 ) temp |= 1;
		}
	return	temp;
}

unsigned int bit_reflect16( unsigned short arg16 )
{
	unsigned int	temp = 0;
	for (int i = 0; i < 16; i++)
		{
		temp <<= 1;
		if ( (arg16 >> i) & 1 ) temp |= 1;
		}
	return	temp;
}

unsigned int bit_reflect32( unsigned int arg32 )
{
	unsigned int	temp = 0;
	for (int i = 0; i < 32; i++)
		{
		temp <<= 1;
		if ( (arg32 >> i) & 1 ) temp |= 1;
		}
	return	temp;
}

unsigned int crc16( unsigned char octet, unsigned short dst16 )
{
	// Algorithm is adapted from pseudocode describing the
	// operation of Intel's x86_64 'crc32' instruction, in
	// "Volume 2A: Instruction Set Reference", of Intel 64  
	// and IA-32 Architecture Software Developer's Manual.

	unsigned int	temp1 = bit_reflect08( octet );
	unsigned int	temp2 = bit_reflect16( dst16 );
	unsigned int	temp3 = temp1 << 24;
	unsigned int	temp4 = temp2 << 16;
	unsigned int	temp5 = temp3 ^ temp4;
	unsigned int	temp6 = mod2( temp5 );
	unsigned int	temp7 = bit_reflect32( temp6 );

	return	temp7;
}

unsigned char	data[ 8 ] =	// From 'Get_Descriptor' transaction 
		{ 0x80, 0x06, 0x00, 0x01, 0x00, 0x00, 0x40, 0x00 };

int main( int argc, char **argv )
{
	// display the packet's data-field
	printf( "\n Packet-Data: " );
	for (int i = 0; i < sizeof( data ); i++) printf( "%02X ", data[i] );
	printf( "\n" );

	// seed the register with all 1's
	unsigned short	crc = 0xFFFF;

	// perform the polynomial division in GF(2)
	for (int i = 0; i < 8; i++) crc = crc16( data[i], crc );

	// complement the register-bits
	crc = (crc ^ 0xFFFF);

	// Report the 16-bit CRC value
	printf( "\n crc16=0x%04X \n\n", crc );
}

//--------------------------------------------------------------------
// REFERENCE: USB Specification, Revision 2.0 (April 2000), page 198.
//--------------------------------------------------------------------