getbits.c

随机数算法

/*  Get bits from central repository of data.
	Enter with starting pointer into source, a destination pointer, the
	endianness and number of bits to get.
	
	Returns with new starting pointer and bits shifted to lsb of
	correct offset depending on endianness.  do bounds checking externally.
*/

#include "randtest.h"

void getbits( SRCPTR *src, unsigned char *dst, int endian, int numbits)
{
	unsigned long bitstore;
	int mscount, bp, shift, mask, sp;
	
/*  figure out how many bytes to deal with  */

	if(numbits & 7)	mscount = numbits/8;
	else mscount = (numbits-1)/8;
	
/*  copy data to destination.
	For big endian, work lsb to msb by going from high offset to low.
	For little endian, lsb to msb goes from low offset to high.
*/
	if (endian)
	{
		bp = mscount;		// start at end
		sp = (numbits + 7 - src->bitpos)/8; // end pointer in source
		if (! (numbits & 7)) sp--;  // adjust for perfect boundary
		shift = (src->bitpos - numbits + 1) % 8;    // find last bit, +1 for shift amount
		if ( shift < 0) shift += 8;

		while( bp >= 0)
		{
			if( !shift) dst[bp] = (src->byteptr)[sp];
			else
			{	// combine two bytes in big endian order
				bitstore = (src->byteptr)[sp-1] << 8 | (src->byteptr)[sp];
				dst[bp] = (bitstore >> shift) & 0xff;  // and keep correct 8 of them
			}
			bp--;
			sp--;
		}
		if( (numbits & 7) )
		{
		  mask = 0xff >> 8 - (numbits %8);  // then clear out
		  dst[0] &= mask;  // unused msb's
		}

/*  bits copied, now update source pointer  */

		src->byteptr += (numbits + 7 - src->bitpos)/8;
		src->bitpos = (src->bitpos - numbits) % 8;
		if( src->bitpos < 0) src->bitpos += 8;
	}
	else  // little endian
	{
		shift = src-> bitpos;
		for( bp = 0; bp <= mscount; bp++)
		{
			if( !shift) dst[bp] = (src->byteptr)[bp];
			else
			{ 	// get two bytes in little endian order
				bitstore = (src->byteptr)[bp+1] << 8 | (src->byteptr)[bp];
				dst[bp] = (bitstore >> shift) & 0xff;
			}
//			printf("dst[%d]=%x\n", bp, dst[bp]);
		}
		mask = (8 - numbits) % 8; // actually (src->bitpos + d - shift)%8, but terms cancel
		if (mask < 0) mask += 8;
		if( mask) dst[mscount] &= (0xff >> mask);

/*  bits copied, now update source pointer  */

		src->byteptr += (numbits + src->bitpos)/8;
		src->bitpos = (src->bitpos + numbits) % 8;
	}
	
//printf("new pointer = %x, offset = %d\n", src->byteptr, src->bitpos); 
}  // end of getbits