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/* bits.c - bitfield extraction code
*
* This file is Copyright (c)2010 by the GPSD project
* BSD terms apply: see the file COPYING in the distribution root for details.
*
* Bitfield extraction functions. In each, start is a bit index (not
* a byte index) and width is a bit width. The width bounded above by
* the bit width of a long long, which is 64 bits in all standard data
* models for 32- and 64-bit processors.
*
* The sbits() function assumes twos-complement arithmetic.
*/
#include <assert.h>
#include "bits.h"
#ifdef DEBUG
#include <stdio.h>
#include "gpsd.h"
#endif /* DEBUG */
#define BITS_PER_BYTE 8
unsigned long long ubits(char buf[], unsigned int start, unsigned int width)
/* extract a (zero-origin) bitfield from the buffer as an unsigned big-endian long long */
{
unsigned long long fld = 0;
unsigned int i;
unsigned end;
/*@i1@*/ assert(width <= sizeof(long long) * BITS_PER_BYTE);
for (i = start / BITS_PER_BYTE;
i < (start + width + BITS_PER_BYTE - 1) / BITS_PER_BYTE; i++) {
fld <<= BITS_PER_BYTE;
fld |= (unsigned char)buf[i];
}
#ifdef DEBUG
(void)printf("%d:%d from %s:\n", start, width, gpsd_hexdump(buf, 32));
#endif
#ifdef DEBUG
(void)printf(" segment=0x%llx,", fld);
#endif /* DEBUG */
end = (start + width) % BITS_PER_BYTE;
if (end != 0) {
fld >>= (BITS_PER_BYTE - end);
#ifdef DEBUG
(void)printf(" after downshifting by %d bits: 0x%llx",
BITS_PER_BYTE - end, fld);
#endif /* UDEBUG */
}
#ifdef DEBUG
(void)printf(" = %lld\n", fld);
#endif /* UDEBUG */
/*@ -shiftimplementation @*/
fld &= ~(-1LL << width);
/*@ +shiftimplementation @*/
#ifdef DEBUG
(void)
printf(" after selecting out the bottom %u bits: 0x%llx = %lld\n",
width, fld, fld);
#endif /* DEBUG */
return fld;
}
signed long long sbits(char buf[], unsigned int start, unsigned int width)
/* extract a bitfield from the buffer as a signed big-endian long */
{
unsigned long long fld = ubits(buf, start, width);
#ifdef SDEBUG
(void)fprintf(stderr, "sbits(%d, %d) extracts %llx\n", start, width, fld);
#endif /* SDEBUG */
/*@ +relaxtypes */
if (fld & (1 << (width - 1))) {
#ifdef SDEBUG
(void)fprintf(stderr, "%llx is signed\n", fld);
#endif /* SDEBUG */
/*@ -shiftimplementation @*/
fld |= (-1LL << (width - 1));
/*@ +shiftimplementation @*/
}
#ifdef SDEBUG
(void)fprintf(stderr, "sbits(%d, %d) returns %lld\n", start, width,
(signed long long)fld);
#endif /* SDEBUG */
return (signed long long)fld;
/*@ -relaxtypes */
}
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