Method getField2

GPRS/ByteVector.cpp:492–514

Read a bit field starting at specified byte and bit, each numbered from 0. Bit numbering is from high to low, like this: getField bitIndex: 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 Note that this is inverted with respect to the numbering scheme used in many GSM specs, which looks like this: GSM specs: 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 Note that some GSM specs use low-to-high and some use high-to-low numbering. G

// Generally, where the BitVector class is used they use low-to-high numbering,
// which is rectified in the FEC classes by the byteswapping the BitVector before being used.
uint64_t ByteVector::getField2(size_t byteIndex, size_t bitIndex, unsigned lengthBits) const
{
	ByteType *dp = mStart + byteIndex;
	int len = (int) lengthBits;
	BVASSERT(bitIndex >= 0 && bitIndex <= 7);
	// Get first byte:
	// This was for bitIndex running from low bit to high bit:
	// int nbits = bitIndex+1;	// Number of bits saved from byte.
	// This is for bitIndex running from 0=>high bit to 7=>low bit:
	int nbits = 8-bitIndex;	// Number of bits saved from byte, ignoring len restriction.
	// Example: bitIndex=3 => 0 | 0 | 0 | X | X | X | X | X  => AND with 0x1f

	uint64_t accum = *dp++ & (0x0ff >> (8-nbits));	// Preserve right-most bits.
	if (len < nbits) { accum >>= (nbits - len); return accum; }
	len -= nbits;

	// Get the full bytes:
	for (; len >= 8; len -= 8) { accum = (accum << 8) | *dp++; }

	// Append high bits of last byte:
	if (len>0) { accum = (accum << len) | (*dp >> (8-len)); }
	return accum;
}

// This is static - there is no 'this' argument.
int ByteVector::compare(const ByteVector &bv1, const ByteVector &bv2)