Method setField2

GPRS/ByteVector.cpp:435–469

Write a bit field starting at specified byte and bit, each numbered from 0

// Write a bit field starting at specified byte and bit, each numbered from 0 
void ByteVector::setField2(size_t byteIndex, size_t bitIndex, uint64_t value,unsigned lengthBits)
{
	BVASSERT(bitIndex >= 0 && bitIndex <= 7);
	// Example: bitIndex = 2, length = 2;
	// 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7
	// 0   0   X   X   0   0   0   0
	// endpos = 4; nbytes = 0; lastbit = 4; nbits = 2; mask = 3; shift = 4;

	unsigned endpos = bitIndex + lengthBits;	// 1 past the 0-based index of the last bit.
	unsigned nbytes = (endpos-1) / 8;		// number of bytes that will be modified, minus 1.
	ByteType *dp = mStart + byteIndex + nbytes;

	unsigned lastbit = endpos % 8;	// index of first bit not to be replaced, or 0.
	// Number of bits to modify in the current byte, starting at the last byte.
	unsigned nbits = lastbit ?  MIN(lengthBits,lastbit) : MIN(lengthBits,8);

	for (int len = lengthBits; len > 0; dp--) {
		// Mask of number of bits to be modified in this byte, starting from LSB.
		unsigned mask = (1 << nbits) - 1;
		ByteType val = value & mask;
		value >>= nbits;
		if (lastbit) {
			// Shift val and mask so they are aligned with the bits to modify in the last byte,
			// noting that we modify the last byte first, since we work backwards.
			int shift = 8 - lastbit;
			mask <<= shift;
			val <<= shift;
		}
		*dp = (*dp & ~mask) | (val & mask);
		len -= nbits;
		nbits = MIN(len,8);

		lastbit = 0;
	}
}

void ByteVector::appendField(uint64_t value,unsigned lengthBits)
{