Assign a hex value to the low nibble of a byte

Question

I am doing an integration and I have the following condition for a field:

If the length of the data is odd, the low nibble of the last byte is assigned the value hex 'F'. This hex 'F' for padding ensures that a whole number of bytes are used for the field and is not included in the length of the item.

I tried appending the hex F, but this is wrong:

data << "%X" % 15

I suppose I need to get the last byte and perform some magic on it, probably some bitwise operation:

low_nibble = data.bytes.last.get_low_nibble
low_nibble = transform_low_nibble_to_hex
data << low_nibble

I will be glad if someone can point me in the right direction.

Answer 1

The binary representation of 0xF is 0b1111. Conveniently, this is all 1s, and an easy way to ensure a bit is 1, is to OR it with 1. So, an easy way to ensure all the bits in a nibble are 1 is to OR the nibble with 0b1111 / 0xF / 15.

Accessing a single nibble is usually inconvenient, but thankfully, there is also an easy way to ensure that a bit stays what it was: OR it with 0. Therefore, in order to ensure that the first nibble of the byte stays what it was, we need to OR the first nibble with 0b0000, and to ensure that the last nibble is 0xF, we need to OR it with 0b1111.

Put that all together, we need to OR the entire last octet with 0b00001111 / 0x0F (which is just 0b1111 / 0xF):

data.bytes.last |= 0xF