Explaining code from https://twitter.com/nkizz11/status/1370046590140973059

hexc2dec.py

def hexc2dec(bufp):
    """
    Returns the decimal value of bufp[1:5],
    where bufp[1:5] is the hexadecimal representation of a 16-bit number
    in two's complement.
    """
    # This variable accumulates our final answer
    newval = 0
    
    divvy = 4096
    for pn in range(1,5):
        # divvy is the power of 16 corresponding to the hex digit at index pn
        # If pn=1, then divvy is 16^3
        # If pn=2, then divvy is 16^2
        # If pn=3, then divvy is 16^1
        # If pn=4, then divvy is 16^0

        # Get the ASCII code of this character
        vally = ord(bufp[pn])
        # 97 is ord("a"), so vally < 97 means that bufp[pn] is a normal digit
        if vally < 97:
            # Since ord("0")=48, ord("1")=49, ..., ord("8")=56, ord("9")=57
            # the value of the hex digit bufp[pn] is ord(bufp[pn])-48
            subby = 48
        else:
            # Since ord("a")=97, ord("b")=98, ..., ord("e")=101, ord("f")=102,
            # the value of the hex digit bufp[pn] is ord(bufp[pn])-87
            subby = 87
        # Update newval with the value of bufp[pn] times divvy,
        # which is the appropriate power of 16
        newval += ( ord(bufp[pn]) - subby )*divvy
        # Divide by 16 to update divvy for the next iteration of the loop
        divvy /= 16
        
        # Because this number is in two's complement,
        # we need to subtract by 65536 (2^16)
        # if the highest bit in the number is set.
        # We check if the highest bit in the number is set by checking if newval
        # is greater than 32767 (2^15-1).
        if newval > 32767:
            newval = newval - 65536

    return newval

print(hexc2dec("?0000")) # 0
print(hexc2dec("?71ab")) # 29099
print(hexc2dec("?f100")) # -3840
print(hexc2dec("?ffff")) # -1