HEX = "0123456789ABCDEF"    

# takes decimal int and return a hex string, the string should have exactly
# length d (number of hex digits), unless more are needed to represent n
def dechex(n,d): 
    hs = ""
    while n>0 or d>0:
        last4 = n%16
        hs =  HEX[last4] + hs
        n = n//16
        d -= 1
    #while
    return hs
#dechex


XEH = {}
i = 0
while i<16:
    XEH[HEX[i]] = i
    i += 1
#while

# hs is a hex string such as "F7B", and return decimal int
def hexdec(hs):  
    n = 0
    i = 0
    while i<len(hs):
        digit = XEH[ hs[i] ]
        n += digit
        if i<=len(hs)-2: n=n*16
        i +=1
    #
    """
    i = len(hs)-1
    while i>=0:
      digit = XEH[ hs[i] ] # 0-15
      n += digit* (16**(len(hs)-1-i))
      i -= 1
    #
    return n
    """
    return n
#hexdec

h = dechex(123,4)
print(h)
print(hexdec(h))