CSC 15 Lab 8b

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For today's lab you are to do the following:

1. Finish Lab 7. For part 2, however, I've set up a network server you
can download from.  You need to use the code found in "client1.py" to contact
the professor's computer running on a server.  You will send the prof a
message, and the prof will send you back a string, ENCRYPTED.  You have
to save it to a file, then decrypt the string.

Use the lettermap.key file for the character map (permutation of size 128)


2.  Below you will find some of the functions to convert between numbers
and binary strings.  At the end there is a function to pack a 16-character
DNA sequence into a single 32-bit two's complement integer.  You need to
write the inverse function, which will convert the number back to a sequence
consisting of 'A', 'C', 'G' and 'T'.



# function to represent a non-negative integer as a string of size bits
def binform(n,bits):  # n is an int, returns string
    if (n> 2**bits-1 or n< -1 * 2**bits): raise Exception("not enough bits")
    s = ""  # binary string to be built
    while n>0:
        d = n%2  # next digit from right end
        s = str(d) + s  # construct s backwards
        n = n/2
    # while n
    ## now sign-extend string to be of size bits
    s = "0"*(bits-len(s)) + s
    return s
## binform


## function to invert a string of bits
def invertbits(s):  # s is a string, returns string
    D = ['1','0']  # inversion map
    t = ""
    i = 0
    while i<len(s):
        t = t + D[int(s[i])]
        i += 1
    return t
## invertbits
# function to convert a number into a two's complement representation


## function to convert complement number x, in size bits, to binary string
def twosform(x,bits): # x is an int, returns string
    if x>=0:
        return binform(x,bits)
    else:
        return invertbits(binform(-x-1, bits))
## twosform

#print twosform(-4,3)  # prints "100"



############### inverse operations

## convert string of bits to integer - non-negative only version
def bitstonn(s):  # s is string, returns int
    ax = 0
    base = 2**(len(s)-1)
    i = 0
    while i<len(s):
        ax = ax + (int(s[i]) * base)
        base = base/2
        i += 1
    # while
    return ax
##

#print bitstonn("1110")   # prints 14

## convert to two's complement number
def bitstotc(s):   # s is string, returns int
    sign = int(s[0])
    if sign==0: return bitstonn(s)
    else: return -(1 + bitstonn(invertbits(s)))
## bitstotc

#print bitstotc("100")  # prints -4


################################################

#########  pack 16-DNA sequence into single 32 bit tc-integer
## 00-A 01-C 10-G 11-T

## hash table for DNA representation
dnahash = {'A':"00", 'C':"01", 'G':"10", 'T':"11"}

def dnatoint(d):  # d is string of dna symbols, returns int
    if len(d)!=16: raise Exception("wrong size")
    s = ""
    for a in d: s = s+dnahash[a]
    return bitstotc(s)
##

#print dnatoint("TAGCTGGTCGTCCAAT")


### inverse function ####### leave this part to students

#print inttodna(dnatoint("TAGCTGGTCGTCCAAT"))