###########     CSC 15 Lab 7b

## This lab and lab7 are due by 2:20pm on Thursday 11/14 (firm)

## In this addendum to lab 7, you will decrypt a file (the sample
#  solution to this lab) that's been encrypted in the same way as
#  lab7, except a permutation was also applied to the final, encrypted
#  form of the ascii string.

# Replace 'lab7' below with the filename prefix of your solution to lab 7:

from lab7 import *  # this imports all definitions from lab7.py, 

# On the class homepage you will find two files, solution2.encP, which is
# the encrypted file that you're supposed to decrypt, and solution2.perm,
# which is a (unencrypted) permutation array.  The following code 
# reads in this array and assigns it to Perm:

import json
fd = open("solution2.perm")
Perm = json.load(fd)
fd.close()
#print(Perm)  # make sure you got it

#### The following are some of the code that I used to produce the
# encrypted file.  However, this time, like most of you, I neglected
# to write enough comments for much of the code (HA!). You will have
# to figure out what parts of the code is pertinent to the assignment
# and write your own code to decrypt the file.  The decrypted program
# will have more comments.

def isperm(P):
    n = len(P)
    Cnt = [0]*n 
    answer = True
    i = 0 
    while answer and i<n:
        if P[i]<0 or P[i]>=n: answer=False
        else: Cnt[P[i]] += 1 
        i += 1
    i = 0
    while answer and i<n:  
        if Cnt[i] != 1: answer = False
        i += 1
    return answer

from random import randint
def genperm(n): 
    P = [0]*n
    i = 1
    while i<n:  
        P[i] = i;
        i += 1
    i = 0
    while i<n-1:
        r = randint(i,n-1)
        P[i],P[r] = P[r],P[i]
        i += 1
    return P

#Perm = genperm(size)  
#fd = open("solution2.perm","w")
#json.dump(Perm,fd)
#fd.close()

def invperm(P):
    Q = [0]*len(P)
    i = 0
    while i<len(Q):
        Q[P[i]] = i
        i += 1
    return Q
#invperm

def permuteS(s,P,m):
    if len(P) != len(s)//m or len(s)%m!=0:
        raise Exception("need "+str(len(s)//m)+" permutation")
    ps = "" 
    pmi = 0  
    i = 0    
    while i+m<=len(s):
        si = P[pmi]*m  
        ps = ps + s[si:si+m]
        pmi += 1
        i += m
    return ps


#### Generic functions reading/writing files in text and in binary:

def writebin(filename,s): # write ascii string s to filename (also string)
    fd = open(filename,"wb")
    fd.write(s.encode("ascii"))
    fd.close()
#writebin

def readbin(filename):  # reads file as ascii-encoded string, returns string
    fd = open(filename,"rb")
    s = fd.read()
    fd.close()
    return s.decode("ascii")
#readbin

# read a plain text file, such as this file, and return contents as string
def readtext(filename): 
  fd = open(filename,"r")
  s = fd.read()
  fd.close()
  return s
#readtext

# write a plain text string s to a plain text file
def writetext(filename,s):
    fd = open(filename,"w")
    fd.write(s)
    fd.close()
#writetext

## Use readtext/writetext for unencrypted text files, use readbin/writebin
#  for encrypted files.


def encryptfileP(filename):
   ms = readtext(filename)
   C = encryptS(ms,6)
   CP = permuteS(C,Perm,7) 
   writebin(filename+".encP",CP)