Encrypt message code review March 10, 2018 8:00 PM

EncryptMessage_codereview.cpp

#include <iostream>
#include <string>

using namespace std;

string decrypt( const string& word ) 
{
  // your code goes here
  
  string source(word.length(),' ');
  int i = 0;
  int prefixSum = 0; 
  
  for(char c : word){
    source[i] = word[i] - 1 - prefixSum; 
    prefixSum += source[i];
    
    while(source[i] <'a' || source[i] > 'z')
      source[i] += 26;
    
    i++;
  }
  
  return source;
}

int main() {
  return 0;
}

/*
a b c d e f g h i j k l m n o p q r s t u v w x y z
d n o t q

c 

source[0] = encrypt[0] - 1
source[1] = (encrypt[1] - 1 - source[0]) >=a && <= z 
while(source[i] >= a && <=z)
source[i] += 26

114 + 100 -> 214 -> 26 * n -> a - z
step 2 number it is sum of series of original chars: 
99 + 1
99 + 1 + 114
...

encrypt[i] = 1 + source[0] + source[1] +...+ source[i] - 26 * ni -> a - z

algebra equation on line 29 -> source[i] -> right hand size, move other terms to left side
 source[i] -> induction
 encrypt[i] - 1 - source[0] - ... - source[i - 1] + 26 * ni -> 
*/