Decrypt message - 30 minutes - written in C programming. Great analysis of the algorithm, good name for function and variable names.

DecryptMessage_30minutes.c

#include <stdio.h>
#include <stdlib.h>


char addUntilASCII(int a){
  while(! ('a' <= a && a <= 'z')){
    a += 26;
  }
  return (char) a;
}

void decrypt(char *word, char *out) {
  // your code goes here
  
  int currStep2 = 1;
  int j=0;
  for(int i=0; word[i] != '\0'; i++ ){
    int diff = word[i] - currStep2;
    char letter = addUntilASCII(diff);
    out[j++] = letter;
    currStep2 += letter;
  }
  out[j] = 0;
}

int main() {
  char *s1 = "dnotq";
  char s2[5] = {0};
  decrypt(s1, s2);
  printf("%s\n", s2);
  return 0;
}

/*
Encryption 
  c = 99         r = 114                                      i 
  + 1 _> 100    100 + 114 = 214                               105
  d            -> 214 - x * 26 in range of [97, 97 + 26]     214 + 105 = 319
                 n                                            0 

Decryption 
  d n o t q
  
  c r i m e 
  
  
  n -> r
  
  114 + 100 - x * 26 = 110  <------- you do not want to find 214, what you want to find 114 , 
  take 110 - 100 first, and then add 26 continuously until it is bigger >= 97
  
  114       - x * 26 = 110 - 100 = 10
  
  114    = 10 + x * 26
  
  110 -> 114, we know 114 in the rang eof a - z
  
  10 + 26 * x -> 97 - 97 + 26
  
  dec + prevStep2 - (26x) = enc
  'a' <= dec <= 'z'
  
  o->i
  enc - currStep2 = -103
  addUntilAscii(-103) == 105
  currStep2 = sum( all the previously decoded letters) + 1
  
  99   114           105                        109 
  100  1+ 99 + 114   1 + 99 + 114 + 105
  
  decrypt(word):
    output = []
    currStep2 = 1
    for enc in word:
      diff = enc = currStep2
      letter = addUntilASCII(diff)
      currStep2 += letter
      
  
*/