Sabrina Markon - Algorithms - Ceasar Cipher. We are given a ROT13 string and must decode it to find the secret message.

algorithm-use-javascript-charCodeAt-and-String.fromCharCode-to-decode-ROT13-shifted-message.txt

/*
One of the simplest and most widely known ciphers is a Caesar cipher, also known as a shift cipher.
In a shift cipher the meanings of the letters are shifted by some set amount.

A common modern use is the ROT13 cipher, where the values of the letters are shifted by 13 places. 
Thus 'A' ↔ 'N', 'B' ↔ 'O' and so on.

Write a function which takes a ROT13 encoded string as input and returns a decoded string.

All letters will be uppercase. 
Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on.

HTML Character Codes: http://www.ascii.cl/htmlcodes.htm
*/

function rot13(str) { // LBH QVQ VG!
  
  var newcharstr = '';
  var newchar = '';
  
  for (var i = 0; i < str.length; i++) {
    
    var thechar = str.charCodeAt(i);
      
      // only for upper case A-Z which is character codes 64-90 inclusive:
      if (thechar >= 64 && thechar <= 90) {
      
        // A-Z is a cycle from A-to-Z-to-A-to-Z like 64-to-90-to-64-to-90...
        if (thechar > 77) {
          // second half of the alphabet so subtract 13
          newchar = String.fromCharCode(thechar - 13);
        } else {
          // first half of the alphabet so add 13
          newchar = String.fromCharCode(thechar + 13);
        }        
      
      } else {
        // if it isn't A-Z, just use the same character ie. spaces, punctuation.
        newchar = str[i];
      }

    newcharstr += newchar;
    
  }
  
  return newcharstr;

}

// Change the inputs below to test
rot13("SERR PBQR PNZC");