Digital_cypher_vol_2.cs

using System.Linq;

namespace CodingChallenges;
[TestClass]
public class Digital_cypher_vol_2 {

    /*

    Digital cypher vol 2

    Digital Cypher assigns to each letter of the alphabet unique number. For example:

     a  b  c  d  e  f  g  h  i  j  k  l  m
     1  2  3  4  5  6  7  8  9 10 11 12 13
     n  o  p  q  r  s  t  u  v  w  x  y  z
    14 15 16 17 18 19 20 21 22 23 24 25 26
    Instead of letters in encrypted word we write the corresponding number, eg. The word scout:

     s  c  o  u  t
    19  3 15 21 20

    Then we add to each obtained digit consecutive digits from the key. For example. In case of key equal to 1939 :

     s  c  o  u  t
    19  3 15 21 20
   + 1  9  3  9  1
    ---------------
    20 12 18 30 21

     m  a  s  t  e  r  p  i  e  c  e
    13  1 19 20  5 18 16  9  5  3  5
  +  1  9  3  9  1  9  3  9  1  9  3
    --------------------------------
    14 10 22 29  6 27 19 18  6  12 8

    Task
    Write a function that accepts an array of integers code and a key number. As the result, it should return string containg a decoded message from the code.

    Input / Output
    The code is a array of positive integers.
    The key input is a nonnegative integer.

    Example
    Decode(new int[]{ 20, 12, 18, 30, 21},1939);  ==> "scout"
    Decode(new int[]{  14, 10, 22, 29, 6, 27, 19, 18, 6, 12, 8},1939);  ==>  "masterpiece"


    */

    [TestMethod]
    public void Test() {

        Assert.AreEqual("scout", Decode(new int[] { 20, 12, 18, 30, 21 }, 1939));
        Assert.AreEqual("masterpiece", Decode(new int[] { 14, 10, 22, 29, 6, 27, 19, 18, 6, 12, 8 }, 1939));
    }

    public static string Decode(int[] code, int key) {
        char[] codepage = " abcdefghijklmnopqrstuvwxyz".ToCharArray();
        int[] keyArr = $"{key}".Select(c => c - '0').ToArray();
        int keyLength = keyArr.Length;
        int keyIndex = 0;
        char[] resultArray = new char[code.Length];
        foreach (int codeInd in code.Select((_, ind) => ind)) {
            int rInt = code[codeInd] - keyArr[keyIndex];
            rInt = rInt < 0 || rInt >= codepage.Length ? 0 : rInt;
            resultArray[codeInd] = codepage[rInt];
            keyIndex = ++keyIndex % keyLength;
        }
        return string.Concat(resultArray);
    }

}