Competitive/Collaborative Programming Class

/*
 *  Author: Son Pham
 *    Date: February 18, 2013
 * Purpose: This program untwists twisted messages
 * Problem: 641 -Do the Untwist
 */


import java.util.Scanner;

public class Cryptography {

    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        String read = reader.nextLine();
        String[] split = read.split("\\s+");
        char[] plaincode = new char[28];
        initPCode(plaincode);
        int k = Integer.parseInt(split[0]);
        while(k != 0){
            String message = split[1];
            char[] ciphertext = message.toCharArray();
            int n = ciphertext.length;
            char[] plaintext = new char[n];
            for(int i = 0; i < n; i++){
                int c;
                if(ciphertext[i] == '_')
                    c = 0;
                else if(ciphertext[i] == '.')
                    c = 27;
                else
                    c = (int)ciphertext[i] - 96;
                c = (c+i)%28;
                plaintext[(k*i)%n] = plaincode[c];
            }
            String umessage = "";
            for(char u: plaintext)
                umessage += u;
            System.out.println(umessage);
            read = reader.nextLine();
            split = read.split("\\s+");
            k = Integer.parseInt(split[0]);
        }
        
    }
    
    public static void initPCode(char[] plc){
        plc[0] = '_';
        int i = 1;
        char c = 'a';
        while(i < 27){
            plc[i++] = c++;
        }
        plc[27] = '.';
    }
}

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