ShiftCipher

import java.lang.Math;
import java.util.ArrayList;

public class ShiftCipher {

  private String[][] wordBank = {
    {"a", "i"},
    {"an", "or", "if", "is", "am", "be", "to", "of"},
    {"and", "the", "for", "but", "are", "not", "you"}
  };

  public String encode(String message) {
    int offset = (int)(Math.random()*25) + 1;
    return encode(message, offset) + "\n" + offset;
  }

  public String encode(String message, int offset) {
    String result = "";
    for (int i = 0; i < message.length(); i++) {
      int n = (int)message.toLowerCase().charAt(i);
      if (n >= 97 && n <= 122) {
        n += offset - 97;
        n = n % 26 + 97;
      }
      char c = (char)n;
      result += c;
    }
    return result;
  }

  public String decode(String message) {
    ArrayList<SolvedShift> matches = new ArrayList<SolvedShift>();
    int[] lettersUsed = new int[26];

    for (int i = 1; i < wordBank.length + 1; i++) {
      String[] clumps = findClumps(message, i);
      for (String c : clumps) {
        for (String word : wordBank[i - 1]) {
          int offset = scanWord(c, word);
          if (offset > -1) {
            if (lettersUsed[offset] <= 0) {
              lettersUsed[offset] = matches.size() + 1;
              matches.add(new SolvedShift(decode(message, offset), offset, word));
            } else {
              matches.get(lettersUsed[offset] - 1).addKeyword(word);
            }
            //matches.add(new SolvedShift(decode(message, offset), offset, word));
          }
          //return c + " " + word;
        }
      }
    }
    return formatList(matches.toArray(new SolvedShift[matches.size()]));
  }

  public String decode(String message, int offset) {
    String result = "";
    for (int i = 0; i < message.length(); i++) {
      int n = (int)message.toLowerCase().charAt(i);
      if (n >= 97 && n <= 122) {
        n += (26 - offset) - 97;
        n = n % 26 + 97;
      }
      char c = (char)n;
      result += c;
    }
    return result;
  }


  public String[] findClumps(String message, int length) {
    message += " ";
    ArrayList<String> clumps = new ArrayList<String>();
    int count = 0;
    for (int i = 0; i < message.length(); i++) {
      int n = (int)message.toLowerCase().charAt(i);

      if (n >= 97 && n <= 122) {
        count++;
      } else if (count == length) {
        clumps.add(message.toLowerCase().substring(i - length, i));
        count = 0;
      } else {
        count = 0;
      }
    }
    return clumps.toArray(new String[clumps.size()]);
  }

  public int scanWord(String message, String word) {
    int offset = (int)message.charAt(0) - (int)word.charAt(0);
    if (offset < 0) {
      offset += 26;
    }
    if (decode(message, offset).equals(word)) {
      return offset;
    }
    return -1;
  }

  public String formatList(SolvedShift[] ciphers) {

    for (SolvedShift s:ciphers) {
      s.cleanKeywords();
    }

    for (int i = 0; i < ciphers.length; i++) {
      int index = i;
      for (int n = i; n < ciphers.length; n++) {
        if (ciphers[n].compareTo(ciphers[index]) < 0) {
          index = n;
        }
      }
      SolvedShift temp = ciphers[i];
      ciphers[i] = ciphers[index];
      ciphers[index] = temp;
    }

    String result = "";
    for(SolvedShift s : ciphers) {
      result += "Response: " + s.message + "\nOffset: " + s.offset;
      if (s.keywords.length > 1) {
        result += "\nBased on keywords: ";
        for (String k : s.keywords) {
          result += k + ", ";
        }
        result = result.substring(0, result.length() - 2);
      } else {
        result += "\nBased on keyword: " + s.keywords[0];
      }
      result += "\n\n";
    }
    if (ciphers.length == 0) {
      result = "Decoding failed.";
    }
    return result;
  }

  public void error(String text) {
    throw new IllegalArgumentException(text);
  }

}