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package net.alessandropetrozzelli;

class InvalidISINCharacterException
    extends Exception {
  InvalidISINCharacterException() {
    super();
  }

  InvalidISINCharacterException(String s) {
    super(s);
  }
}

public class Isin {
  static final int numberTableOffset = '0';
  static final int numberTableEndOffset = '9';
  static final int charTableOffset = 'A';
  static final int charTableEndOffset = 'Z';

  private static int getIntValueForISINChar(final char c) throws
      InvalidISINCharacterException {
    int value = 0;

    if ( (c >= numberTableOffset) && (c <= numberTableEndOffset)) {
      value = c - numberTableOffset;
    }
    else if ( (c >= charTableOffset) && (c <= charTableEndOffset)) {
      value = 10 + (c - charTableOffset);
    }
    else {
      throw new InvalidISINCharacterException("Unsupported character: " + c);
    }

    return value;
  }

  private static int getSumOfDigits(final int number) {
    int sumOfDigits=0;
    if (number<=9) {
      sumOfDigits = number;
    } else {
      String s = String.valueOf(number);
      try {
        for (int i = 0; i < s.length(); i++) {
          sumOfDigits += getIntValueForISINChar(s.charAt(i));
        }
      }
      catch (InvalidISINCharacterException ex) {
        System.out.println(ex);
        return -1;
      }
    }
    return sumOfDigits;
  }

  public static int getCheckDigit(final String isin) {
    int checkDigit = 0;

    if ( (isin == null) || (isin.length() != 11)) {
      return -1;
    }
    String incompleteISINcode = isin.toUpperCase();
    StringBuffer buffer = new StringBuffer();

    // convert every single character into number as specified
    try {
      for (int i = 0; i < incompleteISINcode.length(); i++) {
        buffer.append(getIntValueForISINChar(isin.charAt(i)));
      }
    }
    catch (InvalidISINCharacterException ex) {
      System.out.println(ex);
      return -1;
    }

    // sum all digits (multiplying odd digits by 2)
    try {
      for (int i = buffer.length() - 1; i >= 0; i--) {
        if ( (buffer.length() - i) % 2 == 1) {
          checkDigit += getSumOfDigits(getIntValueForISINChar(buffer.charAt(i)) * 2);
        }
        else {
          checkDigit += getIntValueForISINChar(buffer.charAt(i));
        }
      }
    }
    catch (InvalidISINCharacterException ex) {
      // this should never happen
      System.out.println(ex);
      return -1;
    }

    // end result
    checkDigit = (10 - (checkDigit % 10)) % 10;
    return checkDigit;
  }