package au.gov.amsa.ais;
   
   import au.gov.amsa.util.SixBit;
   import au.gov.amsa.util.SixBitException;
   
   /**
    * Utility class for extracting parts of an ais message as unsigned integers,
    * signed integers or strings.
    * 
   * @author dxm
   * 
   */
  public class AisExtractor {
  
      private final boolean[] bitSet;
      private final boolean[] calculated;
      private final int padBits;
      private final String message;
  
      /**
       * Constructor. If message once decoded is less than minLength then throws
       * {@link AisParseException}.
       * 
       * @param message
       * @param minLength
       */
      public AisExtractor(String message, Integer minLength, int padBits) {
          if (message.length() == 0)
              throw new AisParseException("message length cannot be 0");
          if (padBits > 6 || padBits < 0)
              throw new AisParseException("padBits must be between 0 and 6");
          this.message = message;
          boolean[] bits = new boolean[message.length() * 6 - padBits];
          boolean[] calculated = new boolean[message.length()];
          this.bitSet = bits;
          this.calculated = calculated;
          this.padBits = padBits;
          if (minLength != null && bitSet.length < minLength) {
              throw new AisParseException(AisParseException.NOT_CONSISTENT_DECODED_STRING
                      + ", length was " + bitSet.length + " and should be >=" + minLength);
          }
      }
  
      /**
       * Returns the message id field (the first 6 characters of the decoded
       * message).
       * 
       * @return
       */
      public int getMessageId() {
          return getValue(0, 6);
      }
  
      /**
       * Returns an unsigned integer value using the bits from character position
       * start to position stop in the decoded message.
       * 
       * @param from
       * @param to
       * @return
       */
      public synchronized int getValue(int from, int to) {
          try {
              // is synchronized so that values of bitSet and calculated can be
              // lazily
              // calculated and safely published (thread safe).
              SixBit.convertSixBitToBits(message, padBits, bitSet, calculated, from, to);
              return (int) SixBit.getValue(from, to, bitSet);
          } catch (SixBitException | ArrayIndexOutOfBoundsException e) {
              throw new AisParseException(e);
          }
      }
  
      /**
       * Returns a signed integer value using the bits from character position
       * start to position stop in the decoded message.
       * 
       * @param from
       * @param to
       * @return
       */
      public synchronized int getSignedValue(int from, int to) {
          try {
              // is synchronized so that values of bitSet and calculated can be
              // lazily
              // calculated and safely published (thread safe).
              SixBit.convertSixBitToBits(message, padBits, bitSet, calculated, from, to);
              return (int) SixBit.getSignedValue(from, to, bitSet);
          } catch (SixBitException e) {
              throw new AisParseException(e);
          }
      }
  
      public synchronized String getString(int from, int to) {
          try {
              // is synchronized so that values of bitSet and calculated can be
              // lazily
              // calculated and safely published (thread safe).
              SixBit.convertSixBitToBits(message, padBits, bitSet, calculated, from, to);
             return SixBit.getString(from, to, bitSet);
         } catch (SixBitException e) {
             throw new AisParseException(e);
         }
     }
 
 }