import java.util.Calendar;
import java.util.Date;

/**
 * Utility methods for java.util.Date objects.
 * 
 * @author R.Grove
 * @author Chris Mayfield
 * @author Alvin Chao
 * @version 4-5-18
 */
public class DateUtils {

   private static final long MILLIS_PER_DAY = 86400000;

   /**
    * Adds a number of days to a date.
    * 
    * @param date the date (unchanged)
    * @param days number of days to add
    * @return the new date
    */
   public static Date addDays(Date date, int days) {
      Calendar c = Calendar.getInstance();
      c.setTime(date);
      c.add(Calendar.DATE, days);
      return c.getTime();
   }

   /**
    * Compute the number of days from the "from" date to the "to" date. The time
    * fields (hour, minute, second, millisecond) are set to zero before the
    * calculation is made, so that only the year, month, and day are used to
    * calculate the difference.
    * 
    * @param from the date starting the interval
    * @param to the date ending the interval
    * @return number of whole days in the interval
    */
   public static int interval(Date from, Date to) {

      Calendar fromCal = Calendar.getInstance();
      fromCal.setTime(from);
      fromCal.set(Calendar.HOUR, 0);
      fromCal.set(Calendar.MINUTE, 0);
      fromCal.set(Calendar.SECOND, 0);
      fromCal.set(Calendar.MILLISECOND, 0);

      Calendar toCal = Calendar.getInstance();
      toCal.setTime(to);
      toCal.set(Calendar.HOUR, 0);
      toCal.set(Calendar.MINUTE, 0);
      toCal.set(Calendar.SECOND, 0);
      toCal.set(Calendar.MILLISECOND, 0);

      long diff = toCal.getTimeInMillis() - fromCal.getTimeInMillis();
      return (int) (diff / MILLIS_PER_DAY);
   }

}