package com.jstatcom.util;

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

/**
 * Helper class for computing the difference of two dates in years, months,
 * weeks or days. The estimates are precise for ranges up to 2000 years. The
 * method takes care of corrections for leap seconds.
 *  
 */
public class DateDiff {
    // set some constants that allow pretty accurate estimates over
    //  about 2,000 years before or after the present. These are
    //  slight overestimates since that's what we want (and need!)
    private static final double DAY_MILLIS = 1000 * 60 * 60 * 24.0015;

    private static final double WEEK_MILLIS = DAY_MILLIS * 7;

    private static final double MONTH_MILLIS = DAY_MILLIS * 30.43675;

    private static final double YEAR_MILLIS = WEEK_MILLIS * 52.2;

    /**
     * int diff = dd.getDateDiff(Calendar.WEEK_OF_YEAR, then, now);
     * 
     * @param calUnit
     * @param d1
     * @param d2
     * @return integer difference between two dates
     */
    public static int getDateDiff(int calUnit, Date d1, Date d2) {
        // swap if d1 later than d2
        boolean neg = false;
        if (d1.after(d2)) {
            Date temp = d1;
            d1 = d2;
            d2 = temp;
            neg = true;
        }

        // estimate the diff. d1 is now guaranteed <= d2
        int estimate = getEstDiff(calUnit, d1, d2);

        // convert the Dates to GregorianCalendars
        GregorianCalendar c1 = new GregorianCalendar();
        c1.setTime(d1);
        GregorianCalendar c2 = new GregorianCalendar();
        c2.setTime(d2);

        // add 2 units less than the estimate to 1st date,
        //  then serially add units till we exceed 2nd date
        c1.add(calUnit, estimate - 2);
        for (int i = estimate - 1;; i++) {
            c1.add(calUnit, 1);
            if (c1.after(c2))
                return neg ? 1 - i : i - 1;
        }
    }

    private static int getEstDiff(int calUnit, Date d1, Date d2) {
        long diff = d2.getTime() - d1.getTime();
        switch (calUnit) {
        case Calendar.DAY_OF_WEEK_IN_MONTH:
        case Calendar.DAY_OF_MONTH:
            //      case Calendar.DATE : // codes to same int as DAY_OF_MONTH
            return (int) (diff / DAY_MILLIS + .5);
        case Calendar.WEEK_OF_YEAR:
            return (int) (diff / WEEK_MILLIS + .5);
        case Calendar.MONTH:
            return (int) (diff / MONTH_MILLIS + .5);
        case Calendar.YEAR:
            return (int) (diff / YEAR_MILLIS + .5);
        default:
            return 0;
        } /* endswitch */
    }

}