main/classes/core/src/com/ibm/icu/impl/Grego.java

/**
 *******************************************************************************
 * Copyright (C) 2003-2008, International Business Machines Corporation and
 * others. All Rights Reserved.
 *******************************************************************************
 * Partial port from ICU4C's Grego class in i18n/gregoimp.h.
 *
 * Methods ported, or moved here from OlsonTimeZone, initially
 * for work on Jitterbug 5470:
 *   tzdata2006n Brazil incorrect fall-back date 2009-mar-01
 * Only the methods necessary for that work are provided - this is not a full
 * port of ICU4C's Grego class (yet).
 *
 * These utilities are used by both OlsonTimeZone and SimpleTimeZone.
 */

package com.ibm.icu.impl;

import com.ibm.icu.util.Calendar;

/**
 * A utility class providing proleptic Gregorian calendar functions
 * used by time zone and calendar code.  Do not instantiate.
 *
 * Note:  Unlike GregorianCalendar, all computations performed by this
 * class occur in the pure proleptic GregorianCalendar.
 */
public class Grego {

    // Max/min milliseconds 
    public static final long MIN_MILLIS = -184303902528000000L;
    public static final long MAX_MILLIS = 183882168921600000L;

    public static final int MILLIS_PER_SECOND = 1000;
    public static final int MILLIS_PER_MINUTE = 60*MILLIS_PER_SECOND;
    public static final int MILLIS_PER_HOUR = 60*MILLIS_PER_MINUTE;
    public static final int MILLIS_PER_DAY = 24*MILLIS_PER_HOUR;
    
    //  January 1, 1 CE Gregorian
    private static final int JULIAN_1_CE = 1721426;

    //  January 1, 1970 CE Gregorian
    private static final int JULIAN_1970_CE = 2440588;

    private static final int[] MONTH_LENGTH = new int[] {
        31,28,31,30,31,30,31,31,30,31,30,31,
        31,29,31,30,31,30,31,31,30,31,30,31
    };

    private static final int[] DAYS_BEFORE = new int[] {
        0,31,59,90,120,151,181,212,243,273,304,334,
        0,31,60,91,121,152,182,213,244,274,305,335 };

    /**
     * Return true if the given year is a leap year.
     * @param year Gregorian year, with 0 == 1 BCE, -1 == 2 BCE, etc.
     * @return true if the year is a leap year
     */
    public static final boolean isLeapYear(int year) {
        // year&0x3 == year%4
        return ((year&0x3) == 0) && ((year%100 != 0) || (year%400 == 0));
    }

    /**
     * Return the number of days in the given month.
     * @param year Gregorian year, with 0 == 1 BCE, -1 == 2 BCE, etc.
     * @param month 0-based month, with 0==Jan
     * @return the number of days in the given month
     */
    public static final int monthLength(int year, int month) {
        return MONTH_LENGTH[month + (isLeapYear(year) ? 12 : 0)];
    }

    /**
     * Return the length of a previous month of the Gregorian calendar.
     * @param year Gregorian year, with 0 == 1 BCE, -1 == 2 BCE, etc.
     * @param month 0-based month, with 0==Jan
     * @return the number of days in the month previous to the given month
     */
    public static final int previousMonthLength(int year, int month) {
        return (month > 0) ? monthLength(year, month-1) : 31;
    }

    /**
     * Convert a year, month, and day-of-month, given in the proleptic
     * Gregorian calendar, to 1970 epoch days.
     * @param year Gregorian year, with 0 == 1 BCE, -1 == 2 BCE, etc.
     * @param month 0-based month, with 0==Jan
     * @param dom 1-based day of month
     * @return the day number, with day 0 == Jan 1 1970
     */
    public static long fieldsToDay(int year, int month, int dom) {
        int y = year - 1;
        long julian =
            365 * y + floorDivide(y, 4) + (JULIAN_1_CE - 3) +    // Julian cal
            floorDivide(y, 400) - floorDivide(y, 100) + 2 +   // => Gregorian cal
            DAYS_BEFORE[month + (isLeapYear(year) ? 12 : 0)] + dom; // => month/dom
        return julian - JULIAN_1970_CE; // JD => epoch day
    }

    /**
     * Return the day of week on the 1970-epoch day
     * @param day the 1970-epoch day (integral value)
     * @return the day of week
     */
    public static int dayOfWeek(long day) {
        long[] remainder = new long[1];
        floorDivide(day + Calendar.THURSDAY, 7, remainder);
        int dayOfWeek = (int)remainder[0];
        dayOfWeek = (dayOfWeek == 0) ? 7 : dayOfWeek;
        return dayOfWeek;
    }

    public static int[] dayToFields(long day, int[] fields) {
        if (fields == null || fields.length < 5) {
            fields = new int[5];
        }
        // Convert from 1970 CE epoch to 1 CE epoch (Gregorian calendar)
        day += JULIAN_1970_CE - JULIAN_1_CE;

        long[] rem = new long[1];
        long n400 = floorDivide(day, 146097, rem);
        long n100 = floorDivide(rem[0], 36524, rem);
        long n4 = floorDivide(rem[0], 1461, rem);
        long n1 = floorDivide(rem[0], 365, rem);

        int year = (int)(400 * n400 + 100 * n100 + 4 * n4 + n1);
        int dayOfYear = (int)rem[0];
        if (n100 == 4 || n1 == 4) {
            dayOfYear = 365;    // Dec 31 at end of 4- or 400-yr cycle
        }
        else {
            ++year;
        }

        boolean isLeap = isLeapYear(year);
        int correction = 0;
        int march1 = isLeap ? 60 : 59;  // zero-based DOY for March 1
        if (dayOfYear >= march1) {
            correction = isLeap ? 1 : 2;
        }
        int month = (12 * (dayOfYear + correction) + 6) / 367;  // zero-based month
        int dayOfMonth = dayOfYear - DAYS_BEFORE[isLeap ? month + 12 : month] + 1; // one-based DOM
        int dayOfWeek = (int)((day + 2) % 7);  // day 0 is Monday(2)
        if (dayOfWeek < 1 /* Sunday */) {
            dayOfWeek += 7;
        }
        dayOfYear++; // 1-based day of year

        fields[0] = year;
        fields[1] = month;
        fields[2] = dayOfMonth;
        fields[3] = dayOfWeek;
        fields[4] = dayOfYear;

        return fields;
    }

    /*
     * Convert long time to date/time fields
     * 
     * result[0] : year
     * result[1] : month
     * result[2] : dayOfMonth
     * result[3] : dayOfWeek
     * result[4] : dayOfYear
     * result[5] : millisecond in day
     */
    public static int[] timeToFields(long time, int[] fields) {
        if (fields == null || fields.length < 6) {
            fields = new int[6];
        }
        long[] remainder = new long[1];
        long day = floorDivide(time, 24*60*60*1000 /* milliseconds per day */, remainder);
        dayToFields(day, fields);
        fields[5] = (int)remainder[0];
        return fields;
    }

    public static long floorDivide(long numerator, long denominator) {
        // We do this computation in order to handle
        // a numerator of Long.MIN_VALUE correctly
        return (numerator >= 0) ?
            numerator / denominator :
            ((numerator + 1) / denominator) - 1;
    }

    private static long floorDivide(long numerator, long denominator, long[] remainder) {
        if (numerator >= 0) {
            remainder[0] = numerator % denominator;
            return numerator / denominator;
        }
        long quotient = ((numerator + 1) / denominator) - 1;
        remainder[0] = numerator - (quotient * denominator);
        return quotient;
    }

    /*
     * Returns the ordinal number for the specified day of week in the month.
     * The valid return value is 1, 2, 3, 4 or -1.
     */
    public static int getDayOfWeekInMonth(int year, int month, int dayOfMonth) {
        int weekInMonth = (dayOfMonth + 6)/7;
        if (weekInMonth == 4) {
            if (dayOfMonth + 7 > monthLength(year, month)) {
                weekInMonth = -1;
            }
        } else if (weekInMonth == 5) {
            weekInMonth = -1;
        }
        return weekInMonth;
    }
}