TimeUnit.java

package org.araqne.logdb;

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

public enum TimeUnit {
	Second(Calendar.SECOND, 1000L, "s"), Minute(Calendar.MINUTE, 60 * 1000L, "m"), Hour(Calendar.HOUR_OF_DAY, 60 * 60 * 1000L,
			"h"), Day(Calendar.DAY_OF_MONTH, 86400 * 1000L, "d"), Week(Calendar.WEEK_OF_YEAR, 7 * 86400 * 1000L, "w"), Month(
			Calendar.MONTH, 30 * 86400 * 1000L, "mon"), Year(Calendar.YEAR, 365 * 86400 * 1000L, "y");

	private int calendarField;
	private long millis;
	private String acronym;

	private static final int[] mon2Spans = new int[] { 1, 3, 5, 7, 9, 11 };
	private static final int[] mon3Spans = new int[] { 1, 4, 7, 10 };
	private static final int[] mon4Spans = new int[] { 1, 5, 9 };
	private static final int[] mon6Spans = new int[] { 1, 7 };

	private static long timeZoneOffset;
	private static long timeZoneShift;

	static {
		timeZoneOffset = TimeZone.getDefault().getRawOffset();

		// base to Monday, 00:00:00
		timeZoneShift = timeZoneOffset + 86400000 * 3;
	}

	private TimeUnit(int calendarField, long millis, String acronym) {
		this.calendarField = calendarField;
		this.millis = millis;
		this.acronym = acronym;
	}

	public long getMillis() {
		return millis;
	}

	public int getCalendarField() {
		return calendarField;
	}

	@Override
	public String toString() {
		return acronym;
	}

	public static Date getKey(Date date, TimeSpan timeSpan) {
		TimeUnit spanField = timeSpan.unit;

		if (spanField == TimeUnit.Month) {
			Calendar c = Calendar.getInstance();
			c.setTime(date);
			int mon = c.get(Calendar.MONTH);

			if (timeSpan.amount == 2)
				c.set(Calendar.MONTH, mon2Spans[mon / 2] - 1);
			else if (timeSpan.amount == 3)
				c.set(Calendar.MONTH, mon3Spans[mon / 3] - 1);
			else if (timeSpan.amount == 4)
				c.set(Calendar.MONTH, mon4Spans[mon / 4] - 1);
			else if (timeSpan.amount == 6)
				c.set(Calendar.MONTH, mon6Spans[mon / 6] - 1);

			c.set(Calendar.DAY_OF_MONTH, 1);
			c.set(Calendar.HOUR_OF_DAY, 0);
			c.set(Calendar.MINUTE, 0);
			c.set(Calendar.SECOND, 0);
			c.set(Calendar.MILLISECOND, 0);
			return c.getTime();
		} else if (spanField == TimeUnit.Year) {
			Calendar c = Calendar.getInstance();
			c.setTime(date);
			c.set(Calendar.MONTH, 0);
			c.set(Calendar.DAY_OF_MONTH, 1);
			c.set(Calendar.HOUR_OF_DAY, 0);
			c.set(Calendar.MINUTE, 0);
			c.set(Calendar.SECOND, 0);
			c.set(Calendar.MILLISECOND, 0);
			return c.getTime();
		} else {
			long time = date.getTime();

			int spanAmount = timeSpan.amount;
			time += timeZoneShift;
			time -= time % (spanField.millis * spanAmount);
			time -= timeZoneShift;
			return new Date(time);
		}
	}
}