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JRobin is a 100% pure Java alternative to RRDTool, with about exactly the same specifications. If you provide the same data to RRDTool and JRobin, you will get exactly the same results and graphs. All standard RRDTool operations are supported.

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/*******************************************************************************
 * Copyright (c) 2001-2005 Sasa Markovic and Ciaran Treanor.
 * Copyright (c) 2011 The OpenNMS Group, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *******************************************************************************/
package org.jrobin.graph;

class ValueScaler {
	static final String UNIT_UNKNOWN = "?";
	static final String UNIT_SYMBOLS[] = {
			"a", "f", "p", "n", "u", "m", " ", "k", "M", "G", "T", "P", "E"
	};
	static final int SYMB_CENTER = 6;

	private final double base;
	private double magfact = -1; // nothing scaled before, rescale
	private String unit;

	ValueScaler(double base) {
		this.base = base;
	}

	Scaled scale(double value, boolean mustRescale) {
		Scaled scaled;
		if (mustRescale) {
			scaled = rescale(value);
		}
		else if (magfact >= 0) {
			// already scaled, need not rescale
			scaled = new Scaled(value / magfact, unit);
		}
		else {
			// scaling not requested, but never scaled before - must rescale anyway
			scaled = rescale(value);
			// if zero, scale again on the next try
			if (scaled.value == 0.0 || Double.isNaN(scaled.value)) {
				magfact = -1.0;
			}
		}
		return scaled;
	}

	private Scaled rescale(double value) {
		int sindex;
		if (value == 0.0 || Double.isNaN(value)) {
			sindex = 0;
			magfact = 1.0;
		}
		else {
			sindex = (int) (Math.floor(Math.log(Math.abs(value)) / Math.log(base)));
			magfact = Math.pow(base, sindex);
		}
		if (sindex <= SYMB_CENTER && sindex >= -SYMB_CENTER) {
			unit = UNIT_SYMBOLS[sindex + SYMB_CENTER];
		}
		else {
			unit = UNIT_UNKNOWN;
		}
		return new Scaled(value / magfact, unit);
	}

	static class Scaled {
		double value;
		String unit;

		public Scaled(double value, String unit) {
			this.value = value;
			this.unit = unit;
		}

		void dump() {
			System.out.println("[" + value + unit + "]");
		}
	}
}