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Classes for using and formatting data and data-rate values in the JSR 363 Units of Measurement API.
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package com.pervasivecode.utils.measure;

import java.math.BigDecimal;
import javax.measure.Quantity;
import javax.measure.Unit;
import tec.uom.se.unit.MetricPrefix;

/**
 * Instances select an appropriate SI prefix for a given {@link Quantity} and return the same
 * {@link Quantity} transformed to use that prefix.
 * 
 * Examples: given 10,000 meters, return 10 kilometers. Given 1,000,000 joules, return 1 megajoule.
 */
public class SiPrefixSelector implements QuantityPrefixSelector {

  /**
   * Apply an appropriate prefix from the SI set of prefixes for a given measurement.
   *
   * @param measure A value to scale with a prefix.
   * @param formatUnit The base "ones" unit of the provided measure.
   * @return The measurement, transformed with an appropriate prefix from the SI system.
   */
  @Override
  public > Quantity selectBestPrefix(Quantity measure,
      Unit formatUnit) {

    BigDecimal asBig = BigDecimal.valueOf(
        measure.to(formatUnit).getValue().doubleValue());

    if (asBig.compareTo(SiThousandPrefixes.YOTTA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.YOTTA(formatUnit));
    }
    if (asBig.compareTo(SiThousandPrefixes.ZETTA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.ZETTA(formatUnit));
    }

    if (asBig.compareTo(SiThousandPrefixes.EXA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.EXA(formatUnit));
    }
    if (asBig.compareTo(SiThousandPrefixes.PETA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.PETA(formatUnit));
    }
    if (asBig.compareTo(SiThousandPrefixes.TERA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.TERA(formatUnit));
    }
    if (asBig.compareTo(SiThousandPrefixes.GIGA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.GIGA(formatUnit));
    }
    if (asBig.compareTo(SiThousandPrefixes.MEGA_FACTOR) >= 0) {
      return measure.to(MetricPrefix.MEGA(formatUnit));
    }
    if (asBig.compareTo(SiThousandPrefixes.KILO_FACTOR) >= 0) {
      return measure.to(MetricPrefix.KILO(formatUnit));
    }

    if (asBig.compareTo(BigDecimal.ONE) < 0) {
      if (asBig.compareTo(SiThousandPrefixes.MILLI_FACTOR) >= 0) {
        return measure.to(MetricPrefix.MILLI(formatUnit));
      }
      if (asBig.compareTo(SiThousandPrefixes.MICRO_FACTOR) >= 0) {
        return measure.to(MetricPrefix.MICRO(formatUnit));
      }
      if (asBig.compareTo(SiThousandPrefixes.NANO_FACTOR) >= 0) {
        return measure.to(MetricPrefix.NANO(formatUnit));
      }
      if (asBig.compareTo(SiThousandPrefixes.PICO_FACTOR) >= 0) {
        return measure.to(MetricPrefix.PICO(formatUnit));
      }
      if (asBig.compareTo(SiThousandPrefixes.FEMTO_FACTOR) >= 0) {
        return measure.to(MetricPrefix.FEMTO(formatUnit));
      }
      if (asBig.compareTo(SiThousandPrefixes.ATTO_FACTOR) >= 0) {
        return measure.to(MetricPrefix.ATTO(formatUnit));
      }
      if (asBig.compareTo(SiThousandPrefixes.ZEPTO_FACTOR) >= 0) {
        return measure.to(MetricPrefix.ZEPTO(formatUnit));
      }
      return measure.to(MetricPrefix.YOCTO(formatUnit));
    }

    return measure.to(formatUnit);
  }
}