/*
 * Units of Measurement Implementation for Java SE
 * Copyright (c) 2005-2017, Jean-Marie Dautelle, Werner Keil, V2COM.
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions
 *    and the following disclaimer in the documentation and/or other materials provided with the distribution.
 *
 * 3. Neither the name of JSR-363 nor the names of its contributors may be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
 * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package tec.uom.se.format;

import static tec.uom.se.unit.MetricPrefix.*;

import java.io.IOException;
import java.lang.CharSequence;
import java.text.FieldPosition;
import java.text.ParsePosition;
import java.util.HashMap;
import java.util.Map;

import tec.uom.se.AbstractUnit;
import tec.uom.se.function.AddConverter;
import tec.uom.se.function.MultiplyConverter;
import tec.uom.se.function.RationalConverter;
import tec.uom.se.unit.AlternateUnit;
import tec.uom.se.unit.BaseUnit;
import tec.uom.se.unit.ProductUnit;
import tec.uom.se.unit.TransformedUnit;
import tec.uom.se.unit.Units;
import tec.uom.se.unit.MetricPrefix;

import javax.measure.Unit;
import javax.measure.UnitConverter;
import javax.measure.Quantity;
import javax.measure.format.ParserException;
import javax.measure.format.UnitFormat;

/**
 * <p>
 * This class implements the {@link UnitFormat} interface for formatting and parsing {@link Unit units}.
 * </p>
 * 
 * <p>
 * For all SI units, the 20 SI prefixes used to form decimal multiples and sub-multiples of SI units are recognized. {@link Units} are directly
 * recognized. For example:<br>
 * <code>
 *        AbstractUnit.parse("m°C").equals(MetricPrefix.MILLI(Units.CELSIUS))
 *        AbstractUnit.parse("kW").equals(MetricPrefix.KILO(Units.WATT))
 *        AbstractUnit.parse("ft").equals(Units.METRE.multiply(0.3048))</code>
 * </p>
 *
 * @author <a href="mailto:jean-marie@dautelle.com">Jean-Marie Dautelle</a>
 * @author <a href="mailto:units@catmedia.us">Werner Keil</a>
 * @author Eric Russell
 * @version 1.0.3, June 7, 2017
 * @since 1.0
 */
public abstract class SimpleUnitFormat extends AbstractUnitFormat {
  /**
     * 
     */
  // private static final long serialVersionUID = 4149424034841739785L;

  /**
   * Flavor of this format
   *
   * @author Werner
   *
   */
  public enum Flavor {
    Default, ASCII
  }

  /**
   * Holds the standard unit format.
   */
  private static final DefaultFormat DEFAULT = new DefaultFormat();

  /**
   * Holds the ASCIIFormat unit format.
   */
  private static final ASCIIFormat ASCII = new ASCIIFormat();

  /**
   * Returns the unit format for the default locale (format used by {@link AbstractUnit#parse(CharSequence) AbstractUnit.parse(CharSequence)} and
   * {@link Unit#toString() Unit.toString()}).
   *
   * @return the default unit format (locale sensitive).
   */
  public static SimpleUnitFormat getInstance() {
    return getInstance(Flavor.Default);
  }

  /**
   * Returns the {@link SimpleUnitFormat} in the desired {@link Flavor}
   *
   * @return the instance for the given {@link Flavor}.
   */
  public static SimpleUnitFormat getInstance(Flavor flavor) {
    switch (flavor) {
      case ASCII:
        return SimpleUnitFormat.ASCII;
      default:
        return DEFAULT;
    }
  }

  /**
   * Base constructor.
   */
  protected SimpleUnitFormat() {
  }

  /**
   * Formats the specified unit.
   *
   * @param unit
   *          the unit to format.
   * @param appendable
   *          the appendable destination.
   * @throws IOException
   *           if an error occurs.
   */
  public abstract Appendable format(Unit<?> unit, Appendable appendable) throws IOException;

  /**
   * Parses a sequence of character to produce a unit or a rational product of unit.
   *
   * @param csq
   *          the <code>CharSequence</code> to parse.
   * @param pos
   *          an object holding the parsing index and error position.
   * @return an {@link Unit} parsed from the character sequence.
   * @throws IllegalArgumentException
   *           if the character sequence contains an illegal syntax.
   */
  @SuppressWarnings("rawtypes")
  public abstract Unit<? extends Quantity> parseProductUnit(CharSequence csq, ParsePosition pos) throws ParserException;

  /**
   * Parses a sequence of character to produce a single unit.
   *
   * @param csq
   *          the <code>CharSequence</code> to parse.
   * @param pos
   *          an object holding the parsing index and error position.
   * @return an {@link Unit} parsed from the character sequence.
   * @throws IllegalArgumentException
   *           if the character sequence does not contain a valid unit identifier.
   */
  @SuppressWarnings("rawtypes")
  public abstract Unit<? extends Quantity> parseSingleUnit(CharSequence csq, ParsePosition pos) throws ParserException;

  /**
   * Attaches a system-wide label to the specified unit. For example: <code> SimpleUnitFormat.getInstance().label(DAY.multiply(365), "year");
   * SimpleUnitFormat.getInstance().label(METER.multiply(0.3048), "ft"); </code> If the specified label is already associated to an unit the previous
   * association is discarded or ignored.
   *
   * @param unit
   *          the unit being labeled.
   * @param label
   *          the new label for this unit.
   * @throws IllegalArgumentException
   *           if the label is not a {@link SimpleUnitFormat#isValidIdentifier(String)} valid identifier.
   */
  public abstract void label(Unit<?> unit, String label);

  public boolean isLocaleSensitive() {
    return false;
  }

  /**
   * Attaches a system-wide alias to this unit. Multiple aliases may be attached to the same unit. Aliases are used during parsing to recognize
   * different variants of the same unit. For example: <code> SimpleUnitFormat.getInstance().alias(METER.multiply(0.3048), "foot");
   * SimpleUnitFormat.getInstance().alias(METER.multiply(0.3048), "feet"); SimpleUnitFormat.getInstance().alias(METER, "meter");
   * SimpleUnitFormat.getInstance().alias(METER, "metre"); </code> If the specified label is already associated to an unit the previous association is
   * discarded or ignored.
   *
   * @param unit
   *          the unit being aliased.
   * @param alias
   *          the alias attached to this unit.
   * @throws IllegalArgumentException
   *           if the label is not a {@link SimpleUnitFormat#isValidIdentifier(String)} valid identifier.
   */
  public abstract void alias(Unit<?> unit, String alias);

  /**
   * Indicates if the specified name can be used as unit identifier.
   *
   * @param name
   *          the identifier to be tested.
   * @return <code>true</code> if the name specified can be used as label or alias for this format;<code>false</code> otherwise.
   */
  public abstract boolean isValidIdentifier(String name);

  /**
   * Formats an unit and appends the resulting text to a given string buffer (implements <code>java.text.Format</code>).
   *
   * @param unit
   *          the unit to format.
   * @param toAppendTo
   *          where the text is to be appended
   * @param pos
   *          the field position (not used).
   * @return <code>toAppendTo</code>
   */
  public final StringBuffer format(Object unit, final StringBuffer toAppendTo, FieldPosition pos) {
    try {
      Object dest = toAppendTo;
      if (dest instanceof Appendable) {
        format((Unit<?>) unit, (Appendable) dest);
      } else { // When retroweaver is used to produce 1.4 binaries.
        format((Unit<?>) unit, new Appendable() {

          public Appendable append(char arg0) throws IOException {
            toAppendTo.append(arg0);
            return null;
          }

          public Appendable append(CharSequence arg0) throws IOException {
            toAppendTo.append(arg0);
            return null;
          }

          public Appendable append(CharSequence arg0, int arg1, int arg2) throws IOException {
            toAppendTo.append(arg0.subSequence(arg1, arg2));
            return null;
          }
        });
      }
      return toAppendTo;
    } catch (IOException e) {
      throw new Error(e); // Should never happen.
    }
  }

  /**
   * Parses the text from a string to produce an object (implements <code>java.text.Format</code>).
   *
   * @param source
   *          the string source, part of which should be parsed.
   * @param pos
   *          the cursor position.
   * @return the corresponding unit or <code>null</code> if the string cannot be parsed.
   */
  public final Unit<?> parseObject(String source, ParsePosition pos) throws ParserException {
    // int start = pos.getIndex();
    return parseProductUnit(source, pos);
    /*
     * } catch (ParserException e) { pos.setIndex(start);
     * pos.setErrorIndex(e.getPosition()); return null; }
     */
  }

  /**
   * This class represents an exponent with both a power (numerator) and a root (denominator).
   */
  private static class Exponent {
    public final int pow;
    public final int root;

    public Exponent(int pow, int root) {
      this.pow = pow;
      this.root = root;
    }
  }

  /**
   * This class represents the standard format.
   */
  protected static class DefaultFormat extends SimpleUnitFormat {

    /**
     * Holds the name to unit mapping.
     */
    final HashMap<String, Unit<?>> _nameToUnit = new HashMap<>();

    /**
     * Holds the unit to name mapping.
     */
    final HashMap<Unit<?>, String> _unitToName = new HashMap<>();

    @Override
    public void label(Unit<?> unit, String label) {
      if (!isValidIdentifier(label))
        throw new IllegalArgumentException("Label: " + label + " is not a valid identifier.");
      synchronized (this) {
        _nameToUnit.put(label, unit);
        _unitToName.put(unit, label);
      }
    }

    @Override
    public void alias(Unit<?> unit, String alias) {
      if (!isValidIdentifier(alias))
        throw new IllegalArgumentException("Alias: " + alias + " is not a valid identifier.");
      synchronized (this) {
        _nameToUnit.put(alias, unit);
      }
    }

    @Override
    public boolean isValidIdentifier(String name) {
      if ((name == null) || (name.length() == 0))
        return false;
      /*
       * for (int i = 0; i < name.length(); i++) { if
       * (!isUnitIdentifierPart(name.charAt(i))) return false; }
       */
      return isUnitIdentifierPart(name.charAt(0));
    }

    static boolean isUnitIdentifierPart(char ch) {
      return Character.isLetter(ch)
          || (!Character.isWhitespace(ch) && !Character.isDigit(ch) && (ch != '\u00b7') && (ch != '*') && (ch != '/') && (ch != '(') && (ch != ')')
              && (ch != '[') && (ch != ']') && (ch != '\u00b9') && (ch != '\u00b2') && (ch != '\u00b3') && (ch != '^') && (ch != '+') && (ch != '-'));
    }

    // Returns the name for the specified unit or null if product unit.
    protected String nameFor(Unit<?> unit) {
      // Searches label database.
      String label = _unitToName.get(unit);
      if (label != null)
        return label;
      if (unit instanceof BaseUnit)
        return ((BaseUnit<?>) unit).getSymbol();
      if (unit instanceof AlternateUnit)
        return ((AlternateUnit<?>) unit).getSymbol();
      if (unit instanceof TransformedUnit) {
        TransformedUnit<?> tfmUnit = (TransformedUnit<?>) unit;
        Unit<?> baseUnit = tfmUnit.getParentUnit();
        UnitConverter cvtr = tfmUnit.getConverter(); // tfmUnit.getSystemConverter();
        StringBuilder result = new StringBuilder();
        String baseUnitName = baseUnit.toString();
        String prefix = prefixFor(cvtr);
        if ((baseUnitName.indexOf('\u00b7') >= 0) || (baseUnitName.indexOf('*') >= 0) || (baseUnitName.indexOf('/') >= 0)) {
          // We could use parentheses whenever baseUnits is an
          // instanceof ProductUnit, but most ProductUnits have
          // aliases,
          // so we'd end up with a lot of unnecessary parentheses.
          result.append('(');
          result.append(baseUnitName);
          result.append(')');
        } else {
          result.append(baseUnitName);
        }
        if (prefix != null) {
          result.insert(0, prefix);
        } else {
          if (cvtr instanceof AddConverter) {
            result.append('+');
            result.append(((AddConverter) cvtr).getOffset());
          } else if (cvtr instanceof RationalConverter) {
            double dividend = ((RationalConverter) cvtr).getDividend().doubleValue();
            if (dividend != 1) {
              result.append('*');
              result.append(dividend);
            }
            double divisor = ((RationalConverter) cvtr).getDivisor().doubleValue();
            if (divisor != 1) {
              result.append('/');
              result.append(divisor);
            }
          } else if (cvtr instanceof MultiplyConverter) {
            result.append('*');
            result.append(((MultiplyConverter) cvtr).getFactor());
          } else { // Other converters.
            return "[" + baseUnit + "?]";
          }
        }
        return result.toString();
      }
      // Compound unit.
      // if (unit instanceof CompoundUnit) {
      // CompoundUnit<?> cpdUnit = (CompoundUnit<?>) unit;
      // return nameFor(cpdUnit.getHigher()).toString() + ":"
      // + nameFor(cpdUnit.getLower());
      // }
      return null; // Product unit.
    }

    // Returns the prefix for the specified unit converter.
    protected String prefixFor(UnitConverter converter) {
      for (int i = 0; i < CONVERTERS.length; i++) {
        if (CONVERTERS[i].equals(converter)) {
          return PREFIXES[i];
        }
      }
      return null; // TODO or return blank?
    }

    // Returns the unit for the specified name.
    protected Unit<?> unitFor(String name) {
      Unit<?> unit = _nameToUnit.get(name);
      if (unit != null)
        return unit;
      unit = SYMBOL_TO_UNIT.get(name);
      return unit;
    }

    // //////////////////////////
    // Parsing.
    @SuppressWarnings({ "rawtypes", "unchecked" })
    public Unit<? extends Quantity> parseSingleUnit(CharSequence csq, ParsePosition pos) throws ParserException {
      int startIndex = pos.getIndex();
      String name = readIdentifier(csq, pos);
      Unit unit = unitFor(name);
      check(unit != null, name + " not recognized", csq, startIndex);
      return unit;
    }

    @SuppressWarnings({ "rawtypes", "unchecked" })
    @Override
    public Unit<? extends Quantity> parseProductUnit(CharSequence csq, ParsePosition pos) throws ParserException {
      Unit result = AbstractUnit.ONE;
      int token = nextToken(csq, pos);
      switch (token) {
        case IDENTIFIER:
          result = parseSingleUnit(csq, pos);
          break;
        case OPEN_PAREN:
          pos.setIndex(pos.getIndex() + 1);
          result = parseProductUnit(csq, pos);
          token = nextToken(csq, pos);
          check(token == CLOSE_PAREN, "')' expected", csq, pos.getIndex());
          pos.setIndex(pos.getIndex() + 1);
          break;
      }
      token = nextToken(csq, pos);
      while (true) {
        switch (token) {
          case EXPONENT:
            Exponent e = readExponent(csq, pos);
            if (e.pow != 1) {
              result = result.pow(e.pow);
            }
            if (e.root != 1) {
              result = result.root(e.root);
            }
            break;
          case MULTIPLY:
            pos.setIndex(pos.getIndex() + 1);
            token = nextToken(csq, pos);
            if (token == INTEGER) {
              long n = readLong(csq, pos);
              if (n != 1) {
                result = result.multiply(n);
              }
            } else if (token == FLOAT) {
              double d = readDouble(csq, pos);
              if (d != 1.0) {
                result = result.multiply(d);
              }
            } else {
              result = result.multiply(parseProductUnit(csq, pos));
            }
            break;
          case DIVIDE:
            pos.setIndex(pos.getIndex() + 1);
            token = nextToken(csq, pos);
            if (token == INTEGER) {
              long n = readLong(csq, pos);
              if (n != 1) {
                result = result.divide(n);
              }
            } else if (token == FLOAT) {
              double d = readDouble(csq, pos);
              if (d != 1.0) {
                result = result.divide(d);
              }
            } else {
              result = result.divide(parseProductUnit(csq, pos));
            }
            break;
          case PLUS:
            pos.setIndex(pos.getIndex() + 1);
            token = nextToken(csq, pos);
            if (token == INTEGER) {
              long n = readLong(csq, pos);
              if (n != 1) {
                result = result.shift(n);
              }
            } else if (token == FLOAT) {
              double d = readDouble(csq, pos);
              if (d != 1.0) {
                result = result.shift(d);
              }
            } else {
              throw new ParserException("not a number", pos.getIndex());
            }
            break;
          case EOF:
          case CLOSE_PAREN:
            return result;
          default:
            throw new ParserException("unexpected token " + token, pos.getIndex());
        }
        token = nextToken(csq, pos);
      }
    }

    private static final int EOF = 0;
    private static final int IDENTIFIER = 1;
    private static final int OPEN_PAREN = 2;
    private static final int CLOSE_PAREN = 3;
    private static final int EXPONENT = 4;
    private static final int MULTIPLY = 5;
    private static final int DIVIDE = 6;
    private static final int PLUS = 7;
    private static final int INTEGER = 8;
    private static final int FLOAT = 9;

    private int nextToken(CharSequence csq, ParsePosition pos) {
      final int length = csq.length();
      while (pos.getIndex() < length) {
        char c = csq.charAt(pos.getIndex());
        if (isUnitIdentifierPart(c)) {
          return IDENTIFIER;
        } else if (c == '(') {
          return OPEN_PAREN;
        } else if (c == ')') {
          return CLOSE_PAREN;
        } else if ((c == '^') || (c == '\u00b9') || (c == '\u00b2') || (c == '\u00b3')) {
          return EXPONENT;
        } else if (c == '*') {
          char c2 = csq.charAt(pos.getIndex() + 1);
          if (c2 == '*') {
            return EXPONENT;
          } else {
            return MULTIPLY;
          }
        } else if (c == '\u00b7') {
          return MULTIPLY;
        } else if (c == '/') {
          return DIVIDE;
        } else if (c == '+') {
          return PLUS;
        } else if ((c == '-') || Character.isDigit(c)) {
          int index = pos.getIndex() + 1;
          while ((index < length) && (Character.isDigit(c) || (c == '-') || (c == '.') || (c == 'E'))) {
            c = csq.charAt(index++);
            if (c == '.') {
              return FLOAT;
            }
          }
          return INTEGER;
        }
        pos.setIndex(pos.getIndex() + 1);
      }
      return EOF;
    }

    private void check(boolean expr, String message, CharSequence csq, int index) throws ParserException {
      if (!expr) {
        throw new ParserException(message + " (in " + csq + " at index " + index + ")", index);
      }
    }

    private Exponent readExponent(CharSequence csq, ParsePosition pos) {
      char c = csq.charAt(pos.getIndex());
      if (c == '^') {
        pos.setIndex(pos.getIndex() + 1);
      } else if (c == '*') {
        pos.setIndex(pos.getIndex() + 2);
      }
      final int length = csq.length();
      int pow = 0;
      boolean isPowNegative = false;
      int root = 0;
      boolean isRootNegative = false;
      boolean isRoot = false;
      while (pos.getIndex() < length) {
        c = csq.charAt(pos.getIndex());
        if (c == '\u00b9') {
          if (isRoot) {
            root = root * 10 + 1;
          } else {
            pow = pow * 10 + 1;
          }
        } else if (c == '\u00b2') {
          if (isRoot) {
            root = root * 10 + 2;
          } else {
            pow = pow * 10 + 2;
          }
        } else if (c == '\u00b3') {
          if (isRoot) {
            root = root * 10 + 3;
          } else {
            pow = pow * 10 + 3;
          }
        } else if (c == '-') {
          if (isRoot) {
            isRootNegative = true;
          } else {
            isPowNegative = true;
          }
        } else if ((c >= '0') && (c <= '9')) {
          if (isRoot) {
            root = root * 10 + (c - '0');
          } else {
            pow = pow * 10 + (c - '0');
          }
        } else if (c == ':') {
          isRoot = true;
        } else {
          break;
        }
        pos.setIndex(pos.getIndex() + 1);
      }
      if (pow == 0)
        pow = 1;
      if (root == 0)
        root = 1;
      return new Exponent(isPowNegative ? -pow : pow, isRootNegative ? -root : root);
    }

    private long readLong(CharSequence csq, ParsePosition pos) {
      final int length = csq.length();
      int result = 0;
      boolean isNegative = false;
      while (pos.getIndex() < length) {
        char c = csq.charAt(pos.getIndex());
        if (c == '-') {
          isNegative = true;
        } else if ((c >= '0') && (c <= '9')) {
          result = result * 10 + (c - '0');
        } else {
          break;
        }
        pos.setIndex(pos.getIndex() + 1);
      }
      return isNegative ? -result : result;
    }

    private double readDouble(CharSequence csq, ParsePosition pos) {
      final int length = csq.length();
      int start = pos.getIndex();
      int end = start + 1;
      while (end < length) {
        if ("0123456789+-.E".indexOf(csq.charAt(end)) < 0) {
          break;
        }
        end += 1;
      }
      pos.setIndex(end + 1);
      return Double.parseDouble(csq.subSequence(start, end).toString());
    }

    private String readIdentifier(CharSequence csq, ParsePosition pos) {
      final int length = csq.length();
      int start = pos.getIndex();
      int i = start;
      while ((++i < length) && isUnitIdentifierPart(csq.charAt(i))) {
      }
      pos.setIndex(i);
      return csq.subSequence(start, i).toString();
    }

    // //////////////////////////
    // Formatting.

    @Override
    public Appendable format(Unit<?> unit, Appendable appendable) throws IOException {
      String name = nameFor(unit);
      if (name != null) {
        return appendable.append(name);
      }
      if (!(unit instanceof ProductUnit)) {
        throw new IllegalArgumentException("Cannot format given Object as a Unit");
      }

      // Product unit.
      ProductUnit<?> productUnit = (ProductUnit<?>) unit;
      int invNbr = 0;

      // Write positive exponents first.
      boolean start = true;
      for (int i = 0; i < productUnit.getUnitCount(); i++) {
        int pow = productUnit.getUnitPow(i);
        if (pow >= 0) {
          if (!start) {
            appendable.append('\u00b7'); // Separator.
          }
          name = nameFor(productUnit.getUnit(i));
          int root = productUnit.getUnitRoot(i);
          append(appendable, name, pow, root);
          start = false;
        } else {
          invNbr++;
        }
      }

      // Write negative exponents.
      if (invNbr != 0) {
        if (start) {
          appendable.append('1'); // e.g. 1/s
        }
        appendable.append('/');
        if (invNbr > 1) {
          appendable.append('(');
        }
        start = true;
        for (int i = 0; i < productUnit.getUnitCount(); i++) {
          int pow = productUnit.getUnitPow(i);
          if (pow < 0) {
            name = nameFor(productUnit.getUnit(i));
            int root = productUnit.getUnitRoot(i);
            if (!start) {
              appendable.append('\u00b7'); // Separator.
            }
            append(appendable, name, -pow, root);
            start = false;
          }
        }
        if (invNbr > 1) {
          appendable.append(')');
        }
      }
      return appendable;
    }

    private void append(Appendable appendable, CharSequence symbol, int pow, int root) throws IOException {
      appendable.append(symbol);
      if ((pow != 1) || (root != 1)) {
        // Write exponent.
        if ((pow == 2) && (root == 1)) {
          appendable.append('\u00b2'); // Square
        } else if ((pow == 3) && (root == 1)) {
          appendable.append('\u00b3'); // Cubic
        } else {
          // Use general exponent form.
          appendable.append('^');
          appendable.append(String.valueOf(pow));
          if (root != 1) {
            appendable.append(':');
            appendable.append(String.valueOf(root));
          }
        }
      }
    }

    // private static final long serialVersionUID = 1L;

    @Override
    public Unit<?> parse(CharSequence csq) throws ParserException {
      return parse(csq, 0);
    }

    @Override
    protected SymbolMap getSymbols() {
      return null;
    }

    protected Unit<?> parse(CharSequence csq, int index) throws IllegalArgumentException {
      return parse(csq, new ParsePosition(index));
    }

    @Override
    protected Unit<?> parse(CharSequence csq, ParsePosition cursor) throws IllegalArgumentException {
      return parseObject(csq.toString(), cursor);
    }
  }

  /**
   * This class represents the ASCII format.
   */
  protected final static class ASCIIFormat extends DefaultFormat {

    @Override
    protected String nameFor(Unit<?> unit) {
      // First search if specific ASCII name should be used.
      String name = _unitToName.get(unit);
      if (name != null)
        return name;
      // Else returns default name.
      return DEFAULT.nameFor(unit);
    }

    @Override
    protected Unit<?> unitFor(String name) {
      // First search if specific ASCII name.
      Unit<?> unit = _nameToUnit.get(name);
      if (unit != null)
        return unit;
      // Else returns default mapping.
      return DEFAULT.unitFor(name);
    }

    @Override
    public Appendable format(Unit<?> unit, Appendable appendable) throws IOException {
      String name = nameFor(unit);
      if (name != null)
        return appendable.append(name);
      if (!(unit instanceof ProductUnit))
        throw new IllegalArgumentException("Cannot format given Object as a Unit");

      ProductUnit<?> productUnit = (ProductUnit<?>) unit;
      for (int i = 0; i < productUnit.getUnitCount(); i++) {
        if (i != 0) {
          appendable.append('*'); // Separator.
        }
        name = nameFor(productUnit.getUnit(i));
        int pow = productUnit.getUnitPow(i);
        int root = productUnit.getUnitRoot(i);
        appendable.append(name);
        if ((pow != 1) || (root != 1)) {
          // Use general exponent form.
          appendable.append('^');
          appendable.append(String.valueOf(pow));
          if (root != 1) {
            appendable.append(':');
            appendable.append(String.valueOf(root));
          }
        }
      }
      return appendable;
    }

    @Override
    public boolean isValidIdentifier(String name) {
      if ((name == null) || (name.length() == 0))
        return false;
      // label must not begin with a digit or mathematical operator
      return isUnitIdentifierPart(name.charAt(0)) && isAllASCII(name);
      /*
       * for (int i = 0; i < name.length(); i++) { if
       * (!isAsciiCharacter(name.charAt(i))) return false; } return true;
       */
    }
  }

  /**
   * Holds the unique symbols collection (base units or alternate units).
   */
  private static final Map<String, Unit<?>> SYMBOL_TO_UNIT = new HashMap<>();

  // //////////////////////////////////////////////////////////////////////////
  // Initializes the standard unit database for SI units.

  private static final Unit<?>[] SI_UNITS = { Units.AMPERE, Units.BECQUEREL, Units.CANDELA, Units.COULOMB, Units.FARAD, Units.GRAY, Units.HENRY,
      Units.HERTZ, Units.JOULE, Units.KATAL, Units.KELVIN, Units.LUMEN, Units.LUX, Units.METRE, Units.MOLE, Units.NEWTON, Units.OHM, Units.PASCAL,
      Units.RADIAN, Units.SECOND, Units.SIEMENS, Units.SIEVERT, Units.STERADIAN, Units.TESLA, Units.VOLT, Units.WATT, Units.WEBER };

  private static final String[] PREFIXES = { YOTTA.getSymbol(), ZETTA.getSymbol(), EXA.getSymbol(), PETA.getSymbol(), TERA.getSymbol(),
      GIGA.getSymbol(), MEGA.getSymbol(), KILO.getSymbol(), HECTO.getSymbol(), DEKA.getSymbol(), DECI.getSymbol(), CENTI.getSymbol(),
      MILLI.getSymbol(), MICRO.getSymbol(), NANO.getSymbol(), PICO.getSymbol(), FEMTO.getSymbol(), ATTO.getSymbol(), ZEPTO.getSymbol(),
      YOCTO.getSymbol() };

  // TODO we could try retrieving this dynamically in a static {} method from
  // MetricPrefix if symbols above are also aligned
  private static final UnitConverter[] CONVERTERS = { YOTTA.getConverter(), ZETTA.getConverter(), EXA.getConverter(), PETA.getConverter(),
      TERA.getConverter(), GIGA.getConverter(), MEGA.getConverter(), KILO.getConverter(), HECTO.getConverter(), DEKA.getConverter(),
      DECI.getConverter(), CENTI.getConverter(), MILLI.getConverter(), MICRO.getConverter(), NANO.getConverter(), PICO.getConverter(),
      FEMTO.getConverter(), ATTO.getConverter(), ZEPTO.getConverter(), YOCTO.getConverter() };

  private static String asciiPrefix(String prefix) {
    return prefix == "µ" ? "micro" : prefix;
  }

  // to check if a string only contains US-ASCII characters
  //
  protected static boolean isAllASCII(String input) {
    boolean isASCII = true;
    for (int i = 0; i < input.length(); i++) {
      int c = input.charAt(i);
      if (c > 0x7F) {
        isASCII = false;
        break;
      }
    }
    return isASCII;
  }

  // Initializations
  static {
    for (int i = 0; i < SI_UNITS.length; i++) {
      Unit<?> si = SI_UNITS[i];
      String symbol = (si instanceof BaseUnit) ? ((BaseUnit<?>) si).getSymbol() : ((AlternateUnit<?>) si).getSymbol();
      DEFAULT.label(si, symbol);
      if (isAllASCII(symbol))
        ASCII.label(si, symbol);
      for (int j = 0; j < PREFIXES.length; j++) {
        Unit<?> u = si.transform(CONVERTERS[j]);
        DEFAULT.label(u, PREFIXES[j] + symbol);
        if (PREFIXES[j] == "µ") {
          ASCII.label(u, "micro"); // + symbol);
        }
      }
    }
    // Special case for KILOGRAM.
    DEFAULT.label(Units.GRAM, "g");
    for (int i = 0; i < PREFIXES.length; i++) {
      if (CONVERTERS[i] == KILO.getConverter()) // TODO should it better
        // be equals()?
        continue; // kg is already defined.
      DEFAULT.label(Units.KILOGRAM.transform(CONVERTERS[i].concatenate(MILLI.getConverter())), PREFIXES[i] + "g");
      if (PREFIXES[i] == "µ") {
        ASCII.label(Units.KILOGRAM.transform(CONVERTERS[i].concatenate(MILLI.getConverter())), "microg");
      }
    }

    // Alias and ASCIIFormat for Ohm
    DEFAULT.alias(Units.OHM, "Ohm");
    ASCII.label(Units.OHM, "Ohm");
    for (int i = 0; i < PREFIXES.length; i++) {
      DEFAULT.alias(Units.OHM.transform(CONVERTERS[i]), PREFIXES[i] + "Ohm");
      ASCII.label(Units.OHM.transform(CONVERTERS[i]), asciiPrefix(PREFIXES[i]) + "Ohm");
    }

    // Special case for DEGREE_CELSIUS.
    DEFAULT.label(Units.CELSIUS, "℃");
    DEFAULT.alias(Units.CELSIUS, "°C");
    ASCII.label(Units.CELSIUS, "Celsius");
    for (int i = 0; i < PREFIXES.length; i++) {
      DEFAULT.label(Units.CELSIUS.transform(CONVERTERS[i]), PREFIXES[i] + "℃");
      DEFAULT.alias(Units.CELSIUS.transform(CONVERTERS[i]), PREFIXES[i] + "°C");
      ASCII.label(Units.CELSIUS.transform(CONVERTERS[i]), asciiPrefix(PREFIXES[i]) + "Celsius");
    }

    DEFAULT.label(Units.PERCENT, "%");
    DEFAULT.label(Units.KILOGRAM, "kg");
    ASCII.label(Units.KILOGRAM, "kg");
    DEFAULT.label(Units.METRE, "m");
    ASCII.label(Units.METRE, "m");
    DEFAULT.label(Units.SECOND, "s");
    ASCII.label(Units.SECOND, "s");
    DEFAULT.label(Units.MINUTE, "min");
    DEFAULT.label(Units.HOUR, "h");
    DEFAULT.label(Units.DAY, "day");
    DEFAULT.alias(Units.DAY, "d");
    DEFAULT.label(Units.WEEK, "week");
    DEFAULT.label(Units.YEAR, "year");
    DEFAULT.alias(Units.YEAR, "days365");
    ASCII.label(Units.KILOMETRE_PER_HOUR, "km/h");
    DEFAULT.label(Units.KILOMETRE_PER_HOUR, "km/h");
    DEFAULT.label(Units.CUBIC_METRE, "\u33A5");
    ASCII.label(Units.CUBIC_METRE, "m3");
    ASCII.label(Units.LITRE, "l");
    DEFAULT.label(Units.LITRE, "l");
    DEFAULT.label(MetricPrefix.MICRO(Units.LITRE), "µl");
    ASCII.label(MetricPrefix.MICRO(Units.LITRE), "microL");
    ASCII.label(MetricPrefix.MILLI(Units.LITRE), "mL");
    DEFAULT.label(MetricPrefix.MILLI(Units.LITRE), "ml");
    ASCII.label(MetricPrefix.CENTI(Units.LITRE), "cL");
    DEFAULT.label(MetricPrefix.CENTI(Units.LITRE), "cl");
    ASCII.label(MetricPrefix.DECI(Units.LITRE), "dL");
    DEFAULT.label(MetricPrefix.DECI(Units.LITRE), "dl");
    DEFAULT.label(Units.NEWTON, "N");
    ASCII.label(Units.NEWTON, "N");
    DEFAULT.label(Units.RADIAN, "rad");
    ASCII.label(Units.RADIAN, "rad");

    DEFAULT.label(AbstractUnit.ONE, "one");
    ASCII.label(AbstractUnit.ONE, "one");
  }
}