/*
 * Units of Measurement Reference Implementation
 * Copyright (c) 2005-2021, Jean-Marie Dautelle, Werner Keil, Otavio Santana.
 *
 * 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-385, Indriya nor the names of their 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 tech.units.indriya.format;

import java.io.IOException;
import java.text.FieldPosition;
import java.text.ParsePosition;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import javax.measure.BinaryPrefix;
import javax.measure.MeasurementError;
import javax.measure.MetricPrefix;
import javax.measure.Prefix;
import javax.measure.Quantity;
import javax.measure.Unit;
import javax.measure.UnitConverter;
import javax.measure.format.MeasurementParseException;
import javax.measure.format.UnitFormat;

import static javax.measure.MetricPrefix.MICRO;

import tech.units.indriya.AbstractUnit;
import tech.units.indriya.function.AddConverter;
import tech.units.indriya.function.MultiplyConverter;
import tech.units.indriya.function.RationalNumber;
import tech.units.indriya.unit.AlternateUnit;
import tech.units.indriya.unit.AnnotatedUnit;
import tech.units.indriya.unit.BaseUnit;
import tech.units.indriya.unit.ProductUnit;
import tech.units.indriya.unit.TransformedUnit;
import tech.units.indriya.unit.Units;

import static tech.units.indriya.format.FormatConstants.MIDDLE_DOT;

/**
 * <p>
 * This class implements the {@link UnitFormat} interface for formatting and parsing {@link Unit units}.
 * </p>
 * 
 * <p>
 * For all SI units, the <b>20 SI prefixes</b> used to form decimal multiples and sub-multiples are recognized. As well as the <b>8 binary prefixes</b>.<br>
 * {@link Units} are directly recognized. For example:<br>
 * <code>
 *        UnitFormat format = SimpleUnitFormat.getInstance();<br>
 *        format.parse("m°C").equals(MetricPrefix.MILLI(Units.CELSIUS));<br>
 *        format.parse("kW").equals(MetricPrefix.KILO(Units.WATT));<br>
 *        format.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:werner@units.tech">Werner Keil</a>
 * @author Eric Russell
 * @author Andi Huber
 * @version 2.3, Nov. 27, 2020
 * @since 1.0
 */
public abstract class SimpleUnitFormat extends AbstractUnitFormat {
    /**
     * 
     */
    // private static final long serialVersionUID = 4149424034841739785L;#

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

    // Initializes the standard unit database for SI units.

    private static final Unit<?>[] METRIC_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[] METRIC_PREFIX_SYMBOLS =  
            Stream.of(MetricPrefix.values())
            .map(Prefix::getSymbol)
            .collect(Collectors.toList())
            .toArray(new String[] {});

    // TODO try to consolidate those
    private static final UnitConverter[] METRIC_PREFIX_CONVERTERS =  
            Stream.of(MetricPrefix.values())
            .map(MultiplyConverter::ofPrefix)
            .collect(Collectors.toList())
            .toArray(new UnitConverter[] {});

    private static final String[] BINARY_PREFIX_SYMBOLS =  
            Stream.of(BinaryPrefix.values())
            .map(Prefix::getSymbol)
            .collect(Collectors.toList())
            .toArray(new String[] {});

    private static final UnitConverter[] BINARY_PREFIX_CONVERTERS =  
            Stream.of(BinaryPrefix.values())
            .map(MultiplyConverter::ofPrefix)
            .collect(Collectors.toList())
            .toArray(new UnitConverter[] {});

    private static final String MU = "\u03bc";

    /**
     * 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 MeasurementParseException;

    /**
     * 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 MeasurementParseException;

    /**
     * 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);

    /**
     * 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.
     */
    protected 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 {
            final Object dest = toAppendTo;
            if (dest instanceof Appendable) {
                format((Unit<?>) unit, (Appendable) dest);
            } else { // When retroweaver is used to produce 1.4 binaries. TODO is this still relevant?
                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 MeasurementError(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 MeasurementParseException {
        return parseProductUnit(source, pos);
    }

    /**
     * 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 {
        private static enum Token { EOF, IDENTIFIER, OPEN_PAREN, CLOSE_PAREN, EXPONENT, MULTIPLY, DIVIDE, 
            PLUS, INTEGER, FLOAT };

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

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

            @Override
            public String toString() {
                return "SimpleUnitFormat";
            }

            @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
            protected boolean isValidIdentifier(String name) {
                if ((name == null) || (name.length() == 0))
                    return false;
                return isUnitIdentifierPart(name.charAt(0));
            }

            protected static boolean isUnitIdentifierPart(char ch) {
                return Character.isLetter(ch)
                        || (!Character.isWhitespace(ch) && !Character.isDigit(ch) && (ch != MIDDLE_DOT) && (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;
                    if (tfmUnit.getSymbol() != null) {
                        return tfmUnit.getSymbol();
                    }
                    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(MIDDLE_DOT) >= 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 MultiplyConverter) {
                            Number scaleFactor = ((MultiplyConverter) cvtr).getFactor();
                            if(scaleFactor instanceof RationalNumber) {

                                RationalNumber rational = (RationalNumber)scaleFactor;
                                RationalNumber reciprocal = rational.reciprocal();
                                if(reciprocal.isInteger()) {
                                    result.append('/');
                                    result.append(reciprocal.toString()); // renders as integer
                                } else {
                                    result.append('*');
                                    result.append(scaleFactor);  
                                }

                            } else {
                                result.append('*');
                                result.append(scaleFactor);
                            }

                        } else { // Other converters.
                            return "[" + baseUnit + "?]";
                        }
                    }
                    return result.toString();
                }
                if (unit instanceof AnnotatedUnit<?>) {
                    AnnotatedUnit<?> annotatedUnit = (AnnotatedUnit<?>) unit;
                    final StringBuilder annotable = new StringBuilder(nameFor(annotatedUnit.getActualUnit()));
                    if (annotatedUnit.getAnnotation() != null) {
                        annotable.append('{'); // TODO maybe also configure this one similar to mix delimiter
                        annotable.append(annotatedUnit.getAnnotation());
                        annotable.append('}');
                    }
                    return annotable.toString();
                }
                // mixed unit.
                /*      if (unit instanceof MixedUnit) {
        MixedUnit<?> mixUnit = (MixedUnit<?>) unit;
        final StringBuilder mixer = new StringBuilder();
        final int partSize = mixUnit.getUnits().size();
        int pos = 0;
        for (Unit<?> part : mixUnit.getUnits()) {
            mixer.append(nameFor(part));
            pos++;
            if (mixer.length() > 0 && pos < partSize) 
                mixer.append(";"); // FIXME we need a more flexible pattern here
        }
        return mixer.toString();
      }
                 */
                return null; // Product unit.
            }

            // Returns the prefix for the specified unit converter.
            protected String prefixFor(UnitConverter converter) {
                for (int i = 0; i < METRIC_PREFIX_CONVERTERS.length; i++) {
                    if (METRIC_PREFIX_CONVERTERS[i].equals(converter)) {
                        return METRIC_PREFIX_SYMBOLS[i];
                    }
                }
                for (int j = 0; j < BINARY_PREFIX_CONVERTERS.length; j++) {
                    if (BINARY_PREFIX_CONVERTERS[j].equals(converter)) {
                        return BINARY_PREFIX_SYMBOLS[j];
                    }
                }
                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;
                } else {
                    unit = SYMBOL_TO_UNIT.get(name);
                }
                return unit;
            }

            // //////////////////////////
            // Parsing.
            @SuppressWarnings({ "rawtypes", "unchecked" })
            public Unit<? extends Quantity> parseSingleUnit(CharSequence csq, ParsePosition pos) throws MeasurementParseException {
                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 MeasurementParseException {
                Unit result = null;
                if (csq == null) {
                        throw new MeasurementParseException("Cannot parse null", csq, pos.getIndex());
                } else {
                        result = unitFor(csq.toString());
                        if (result != null)
                                return result;
                }
                result = AbstractUnit.ONE;
                Token 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 == Token.CLOSE_PAREN, "')' expected", csq, pos.getIndex());
                    pos.setIndex(pos.getIndex() + 1);
                    break;
                default:
                    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 == Token.INTEGER) {
                            long n = readLong(csq, pos);
                            if (n != 1) {
                                result = result.multiply(n);
                            }
                        } else if (token == 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 == Token.INTEGER) {
                            long n = readLong(csq, pos);
                            if (n != 1) {
                                result = result.divide(n);
                            }
                        } else if (token == 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 == Token.INTEGER) {
                            long n = readLong(csq, pos);
                            if (n != 1) {
                                result = result.shift(n);
                            }
                        } else if (token == Token.FLOAT) {
                            double d = readDouble(csq, pos);
                            if (d != 1.0) {
                                result = result.shift(d);
                            }
                        } else {
                            throw new MeasurementParseException("not a number", csq, pos.getIndex());
                        }
                        break;
                    case EOF:
                    case CLOSE_PAREN:
                        return result;
                    default:
                        throw new MeasurementParseException("unexpected token " + token, csq, pos.getIndex());
                    }
                    token = nextToken(csq, pos);
                }
            }

            private static Token nextToken(CharSequence csq, ParsePosition pos) {
                final int length = csq.length();
                while (pos.getIndex() < length) {
                    char c = csq.charAt(pos.getIndex());
                    if (isUnitIdentifierPart(c)) {
                        return Token.IDENTIFIER;
                    } else if (c == '(') {
                        return Token.OPEN_PAREN;
                    } else if (c == ')') {
                        return Token.CLOSE_PAREN;
                    } else if ((c == '^') || (c == '\u00b9') || (c == '\u00b2') || (c == '\u00b3')) {
                        return Token.EXPONENT;
                    } else if (c == '*') {
                        if (csq.length() == pos.getIndex() + 1) {
                            throw new MeasurementParseException("unexpected token " + Token.EOF, csq, pos.getIndex()); // return ;
                        }
                        char c2 = csq.charAt(pos.getIndex() + 1);
                        return c2 == '*' ? Token.EXPONENT : Token.MULTIPLY;
                    } else if (c == MIDDLE_DOT) {
                        return Token.MULTIPLY;
                    } else if (c == '/') {
                        return Token.DIVIDE;
                    } else if (c == '+') {
                        return Token.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 Token.FLOAT;
                            }
                        }
                        return Token.INTEGER;
                    }
                    pos.setIndex(pos.getIndex() + 1);
                }
                return Token.EOF;
            }

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

            private static 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;
                boolean parseRoot = false;

                POWERLOOP: while (pos.getIndex() < length) {
                    c = csq.charAt(pos.getIndex());
                    switch(c) {
                    case '-': isPowNegative = true; break;
                    case '\u00b9': pow = pow * 10 + 1; break;
                    case '\u00b2': pow = pow * 10 + 2; break;
                    case '\u00b3': pow = pow * 10 + 3; break;
                    case ':': parseRoot = true; break POWERLOOP; 
                    default: 
                        if (c >= '0' && c <= '9') pow = pow * 10 + (c - '0');  
                        else break POWERLOOP; 
                    }
                    pos.setIndex(pos.getIndex() + 1);
                }
                if (pow == 0) pow = 1;

                int root = 0;
                boolean isRootNegative = false;
                if (parseRoot) {
                    pos.setIndex(pos.getIndex() + 1);
                    ROOTLOOP: while (pos.getIndex() < length) {
                        c = csq.charAt(pos.getIndex());
                        switch(c) {
                        case '-': isRootNegative = true; break;
                        case '\u00b9': root = root * 10 + 1; break;
                        case '\u00b2': root = root * 10 + 2; break;
                        case '\u00b3': root = root * 10 + 3; break;
                        default: 
                            if (c >= '0' && c <= '9') root = root * 10 + (c - '0');  
                            else break ROOTLOOP; 
                        }
                        pos.setIndex(pos.getIndex() + 1);
                    }
                }
                if (root == 0) root = 1;

                return new Exponent(isPowNegative ? -pow : pow, isRootNegative ? -root : root);
            }

            private static 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 static 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 static 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;

                // Special case: self-powered product unit
                if (productUnit.getUnitCount() == 1 && productUnit.getUnit(0) instanceof ProductUnit) {
                    final ProductUnit<?> powerUnit = (ProductUnit<?>) productUnit.getUnit(0);
                    // is the sub-unit known under a given label?
                    if (nameFor(powerUnit) == null)
                        // apply the power to the sub-units and format those instead
                        return format(ProductUnit.ofPow(powerUnit, productUnit.getUnitPow(0)), appendable);
                }

                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(MIDDLE_DOT); // 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(MIDDLE_DOT); // Separator.
                            }
                            append(appendable, name, -pow, root);
                            start = false;
                        }
                    }
                    if (invNbr > 1) {
                        appendable.append(')');
                    }
                }
                return appendable;
            }

            private static 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 MeasurementParseException {
                return parse(csq, 0);
            }

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

            @Override
            public 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 String toString() {
            return "SimpleUnitFormat - ASCII";
        }

        @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
        protected 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<>();

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

    private static String asciiSymbol(String s) {
        return "Ω".equals(s) ? "Ohm" : s;
    }

    /** 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;
    }
    
    // -- INIT
    
    static {
        // Hack, somehow µg is not found.
        SYMBOL_TO_UNIT.put(MetricPrefix.MICRO.getSymbol() + "g", MICRO(Units.GRAM));
        SYMBOL_TO_UNIT.put("μg", MICRO(Units.GRAM));
        SYMBOL_TO_UNIT.put(MU + "g", MICRO(Units.GRAM));
    }
    
    /**
     * Holds the standard unit format.
     */
    private static final DefaultFormat DEFAULT = initDefaultFormat(new DefaultFormat());

    /**
     * Holds the ASCIIFormat unit format.
     */
    private static final ASCIIFormat ASCII = initASCIIFormat(new ASCIIFormat());
    
    // -- FACTORIES
    
    protected static DefaultFormat initDefaultFormat(final DefaultFormat defaultFormat) {
    
        for (int i = 0; i < METRIC_UNITS.length; i++) {
            Unit<?> si = METRIC_UNITS[i];
            String symbol = (si instanceof BaseUnit) ? ((BaseUnit<?>) si).getSymbol() : ((AlternateUnit<?>) si).getSymbol();
            defaultFormat.label(si, symbol);
            for (int j = 0; j < METRIC_PREFIX_SYMBOLS.length; j++) {
                Unit<?> u = si.prefix(MetricPrefix.values()[j]);
                defaultFormat.label(u, METRIC_PREFIX_SYMBOLS[j] + symbol);
                if ( "µ".equals(METRIC_PREFIX_SYMBOLS[j]) ) {
                    defaultFormat.label(u, MU + symbol);
                }
            } // TODO what about BINARY_PREFIX here?
        }

        // -- GRAM/KILOGRAM

        defaultFormat.label(Units.GRAM, "g");
        for(MetricPrefix prefix : MetricPrefix.values()) {
            switch (prefix) {
            case KILO:
                defaultFormat.label(Units.KILOGRAM, "kg");
                break;
            case MICRO:
                defaultFormat.label(Units.GRAM.prefix(prefix), prefix.getSymbol()+"g");
                break;
            default:
                defaultFormat.label(Units.GRAM.prefix(prefix), prefix.getSymbol()+"g");
                break;
            }
        }

        defaultFormat.label(MICRO(Units.GRAM), MetricPrefix.MICRO.getSymbol() + "g");

        // Alias and ASCIIFormat for Ohm
        defaultFormat.alias(Units.OHM, "Ohm");
        for (int i = 0; i < METRIC_PREFIX_SYMBOLS.length; i++) {
            defaultFormat.alias(Units.OHM.prefix(MetricPrefix.values()[i]), METRIC_PREFIX_SYMBOLS[i] + "Ohm");
        }

        // Special case for DEGREE_CELSIUS.
        defaultFormat.label(Units.CELSIUS, "℃");
        defaultFormat.alias(Units.CELSIUS, "°C");
        for (int i = 0; i < METRIC_PREFIX_SYMBOLS.length; i++) {
            defaultFormat.label(Units.CELSIUS.prefix(MetricPrefix.values()[i]), METRIC_PREFIX_SYMBOLS[i] + "℃");
            defaultFormat.alias(Units.CELSIUS.prefix(MetricPrefix.values()[i]), METRIC_PREFIX_SYMBOLS[i] + "°C");
        }

        defaultFormat.label(Units.PERCENT, "%");
        defaultFormat.label(Units.METRE, "m");
        defaultFormat.label(Units.SECOND, "s");
        defaultFormat.label(Units.MINUTE, "min");
        defaultFormat.label(Units.HOUR, "h");
        defaultFormat.label(Units.DAY, "day");
        defaultFormat.alias(Units.DAY, "d");
        defaultFormat.label(Units.WEEK, "week");
        defaultFormat.label(Units.YEAR, "year");
        defaultFormat.alias(Units.YEAR, "days365");
        defaultFormat.label(Units.MONTH, "mo");
        defaultFormat.alias(Units.MONTH, "mon");
        defaultFormat.alias(Units.MONTH, "month");
        defaultFormat.label(Units.KILOMETRE_PER_HOUR, "km/h");
        defaultFormat.label(Units.CUBIC_METRE, "\u33A5");

        // -- LITRE

        defaultFormat.label(Units.LITRE, "l");
        for(Prefix prefix : MetricPrefix.values()) {
            defaultFormat.label(Units.LITRE.prefix(prefix), prefix.getSymbol()+"l");
        }   
        defaultFormat.label(Units.NEWTON, "N");
        defaultFormat.label(Units.RADIAN, "rad");

        defaultFormat.label(AbstractUnit.ONE, "one");
        
        defaultFormat.alias(Units.SQUARE_METRE, "m2");
        defaultFormat.alias(Units.CUBIC_METRE, "m3");
        
        return defaultFormat;
    }
    
    protected static ASCIIFormat initASCIIFormat(final ASCIIFormat asciiFormat) {
        
        for (int i = 0; i < METRIC_UNITS.length; i++) {
            Unit<?> si = METRIC_UNITS[i];
            String symbol = (si instanceof BaseUnit) ? ((BaseUnit<?>) si).getSymbol() : ((AlternateUnit<?>) si).getSymbol();
            if (isAllASCII(symbol))
                asciiFormat.label(si, symbol);
            for (int j = 0; j < METRIC_PREFIX_SYMBOLS.length; j++) {
                Unit<?> u = si.prefix(MetricPrefix.values()[j]);
                if ( "µ".equals(METRIC_PREFIX_SYMBOLS[j]) ) {
                    asciiFormat.label(u, "micro" + asciiSymbol(symbol));
                }
            } // TODO what about BINARY_PREFIX here?
        }

        // -- GRAM/KILOGRAM

        asciiFormat.label(Units.GRAM, "g");
        for(MetricPrefix prefix : MetricPrefix.values()) {
            switch (prefix) {
            case KILO:
                asciiFormat.label(Units.KILOGRAM, "kg");
                break;
            case MICRO:
                asciiFormat.label(MICRO(Units.LITRE), "microg"); // instead of 'µg' -> 'microg'
                break;
            default:
                asciiFormat.label(Units.GRAM.prefix(prefix), prefix.getSymbol()+"g");
                break;
            }
        }

        // Alias and ASCIIFormat for Ohm
        asciiFormat.label(Units.OHM, "Ohm");
        for (int i = 0; i < METRIC_PREFIX_SYMBOLS.length; i++) {
            asciiFormat.label(Units.OHM.prefix(MetricPrefix.values()[i]), asciiPrefix(METRIC_PREFIX_SYMBOLS[i]) + "Ohm");
        }

        // Special case for DEGREE_CELSIUS.
        asciiFormat.label(Units.CELSIUS, "Celsius");
        for (int i = 0; i < METRIC_PREFIX_SYMBOLS.length; i++) {
            asciiFormat.label(Units.CELSIUS.prefix(MetricPrefix.values()[i]), asciiPrefix(METRIC_PREFIX_SYMBOLS[i]) + "Celsius");
        }
        asciiFormat.alias(Units.CELSIUS, "Cel");

        asciiFormat.label(Units.METRE, "m");
        asciiFormat.label(Units.SECOND, "s");
        asciiFormat.label(Units.KILOMETRE_PER_HOUR, "km/h");        
        asciiFormat.alias(Units.SQUARE_METRE, "m2");
        asciiFormat.alias(Units.CUBIC_METRE, "m3");
        
        // -- LITRE

        asciiFormat.label(Units.LITRE, "l");
        for(Prefix prefix : MetricPrefix.values()) {
            if(prefix==MICRO) {
                asciiFormat.label(MICRO(Units.LITRE), "microL"); // instead of 'µL' -> 'microL'
            } else {
                asciiFormat.label(Units.LITRE.prefix(prefix), prefix.getSymbol()+"L");
            }
        }   
        asciiFormat.label(Units.NEWTON, "N");
        asciiFormat.label(Units.RADIAN, "rad");

        asciiFormat.label(AbstractUnit.ONE, "one");
        
        return asciiFormat;
    }
}