/**
 * Copyright (C) 2019 Adrien Hopkins
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
package sevenUnits.unit;

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.function.DoubleUnaryOperator;

import sevenUnits.utils.DecimalComparison;
import sevenUnits.utils.NameSymbol;
import sevenUnits.utils.Nameable;
import sevenUnits.utils.ObjectProduct;

/**
 * A unit that is composed of base units.
 * 
 * @author Adrien Hopkins
 * @since 2019-10-16
 */
public abstract class Unit implements Nameable {
	/**
	 * Returns a unit from its base and the functions it uses to convert to and
	 * from its base.
	 * 
	 * <p>
	 * For example, to get a unit representing the degree Celsius, the following
	 * code can be used:
	 * 
	 * {@code Unit.fromConversionFunctions(SI.KELVIN, tempK -> tempK - 273.15, tempC -> tempC + 273.15);}
	 * </p>
	 * 
	 * @param base          unit's base
	 * @param converterFrom function that accepts a value expressed in the unit's
	 *                      base and returns that value expressed in this unit.
	 * @param converterTo   function that accepts a value expressed in the unit
	 *                      and returns that value expressed in the unit's base.
	 * @return a unit that uses the provided functions to convert.
	 * @since 2019-05-22
	 * @throws NullPointerException if any argument is null
	 */
	public static final Unit fromConversionFunctions(
			final ObjectProduct<BaseUnit> base,
			final DoubleUnaryOperator converterFrom,
			final DoubleUnaryOperator converterTo) {
		return new FunctionalUnit(base, converterFrom, converterTo);
	}

	/**
	 * Returns a unit from its base and the functions it uses to convert to and
	 * from its base.
	 * 
	 * <p>
	 * For example, to get a unit representing the degree Celsius, the following
	 * code can be used:
	 * 
	 * {@code Unit.fromConversionFunctions(SI.KELVIN, tempK -> tempK - 273.15, tempC -> tempC + 273.15);}
	 * </p>
	 * 
	 * @param base          unit's base
	 * @param converterFrom function that accepts a value expressed in the unit's
	 *                      base and returns that value expressed in this unit.
	 * @param converterTo   function that accepts a value expressed in the unit
	 *                      and returns that value expressed in the unit's base.
	 * @param ns            names and symbol of unit
	 * @return a unit that uses the provided functions to convert.
	 * @since 2019-05-22
	 * @throws NullPointerException if any argument is null
	 */
	public static final Unit fromConversionFunctions(
			final ObjectProduct<BaseUnit> base,
			final DoubleUnaryOperator converterFrom,
			final DoubleUnaryOperator converterTo, final NameSymbol ns) {
		return new FunctionalUnit(base, converterFrom, converterTo, ns);
	}

	/**
	 * The combination of units that this unit is based on.
	 * 
	 * @since 2019-10-16
	 */
	private final ObjectProduct<BaseUnit> unitBase;

	/**
	 * This unit's name(s) and symbol
	 * 
	 * @since 2020-09-07
	 */
	private final NameSymbol nameSymbol;

	/**
	 * Cache storing the result of getDimension()
	 *
	 * @since 2019-10-16
	 */
	private transient ObjectProduct<BaseDimension> dimension = null;

	/**
	 * A constructor that constructs {@code BaseUnit} instances.
	 * 
	 * @since 2019-10-16
	 */
	Unit(final NameSymbol nameSymbol) {
		if (this instanceof BaseUnit) {
			this.unitBase = ObjectProduct.oneOf((BaseUnit) this);
		} else
			throw new AssertionError();
		this.nameSymbol = nameSymbol;
	}

	/**
	 * Creates the {@code Unit}.
	 * 
	 * @param unitBase base of unit
	 * @param ns       names and symbol of unit
	 * @since 2019-10-16
	 * @throws NullPointerException if unitBase or ns is null
	 */
	protected Unit(ObjectProduct<BaseUnit> unitBase, NameSymbol ns) {
		this.unitBase = Objects.requireNonNull(unitBase,
				"unitBase may not be null");
		this.nameSymbol = Objects.requireNonNull(ns, "ns may not be null");
	}

	/**
	 * Checks if a value expressed in this unit can be converted to a value
	 * expressed in {@code other}
	 * 
	 * @param other unit or unitlike form to test with
	 * @return true if they are compatible
	 * @since 2019-01-13
	 * @since v0.1.0
	 * @throws NullPointerException if other is null
	 */
	public final boolean canConvertTo(final Unit other) {
		Objects.requireNonNull(other, "other must not be null.");
		return Objects.equals(this.getBase(), other.getBase());
	}

	/**
	 * Converts from a value expressed in this unit's base unit to a value
	 * expressed in this unit.
	 * <p>
	 * This must be the inverse of {@code convertToBase}, so
	 * {@code convertFromBase(convertToBase(value))} must be equal to
	 * {@code value} for any value, ignoring precision loss by roundoff error.
	 * </p>
	 * <p>
	 * If this unit <i>is</i> a base unit, this method should return
	 * {@code value}.
	 * </p>
	 * 
	 * @implSpec This method is used by {@link #convertTo}, and its behaviour
	 *           affects the behaviour of {@code convertTo}.
	 * 
	 * @param value value expressed in <b>base</b> unit
	 * @return value expressed in <b>this</b> unit
	 * @since 2018-12-22
	 * @since v0.1.0
	 */
	protected abstract double convertFromBase(double value);

	/**
	 * Converts a value expressed in this unit to a value expressed in
	 * {@code other}.
	 * 
	 * @implSpec If unit conversion is possible, this implementation returns
	 *           {@code other.convertFromBase(this.convertToBase(value))}.
	 *           Therefore, overriding either of those methods will change the
	 *           output of this method.
	 * 
	 * @param other unit to convert to
	 * @param value value to convert
	 * @return converted value
	 * @since 2019-05-22
	 * @throws IllegalArgumentException if {@code other} is incompatible for
	 *                                  conversion with this unit (as tested by
	 *                                  {@link Unit#canConvertTo}).
	 * @throws NullPointerException     if other is null
	 */
	public final double convertTo(final Unit other, final double value) {
		Objects.requireNonNull(other, "other must not be null.");
		if (this.canConvertTo(other))
			return other.convertFromBase(this.convertToBase(value));
		else
			throw new IllegalArgumentException(
					String.format("Cannot convert from %s to %s.", this, other));
	}

	/**
	 * Converts from a value expressed in this unit to a value expressed in this
	 * unit's base unit.
	 * <p>
	 * This must be the inverse of {@code convertFromBase}, so
	 * {@code convertToBase(convertFromBase(value))} must be equal to
	 * {@code value} for any value, ignoring precision loss by roundoff error.
	 * </p>
	 * <p>
	 * If this unit <i>is</i> a base unit, this method should return
	 * {@code value}.
	 * </p>
	 * 
	 * @implSpec This method is used by {@link #convertTo}, and its behaviour
	 *           affects the behaviour of {@code convertTo}.
	 * 
	 * @param value value expressed in <b>this</b> unit
	 * @return value expressed in <b>base</b> unit
	 * @since 2018-12-22
	 * @since v0.1.0
	 */
	protected abstract double convertToBase(double value);

	/**
	 * @return combination of units that this unit is based on
	 * @since 2018-12-22
	 * @since v0.1.0
	 */
	public final ObjectProduct<BaseUnit> getBase() {
		return this.unitBase;
	}

	/**
	 * @return dimension measured by this unit
	 * @since 2018-12-22
	 * @since v0.1.0
	 */
	public final ObjectProduct<BaseDimension> getDimension() {
		if (this.dimension == null) {
			final Map<BaseUnit, Integer> mapping = this.unitBase.exponentMap();
			final Map<BaseDimension, Integer> dimensionMap = new HashMap<>();

			for (final BaseUnit key : mapping.keySet()) {
				dimensionMap.put(key.getBaseDimension(), mapping.get(key));
			}

			this.dimension = ObjectProduct.fromExponentMapping(dimensionMap);
		}
		return this.dimension;
	}

	/**
	 * @return the nameSymbol
	 * @since 2020-09-07
	 */
	@Override
	public final NameSymbol getNameSymbol() {
		return this.nameSymbol;
	}

	/**
	 * Returns true iff this unit is metric.
	 * <p>
	 * "Metric" is defined by three conditions:
	 * <ul>
	 * <li>Must be an instance of {@link LinearUnit}.</li>
	 * <li>Must be based on the SI base units (as determined by getBase())</li>
	 * <li>The conversion factor must be a power of 10.</li>
	 * </ul>
	 * <p>
	 * Note that this definition excludes some units that many would consider
	 * "metric", such as the degree Celsius (fails the first condition),
	 * calories, minutes and hours (fail the third condition).
	 * <p>
	 * All SI units (as designated by the BIPM) except the degree Celsius are
	 * considered "metric" by this definition.
	 * 
	 * @since 2020-08-27
	 */
	public final boolean isMetric() {
		// first condition - check that it is a linear unit
		if (!(this instanceof LinearUnit))
			return false;
		final LinearUnit linear = (LinearUnit) this;

		// second condition - check that
		for (final BaseUnit b : linear.getBase().getBaseSet()) {
			if (!Metric.BaseUnits.BASE_UNITS.contains(b))
				return false;
		}

		// third condition - check that conversion factor is a power of 10
		return DecimalComparison
				.equals(Math.log10(linear.getConversionFactor()) % 1.0, 0);
	}

	/**
	 * @return a string representing this unit's definition
	 * @since 2022-03-10
	 */
	public String toDefinitionString() {
		if (!this.unitBase.getNameSymbol().isEmpty())
			return "derived from " + this.unitBase.getName();
		else
			return "derived from "
					+ this.getBase().toString(BaseUnit::getShortName);
	}

	/**
	 * @return a string containing both this unit's name and its definition
	 * @since 2022-03-10
	 */
	public final String toFullString() {
		return this.toString() + " (" + this.toDefinitionString() + ")";
	}

	@Override
	public String toString() {
		if (this.nameSymbol.getPrimaryName().isPresent()
				&& this.nameSymbol.getSymbol().isPresent())
			return this.nameSymbol.getPrimaryName().orElseThrow() + " ("
					+ this.nameSymbol.getSymbol().orElseThrow() + ")";
		else
			return this.getName();
	}

	/**
	 * @param ns name(s) and symbol to use
	 * @return a copy of this unit with provided name(s) and symbol
	 * @since 2019-10-21
	 * @throws NullPointerException if ns is null
	 */
	public Unit withName(final NameSymbol ns) {
		return fromConversionFunctions(this.getBase(), this::convertFromBase,
				this::convertToBase,
				Objects.requireNonNull(ns, "ns must not be null."));
	}
}