/**
 * 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 org.unitConverter.unit;

import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;
import java.util.function.DoubleUnaryOperator;

import org.unitConverter.math.ObjectProduct;

/**
 * A unit that is composed of base units.
 * 
 * @author Adrien Hopkins
 * @since 2019-10-16
 */
public abstract class Unit {
	/**
	 * 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;

	/**
	 * The primary name used by this unit.
	 */
	private final Optional<String> primaryName;

	/**
	 * A short symbol used to represent this unit.
	 */
	private final Optional<String> symbol;

	/**
	 * A set of any additional names and/or spellings that the unit uses.
	 */
	private final Set<String> otherNames;

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

	/**
	 * Creates the {@code AbstractUnit}.
	 * 
	 * @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(final ObjectProduct<BaseUnit> unitBase, final NameSymbol ns) {
		this.unitBase = Objects.requireNonNull(unitBase, "unitBase must not be null.");
		this.primaryName = Objects.requireNonNull(ns, "ns must not be null.").getPrimaryName();
		this.symbol = ns.getSymbol();
		this.otherNames = ns.getOtherNames();
	}

	/**
	 * A constructor that constructs {@code BaseUnit} instances.
	 * 
	 * @since 2019-10-16
	 */
	Unit(final String primaryName, final String symbol, final Set<String> otherNames) {
		if (this instanceof BaseUnit) {
			this.unitBase = ObjectProduct.oneOf((BaseUnit) this);
		} else
			throw new AssertionError();
		this.primaryName = Optional.of(primaryName);
		this.symbol = Optional.of(symbol);
		this.otherNames = Collections.unmodifiableSet(
				new HashSet<>(Objects.requireNonNull(otherNames, "additionalNames must not be null.")));
	}

	/**
	 * Checks if a value expressed in this unit can be converted to a value expressed in {@code other}
	 * 
	 * @param other
	 *            unit to test with
	 * @return true if the units 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 additionalNames
	 * @since 2019-10-21
	 */
	public final Set<String> getOtherNames() {
		return this.otherNames;
	}

	/**
	 * @return primaryName
	 * @since 2019-10-21
	 */
	public final Optional<String> getPrimaryName() {
		return this.primaryName;
	}

	/**
	 * @return symbol
	 * @since 2019-10-21
	 */
	public final Optional<String> getSymbol() {
		return this.symbol;
	}

	@Override
	public String toString() {
		return this.getPrimaryName().orElse("Unnamed unit")
				+ (this.getSymbol().isPresent() ? String.format(" (%s)", this.getSymbol().get()) : "")
				+ ", derived from " + this.getBase().toString(u -> u.getSymbol().get())
				+ (this.getOtherNames().isEmpty() ? "" : ", also called " + String.join(", ", this.getOtherNames()));
	}

	/**
	 * @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."));
	}
}