/**
 * Copyright (C) 2018 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.math;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.function.Function;

/**
 * An immutable product of multiple objects of a type, such as base units. The objects can be multiplied and
 * exponentiated.
 * 
 * @author Adrien Hopkins
 * @since 2019-10-16
 */
public final class ObjectProduct<T> {
	/**
	 * Returns an empty ObjectProduct of a certain type
	 * 
	 * @param <T>
	 *            type of objects that can be multiplied
	 * @return empty product
	 * @since 2019-10-16
	 */
	public static final <T> ObjectProduct<T> empty() {
		return new ObjectProduct<>(new HashMap<>());
	}

	/**
	 * Gets an {@code ObjectProduct} from an object-to-integer mapping
	 * 
	 * @param <T>
	 *            type of object in product
	 * @param map
	 *            map mapping objects to exponents
	 * @return object product
	 * @since 2019-10-16
	 */
	public static final <T> ObjectProduct<T> fromExponentMapping(final Map<T, Integer> map) {
		return new ObjectProduct<>(new HashMap<>(map));
	}

	/**
	 * Gets an ObjectProduct that has one of the inputted argument, and nothing else.
	 * 
	 * @param object
	 *            object that will be in the product
	 * @return product
	 * @since 2019-10-16
	 * @throws NullPointerException
	 *             if object is null
	 */
	public static final <T> ObjectProduct<T> oneOf(final T object) {
		Objects.requireNonNull(object, "object must not be null.");
		final Map<T, Integer> map = new HashMap<>();
		map.put(object, 1);
		return new ObjectProduct<>(map);
	}

	/**
	 * The objects that make up the product, mapped to their exponents. This map treats zero as null, and is immutable.
	 * 
	 * @since 2019-10-16
	 */
	final Map<T, Integer> exponents;

	/**
	 * Creates the {@code ObjectProduct}.
	 * 
	 * @param exponents
	 *            objects that make up this product
	 * @since 2019-10-16
	 */
	private ObjectProduct(final Map<T, Integer> exponents) {
		this.exponents = Collections.unmodifiableMap(ZeroIsNullMap.create(new HashMap<>(exponents)));
	}

	/**
	 * Calculates the quotient of two products
	 *
	 * @param other
	 *            other product
	 * @return quotient of two products
	 * @since 2019-10-16
	 * @throws NullPointerException
	 *             if other is null
	 */
	public ObjectProduct<T> dividedBy(final ObjectProduct<T> other) {
		Objects.requireNonNull(other, "other must not be null.");
		// get a list of all objects in both sets
		final Set<T> objects = new HashSet<>();
		objects.addAll(this.getBaseSet());
		objects.addAll(other.getBaseSet());

		// get a list of all exponents
		final Map<T, Integer> map = new HashMap<>(objects.size());
		for (final T key : objects) {
			map.put(key, this.getExponent(key) - other.getExponent(key));
		}

		// create the product
		return new ObjectProduct<>(map);
	}

	// this method relies on the use of ZeroIsNullMap
	@Override
	public boolean equals(final Object obj) {
		if (this == obj)
			return true;
		if (!(obj instanceof ObjectProduct))
			return false;
		final ObjectProduct<?> other = (ObjectProduct<?>) obj;
		return Objects.equals(this.exponents, other.exponents);
	}

	/**
	 * @return immutable map mapping objects to exponents
	 * @since 2019-10-16
	 */
	public Map<T, Integer> exponentMap() {
		return this.exponents;
	}

	/**
	 * @return a set of all of the base objects with non-zero exponents that make up this dimension.
	 * @since 2018-12-12
	 * @since v0.1.0
	 */
	public final Set<T> getBaseSet() {
		final Set<T> dimensions = new HashSet<>();

		// add all dimensions with a nonzero exponent - zero exponents shouldn't be there in the first place
		for (final T dimension : this.exponents.keySet()) {
			if (!this.exponents.get(dimension).equals(0)) {
				dimensions.add(dimension);
			}
		}

		return dimensions;
	}

	/**
	 * Gets the exponent for a specific dimension.
	 * 
	 * @param dimension
	 *            dimension to check
	 * @return exponent for that dimension
	 * @since 2018-12-12
	 * @since v0.1.0
	 */
	public int getExponent(final T dimension) {
		return this.exponents.getOrDefault(dimension, 0);
	}

	@Override
	public int hashCode() {
		return Objects.hash(this.exponents);
	}

	/**
	 * @return true if this product is a single object, i.e. it has one exponent of one and no other nonzero exponents
	 * @since 2019-10-16
	 */
	public boolean isSingleObject() {
		int oneCount = 0;
		boolean twoOrMore = false; // has exponents of 2 or more
		for (final T b : this.getBaseSet()) {
			if (this.getExponent(b) == 1) {
				oneCount++;
			} else if (this.getExponent(b) != 0) {
				twoOrMore = true;
			}
		}
		return oneCount == 1 && !twoOrMore;
	}

	/**
	 * Multiplies this product by another
	 *
	 * @param other
	 *            other product
	 * @return product of two products
	 * @since 2019-10-16
	 * @throws NullPointerException
	 *             if other is null
	 */
	public ObjectProduct<T> times(final ObjectProduct<T> other) {
		Objects.requireNonNull(other, "other must not be null.");
		// get a list of all objects in both sets
		final Set<T> objects = new HashSet<>();
		objects.addAll(this.getBaseSet());
		objects.addAll(other.getBaseSet());

		// get a list of all exponents
		final Map<T, Integer> map = new HashMap<>(objects.size());
		for (final T key : objects) {
			map.put(key, this.getExponent(key) + other.getExponent(key));
		}

		// create the product
		return new ObjectProduct<>(map);
	}

	/**
	 * Returns this product, but to an exponent
	 * 
	 * @param exponent
	 *            exponent
	 * @return result of exponentiation
	 * @since 2019-10-16
	 */
	public ObjectProduct<T> toExponent(final int exponent) {
		final Map<T, Integer> map = new HashMap<>(this.exponents);
		for (final T key : this.exponents.keySet()) {
			map.put(key, this.getExponent(key) * exponent);
		}
		return new ObjectProduct<>(map);
	}

	@Override
	public String toString() {
		return this.toString(Object::toString);
	}

	/**
	 * Converts this product to a string. The objects that make up this product are represented by
	 * {@code objectToString}
	 * 
	 * @param objectToString
	 *            function to convert objects to strings
	 * @return string representation of product
	 * @since 2019-10-16
	 */
	public String toString(final Function<T, String> objectToString) {
		final List<String> positiveStringComponents = new ArrayList<>();
		final List<String> negativeStringComponents = new ArrayList<>();

		// for each base object that makes up this object, add it and its exponent
		for (final T object : this.getBaseSet()) {
			final int exponent = this.exponents.get(object);
			if (exponent > 0) {
				positiveStringComponents.add(String.format("%s^%d", objectToString.apply(object), exponent));
			} else if (exponent < 0) {
				negativeStringComponents.add(String.format("%s^%d", objectToString.apply(object), -exponent));
			}
		}

		final String positiveString = positiveStringComponents.isEmpty() ? "1"
				: String.join(" * ", positiveStringComponents);
		final String negativeString = negativeStringComponents.isEmpty() ? ""
				: " / " + String.join(" * ", negativeStringComponents);

		return positiveString + negativeString;
	}
}