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Provides common utility functions

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/*
 * Copyright (c) CQSE GmbH
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.conqat.lib.commons.datamining;

import java.util.HashSet;
import java.util.Set;

import org.conqat.lib.commons.assertion.CCSMAssert;

/**
 * User represented by a shopping basket, i.e. a set of purchased items. The similarity measure used
 * is cosine similarity (see http://en.wikipedia.org/wiki/Cosine_similarity).
 */
public class ShoppingBasketUser implements IRecommenderUser {

	/** The set of items */
	private Set items;

	/** Constructor */
	public ShoppingBasketUser(Set items) {
		CCSMAssert.isFalse(items.isEmpty(), "Items must not be empty");
		this.items = items;
	}

	/** Returns the items. */
	public Set getItems() {
		return items;
	}

	/** {@inheritDoc} */
	@Override
	public double similarity(IRecommenderUser other) {
		if (!(other instanceof ShoppingBasketUser)) {
			throw new IllegalArgumentException();
		}
		ShoppingBasketUser user = (ShoppingBasketUser) other;
		return similarity(items, user.items);
	}

	/** Computes the cosine similarity of two non-empty sets */
	private static double similarity(Set set1, Set set2) {
		CCSMAssert.isFalse(set1.isEmpty() || set2.isEmpty(), "Sets must not be empty");
		Set intersection = new HashSet<>(set1);
		intersection.retainAll(set2);
		double numerator = intersection.size();
		double denominator = Math.sqrt(set1.size()) * Math.sqrt(set2.size());
		return numerator / denominator;
	}

	/** {@inheritDoc} */
	@Override
	public String toString() {
		return items.toString();
	}
}