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Tensorics is a java framework which uses a tensor as a central object. A tensor represents a set of values placed in an N-dimensional space. Wherever you are tempted to use maps of maps, a tensor might be a good choice ;-) Tensorics provides methods to create, transform and performing calculations with those tensors.

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// @formatter:off
 /*******************************************************************************
 *
 * This file is part of tensorics.
 * 
 * Copyright (c) 2008-2011, CERN. All rights reserved.
 *
 * 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.
 * 
 ******************************************************************************/
// @formatter:on

package org.tensorics.core.iterable.lang;

import java.util.Iterator;
import java.util.stream.Collectors;
import java.util.stream.StreamSupport;

import org.tensorics.core.lang.Tensorics;
import org.tensorics.core.quantity.QuantifiedValue;
import org.tensorics.core.quantity.lang.QuantitySupport;
import org.tensorics.core.quantity.options.QuantityEnvironment;

import com.google.common.collect.Iterables;

/**
 * Contains methods of the tensorics eDSL which deal with iterables of quantities.
 * 
 * @author kfuchsbe
 * @param  the type of the scalar values of the quantities (elements of the field on which all the operations are
 *            based on)
 */
public class QuantityIterableSupport extends QuantitySupport {

    protected QuantityIterableSupport(QuantityEnvironment environment) {
        super(environment);
    }

    public final QuantifiedValue averageOf(Iterable> values) {
        if (Iterables.isEmpty(values)) {
            throw new IllegalArgumentException("Averaging of empty value set is not possible.");
        }
        return calculate(sumOf(values)).dividedBy(sizeOf(values));
    }

    public final QuantifiedValue rmsOf(Iterable> values) {
        if (Iterables.isEmpty(values)) {
            throw new IllegalArgumentException("Rms of empty value set is not possible.");
        }
        return squareRootOf(calculate(sumOfSquaresOf(values)).dividedBy(sizeOf(values)));
    }

    public final QuantifiedValue stdOf(Iterable> values) {
        if (Iterables.isEmpty(values)) {
            throw new IllegalArgumentException("Standard deviation of empty value set is not possible.");
        }
        return squareRootOf(varOf(values));
    }

    public final QuantifiedValue varOf(Iterable> values) {
        if (Iterables.isEmpty(values)) {
            throw new IllegalArgumentException("Variance of empty value set is not possible.");
        }
        final QuantifiedValue average = averageOf(values);

        // @formatter:off
        Iterable> squaredDifferences = StreamSupport.stream(values.spliterator(), false)
           .map(v -> calculate(v).minus(average))
           .map(difference -> calculate(difference).toThePowerOf(two()))
          .collect(Collectors.toList());
        // @formatter:on

        return averageOf(squaredDifferences);
    }

    public final QuantifiedValue sizeOf(Iterable> values) {
        QuantifiedValue one = one();
        QuantifiedValue count = zero();
        for (@SuppressWarnings("unused")
        Object value : values) {
            count = calculate(count).plus(one);
        }
        return count;
    }

    private QuantifiedValue zeroInUnitsOfFirstValueIn(Iterable> values) {
        QuantifiedValue zero = zero();
        Iterator> iterator = values.iterator();
        if (iterator.hasNext()) {
            zero = Tensorics.quantityOf(zero.value(), iterator.next().unit());
        }
        return zero;
    }

    public final QuantifiedValue sumOf(Iterable> values) {
        QuantifiedValue sum = zeroInUnitsOfFirstValueIn(values);
        for (QuantifiedValue value : values) {
            sum = calculate(sum).plus(value);
        }
        return sum;
    }

    public final QuantifiedValue sumOfSquaresOf(Iterable> values) {
        // XXX: This method has never been tested
        QuantifiedValue sum = squareOf(zeroInUnitsOfFirstValueIn(values));
        for (QuantifiedValue value : values) {
            sum = calculate(sum).plus(squareOf(value));
        }
        return sum;
    }

    private QuantifiedValue squareOf(QuantifiedValue value) {
        return calculate(value).times(value);
    }

    private QuantifiedValue squareRootOf(QuantifiedValue value) {
        return calculate(value).root(two());
    }

    public final OngoingQuantityIterableValueExtraction valuesOf(Iterable> quantities) {
        return new OngoingQuantityIterableValueExtraction<>(quantities, operationRepository());
    }

}