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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
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 *
 * This file is part of tensorics.
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 * 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.
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// @formatter:on

package org.tensorics.core.tensor.variance;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

/**
 * Represents a pair of dimensions, related to two different tensors (left, right), where either one has to be the
 * covariant equivalent to the second contravariant dimension.
 * 
 * @author kfuchsbe
 */
public final class CoContraDimensionPair {

    private final Class leftDimension;
    private final Class rightDimension;

    private CoContraDimensionPair(Class leftDimension, Class rightDimension) {
        super();
        checkNotNull(leftDimension, "left dimension must not be null.");
        checkNotNull(rightDimension, "right dimension must not be null.");
        if (!Covariants.isCoContraPair(leftDimension, rightDimension)) {
            throw new IllegalArgumentException("The dimensions [" + leftDimension + "," + rightDimension
                    + "] do not form a co- and contravariant pair.");
        }
        this.leftDimension = leftDimension;
        this.rightDimension = rightDimension;
    }

    public static CoContraDimensionPair ofLeftRight(Class leftDimension, Class rightDimension) {
        return new CoContraDimensionPair(leftDimension, rightDimension);
    }

    public Class left() {
        return leftDimension;
    }

    public Class right() {
        return rightDimension;
    }

    /**
     * Retrieves the covariant part of the pair, no matter if it is left or right
     * 
     * @return the covariant part
     */
    public Class covariant() {
        if (Covariants.isCovariant(leftDimension)) {
            return leftDimension;
        }
        return rightDimension;
    }

    /**
     * Retrieves the contravariant part of the pair, no matter if it is left or right
     * 
     * @return the contravariant part
     */
    public Class contravariant() {
        if (Covariants.isContravariant(leftDimension)) {
            return leftDimension;
        }
        return rightDimension;
    }

    public Object toLeft(Object rightCoordinate) {
        return convertFromTo(rightCoordinate, rightDimension, leftDimension);
    }

    public Object toRight(Object leftCoordinate) {
        return convertFromTo(leftCoordinate, leftDimension, rightDimension);
    }

    private Object convertFromTo(Object coordinate, Class fromDimension, Class toDimension) {
        checkArgument(fromDimension.isInstance(coordinate),
                "The coordinate {} is not compatible with the dimension {}. Cannot convert.", //
                coordinate, fromDimension);
        if (Covariants.isCovariant(fromDimension)) {
            Covariant covariant = (Covariant) coordinate;
            return covariant.get();
        } else {
            @SuppressWarnings("unchecked")
            Class> covariantClass = (Class>) toDimension;
            return Covariants.instantiatorFor(covariantClass).create(coordinate);
        }
    }

}