CenteredDifference.h

/* Copyright 2018 Kristofer Björnson
 *
 * 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.
 */

#ifndef COM_DAFER45_TBTK_CENTERED_DIFFERENCE
#define COM_DAFER45_TBTK_CENTERD_DIFFERENCE

#include "TBTK/HoppingAmplitudeList.h"

namespace TBTK{

class CenteredDifference : public HoppingAmplitudeList{
public:
    CenteredDifference(
        unsigned int subindex,
        const Index &index,
        double dx = 1
    );
};

inline CenteredDifference::CenteredDifference(
    unsigned int subindex,
    const Index &index,
    double dx
){
    TBTKAssert(
        subindex < index.getSize(),
        "CenteredDifference::CenteredDifference()",
        "Invalid subindex. The subindex '" << subindex << "' is larger"
        << " than the size of the Index '" << index.getSize() << "'.",
        ""
    );
    TBTKAssert(
        index[subindex] > 0,
        "CenteredDifference::CenteredDifference()",
        "Invalid subindex value. Unable to add a centered difference"
        " for subindex '" << subindex << "' at '" << index.toString()
        << "' since the centered difference would contain a negative"
        << " subindex.",
        "Modify the domain of the differential equation to ensure that"
        << " no negative subindices are needed."
    );

    Index backward = index;
    backward[subindex]--;
    Index forward = index;
    forward[subindex]++;
    add(HoppingAmplitude(1/(2*dx), index, backward));
    add(HoppingAmplitude(-1/(2*dx), index, forward));
}

};  //End of namespace TBTK

#endif