solr/solrj/src/java/org/apache/solr/client/solrj/io/eval/FFTEvaluator.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.solr.client.solrj.io.eval;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

import org.apache.commons.math3.transform.DftNormalization;
import org.apache.commons.math3.transform.FastFourierTransformer;
import org.apache.commons.math3.transform.TransformType;
import org.apache.commons.math3.complex.Complex;
import org.apache.solr.client.solrj.io.stream.expr.StreamExpression;
import org.apache.solr.client.solrj.io.stream.expr.StreamFactory;

public class FFTEvaluator extends RecursiveNumericEvaluator implements OneValueWorker {
  protected static final long serialVersionUID = 1L;

  private static List<String> clabels = new ArrayList<>();

  static {
    clabels.add("real");
    clabels.add("imaginary");
  }

  public FFTEvaluator(StreamExpression expression, StreamFactory factory) throws IOException{
    super(expression, factory);

    if(containedEvaluators.size() < 1){
      throw new IOException(String.format(Locale.ROOT,"Invalid expression %s - expecting at least one value but found %d",expression,containedEvaluators.size()));
    }
  }

  @Override
  public Object doWork(Object v) throws IOException {

    double[] data = ((List<?>)v).stream().mapToDouble(value -> ((Number)value).doubleValue()).toArray();

    FastFourierTransformer fastFourierTransformer = new FastFourierTransformer(DftNormalization.STANDARD);
    Complex[] complex = fastFourierTransformer.transform(data, TransformType.FORWARD);

    double[] real = new double[complex.length];
    double[] imaginary = new double[complex.length];

    for(int i=0; i<real.length; ++i) {
      real[i] = complex[i].getReal();
      imaginary[i] = complex[i].getImaginary();
    }

    double[][] d = new double[2][];
    d[0]=real;
    d[1]=imaginary;

    Matrix matrix = new Matrix(d);
    matrix.setRowLabels(clabels);
    return matrix;
  }
}