main/src/main/java/org/apache/poi/ss/formula/functions/Frequency.java

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   this work for additional information regarding copyright ownership.
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package org.apache.poi.ss.formula.functions;

import org.apache.poi.ss.formula.CacheAreaEval;
import org.apache.poi.ss.formula.eval.EvaluationException;
import org.apache.poi.ss.formula.eval.NumberEval;
import org.apache.poi.ss.formula.eval.ValueEval;

import java.util.Arrays;

/**
 * Implementation of Excel 'Analysis ToolPak' function FREQUENCY()<br>
 * Returns a frequency distribution as a vertical array<p>
 * <p>
 * <b>Syntax</b><br>
 * <b>FREQUENCY</b>(<b>data_array</b>, <b>bins_array</b>)<p>
 * <p>
 * <b>data_array</b> Required. An array of or reference to a set of values for which you want to count frequencies.
 * If data_array contains no values, FREQUENCY returns an array of zeros.<br>
 * <b>bins_array</b> Required. An array of or reference to intervals into which you want to group the values in data_array.
 * If bins_array contains no values, FREQUENCY returns the number of elements in data_array.<br>
 *
 * @author Yegor Kozlov
 */
public class Frequency extends Fixed2ArgFunction {
    public static final Function instance = new Frequency();

    private Frequency() {
        // enforce singleton
    }

    public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval arg0, ValueEval arg1) {
        MatrixFunction.MutableValueCollector collector = new MatrixFunction.MutableValueCollector(false, false);

        double[] values;
        double[] bins;
        try {
            values = collector.collectValues(arg0);
            bins = collector.collectValues(arg1);
        } catch (EvaluationException e) {
            return e.getErrorEval();
        }

        // can bins be not sorted?
        //bins = Arrays.stream(bins).sorted().distinct().toArray();

        int[] histogram = histogram(values, bins);
        NumberEval[] result = Arrays.stream(histogram).boxed().map(NumberEval::new).toArray(NumberEval[]::new);
        return new CacheAreaEval(srcRowIndex, srcColumnIndex,
                srcRowIndex + result.length - 1, srcColumnIndex, result);
    }

    static int findBin(double value, double[] bins) {
        int idx = Arrays.binarySearch(bins, value);
        return idx >= 0 ? idx + 1 : -idx;
    }

    static int[] histogram(double[] values, double[] bins) {
        int[] histogram = new int[bins.length + 1];
        for (double val : values) {
            histogram[findBin(val, bins) - 1]++;
        }
        return histogram;
    }
}