ValueIndexSort.java

import javax.annotation.Nonnull;
import java.util.AbstractMap;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

/**
 * common functions
 */
public final class ValueIndexSort
{
    /**
     * defines ranking index of number
     *
     * @param p_data any number array
     * @return unmodifyable index list (first index, defines the lowest element)
     */
    public static List<Integer> rankindex( @Nonnull final double[] p_data )
    {
        return Collections.unmodifiableList(
            IntStream.range( 0, p_data.length )
                     .parallel()
                     .boxed()
                     .map( i -> new AbstractMap.SimpleImmutableEntry<>( i, p_data[i] )  )
                     .collect( Collectors.toList() )
                     .parallelStream()
                     .sorted( Comparator.comparingDouble( AbstractMap.SimpleImmutableEntry::getValue ) )
                     .map( AbstractMap.SimpleImmutableEntry::getKey )
                     .collect( Collectors.toList() )
        );
    }

}