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The OpenTripPlanner multimodal journey planning system

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package org.opentripplanner.util.lang;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.IntSummaryStatistics;
import java.util.List;

/**
 * A utility class for integer functions.
 */
public final class IntUtils {

  /** The constructor is private to protect this class from being instantiated. */
  private IntUtils() {}

  /**
   * Convert an integer to a String, if the value equals the {@code notSetValue} parameter an empty
   * string is returned.
   */
  public static String intToString(int value, int notSetValue) {
    return value == notSetValue ? "" : Integer.toString(value);
  }

  /**
   * Create a new int array and initialize all values with the given {@code initialValue}.
   */
  public static int[] intArray(int size, int initialValue) {
    int[] array = new int[size];
    Arrays.fill(array, initialValue);
    return array;
  }

  /**
   * Concatenate list a and b and convert them to int arrays.
   */
  public static int[] concat(Collection a, Collection b) {
    List all = new ArrayList<>(a);
    all.addAll(b);
    return all.stream().mapToInt(it -> it).toArray();
  }

  public static double standardDeviation(List v) {
    double average = v.stream().mapToInt(it -> it).average().orElse(0d);

    double sum = 0.0;
    for (double num : v) {
      sum += Math.pow(num - average, 2);
    }

    return Math.sqrt(sum / v.size());
  }
}