JavaAlgorithms/Searches/TernarySearch.java

package Searches;

import static java.lang.String.format;

import java.util.Arrays;
import java.util.Random;
import java.util.stream.Stream;

/**
 * A ternary search algorithm is a technique in computer science for finding the minimum or maximum
 * of a unimodal function The algorithm determines either that the minimum or maximum cannot be in
 * the first third of the domain or that it cannot be in the last third of the domain, then repeats
 * on the remaining third.
 *
 * <p>Worst-case performance Θ(log3(N)) Best-case performance O(1) Average performance Θ(log3(N))
 * Worst-case space complexity O(1)
 *
 * @author Podshivalov Nikita (https://github.com/nikitap492)
 * @see SearchAlgorithm
 * @see IterativeBinarySearch
 */
public class TernarySearch implements SearchAlgorithm {

  /**
   * @param arr The **Sorted** array in which we will search the element.
   * @param value The value that we want to search for.
   * @return The index of the element if found. Else returns -1.
   */
  @Override
  public <T extends Comparable<T>> int find(T[] arr, T value) {
    return ternarySearch(arr, value, 0, arr.length - 1);
  }

  /**
   * @param arr The **Sorted** array in which we will search the element.
   * @param key The value that we want to search for.
   * @param start The starting index from which we will start Searching.
   * @param end The ending index till which we will Search.
   * @return Returns the index of the Element if found. Else returns -1.
   */
  private <T extends Comparable<T>> int ternarySearch(T[] arr, T key, int start, int end) {
    if (start > end) {
      return -1;
    }
    /* First boundary: add 1/3 of length to start */
    int mid1 = start + (end - start) / 3;
    /* Second boundary: add 2/3 of length to start */
    int mid2 = start + 2 * (end - start) / 3;

    if (key.compareTo(arr[mid1]) == 0) {
      return mid1;
    } else if (key.compareTo(arr[mid2]) == 0) {
      return mid2;
    }

    /* Search the first (1/3) rd part of the array.*/

    else if (key.compareTo(arr[mid1]) < 0) {
      return ternarySearch(arr, key, start, --mid1);
    }
    /* Search 3rd (1/3)rd part of the array */

    else if (key.compareTo(arr[mid2]) > 0) {
      return ternarySearch(arr, key, ++mid2, end);
    }
    /* Search middle (1/3)rd part of the array */

    else {
      return ternarySearch(arr, key, mid1, mid2);
    }
  }

  public static void main(String[] args) {
    // just generate data
    Random r = new Random();
    int size = 100;
    int maxElement = 100000;
    Integer[] integers =
        Stream.generate(() -> r.nextInt(maxElement)).limit(size).sorted().toArray(Integer[]::new);

    // the element that should be found
    Integer shouldBeFound = integers[r.nextInt(size - 1)];

    TernarySearch search = new TernarySearch();
    int atIndex = search.find(integers, shouldBeFound);

    System.out.println(
        format(
            "Should be found: %d. Found %d at index %d. An array length %d",
            shouldBeFound, integers[atIndex], atIndex, size));

    int toCheck = Arrays.binarySearch(integers, shouldBeFound);
    System.out.println(
        format(
            "Found by system method at an index: %d. Is equal: %b", toCheck, toCheck == atIndex));
  }
}