JavaAlgorithms/Searches/IterativeTernarySearch.java

package Searches;

import static java.lang.String.format;

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

/**
 * A iterative version of a ternary search algorithm This is better way to implement the ternary
 * search, because a recursive version adds some overhead to a stack. But in java the compile can
 * transform the recursive version to iterative implicitly, so there are no much differences between
 * these two algorithms
 *
 * <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 TernarySearch
 * @since 2018-04-13
 */
public class IterativeTernarySearch implements SearchAlgorithm {

  @Override
  public <T extends Comparable<T>> int find(T[] array, T key) {
    int left = 0;
    int right = array.length - 1;

    while (right > left) {

      int leftCmp = array[left].compareTo(key);
      int rightCmp = array[right].compareTo(key);
      if (leftCmp == 0) return left;
      if (rightCmp == 0) return right;

      int leftThird = left + (right - left) / 3 + 1;
      int rightThird = right - (right - left) / 3 - 1;

      if (array[leftThird].compareTo(key) <= 0) {
        left = leftThird;
      } else {
        right = rightThird;
      }
    }

    return -1;
  }

  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)];

    IterativeTernarySearch search = new IterativeTernarySearch();
    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));
  }
}