JavaAlgorithms/Searches/InterpolationSearch.java

package Searches;

import static java.lang.String.format;

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

/**
 * Interpolation search algorithm implementation
 *
 * <p>Worst-case performance O(n) Best-case performance O(1) Average performance O(log(log(n))) if
 * the elements are uniformly distributed if not O(n) Worst-case space complexity O(1)
 *
 * @author Podshivalov Nikita (https://github.com/nikitap492)
 */
class InterpolationSearch {

  /**
   * @param array is a sorted array
   * @param key is a value what shoulb be found in the array
   * @return an index if the array contains the key unless -1
   */
  public int find(int array[], int key) {
    // Find indexes of two corners
    int start = 0, end = (array.length - 1);

    // Since array is sorted, an element present
    // in array must be in range defined by corner
    while (start <= end && key >= array[start] && key <= array[end]) {
      // Probing the position with keeping
      // uniform distribution in mind.
      int pos = start + (((end - start) / (array[end] - array[start])) * (key - array[start]));

      // Condition of target found
      if (array[pos] == key) return pos;

      // If key is larger, key is in upper part
      if (array[pos] < key) start = pos + 1;

      // If key is smaller, x is in lower part
      else end = pos - 1;
    }
    return -1;
  }

  // Driver method
  public static void main(String[] args) {
    Random r = new Random();
    int size = 100;
    int maxElement = 100000;
    int[] integers = IntStream.generate(() -> r.nextInt(maxElement)).limit(size).sorted().toArray();

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

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

    System.out.println(
        String.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));
  }
}