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package Searches;

import java.util.Arrays;

import java.util.Random;

import java.util.stream.Stream;

import static java.lang.String.format;

/**

* 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));

}

}