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TernarySearch.java
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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));
}
}