tests: add tests for SieveOfEratosthenes
(#4217)
* tests: add tests for SieveOfEratosthenes * feat: throw for inputs <= 0 * refactor: simplify logic in SieveOfEratosthenes.findPrimesTill * refactor: make SieveOfEratosthenes a utility class * docs: update docs, fix typo
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@ -3,55 +3,63 @@ package com.thealgorithms.others;
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import java.util.Arrays;
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/**
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* Sieve of Eratosthenes is an ancient algorithm for finding all prime numbers
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* up to any given limit.
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*
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* @see <a href="https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes">Wiki</a>
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* @brief utility class implementing <a href="https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes">Sieve of Eratosthenes</a>
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*/
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public class SieveOfEratosthenes {
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final public class SieveOfEratosthenes {
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private SieveOfEratosthenes() {
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}
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/**
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* Finds all prime numbers till n.
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*
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* @param n The number till which we have to check for primes. Should be more than 1.
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* @return Array of all prime numbers between 0 to n.
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*/
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public static int[] findPrimesTill(int n) {
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Type[] numbers = new Type[n + 1];
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Arrays.fill(numbers, Type.PRIME);
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numbers[0] = numbers[1] = Type.NOT_PRIME;
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private static void checkInput(int n) {
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if (n <= 0) {
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throw new IllegalArgumentException("n must be positive.");
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}
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}
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private static Type[] sievePrimesTill(int n) {
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checkInput(n);
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Type[] isPrimeArray = new Type[n + 1];
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Arrays.fill(isPrimeArray, Type.PRIME);
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isPrimeArray[0] = isPrimeArray[1] = Type.NOT_PRIME;
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double cap = Math.sqrt(n);
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for (int i = 2; i <= cap; i++) {
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if (numbers[i] == Type.PRIME) {
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if (isPrimeArray[i] == Type.PRIME) {
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for (int j = 2; i * j <= n; j++) {
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numbers[i * j] = Type.NOT_PRIME;
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isPrimeArray[i * j] = Type.NOT_PRIME;
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}
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}
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}
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return isPrimeArray;
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}
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int primesCount = (int) Arrays.stream(numbers).filter(element -> element == Type.PRIME).count();
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int[] primes = new int[primesCount];
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private static int countPrimes(Type[] isPrimeArray) {
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return (int) Arrays.stream(isPrimeArray).filter(element -> element == Type.PRIME).count();
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}
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private static int[] extractPrimes(Type[] isPrimeArray) {
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int numberOfPrimes = countPrimes(isPrimeArray);
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int[] primes = new int[numberOfPrimes];
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int primeIndex = 0;
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for (int i = 0; i < n + 1; i++) {
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if (numbers[i] == Type.PRIME) {
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primes[primeIndex++] = i;
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for (int curNumber = 0; curNumber < isPrimeArray.length; ++curNumber) {
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if (isPrimeArray[curNumber] == Type.PRIME) {
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primes[primeIndex++] = curNumber;
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}
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}
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return primes;
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}
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/**
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* @brief finds all of the prime numbers up to the given upper (inclusive) limit
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* @param n upper (inclusive) limit
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* @exception IllegalArgumentException n is non-positive
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* @return the array of all primes up to the given number (inclusive)
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*/
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public static int[] findPrimesTill(int n) {
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return extractPrimes(sievePrimesTill(n));
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}
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private enum Type {
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PRIME,
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NOT_PRIME,
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}
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public static void main(String[] args) {
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int n = 100;
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System.out.println("Searching for all primes from zero to " + n);
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int[] primes = findPrimesTill(n);
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System.out.println("Found: " + Arrays.toString(primes));
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}
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}
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@ -0,0 +1,46 @@
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package com.thealgorithms.others;
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import static org.junit.jupiter.api.Assertions.assertArrayEquals;
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import static org.junit.jupiter.api.Assertions.assertThrows;
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import org.junit.jupiter.api.Test;
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class SieveOfEratosthenesTest {
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@Test
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public void testfFindPrimesTill1() {
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assertArrayEquals(new int[] {}, SieveOfEratosthenes.findPrimesTill(1));
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}
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@Test
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public void testfFindPrimesTill2() {
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assertArrayEquals(new int[] {2}, SieveOfEratosthenes.findPrimesTill(2));
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}
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@Test
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public void testfFindPrimesTill4() {
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var primesTill4 = new int[] {2, 3};
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assertArrayEquals(primesTill4, SieveOfEratosthenes.findPrimesTill(3));
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assertArrayEquals(primesTill4, SieveOfEratosthenes.findPrimesTill(4));
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}
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@Test
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public void testfFindPrimesTill40() {
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var primesTill40 = new int[] {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37};
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assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(37));
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assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(38));
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assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(39));
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assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(40));
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}
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@Test
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public void testfFindPrimesTill240() {
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var primesTill240 = new int[] {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239};
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assertArrayEquals(primesTill240, SieveOfEratosthenes.findPrimesTill(239));
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assertArrayEquals(primesTill240, SieveOfEratosthenes.findPrimesTill(240));
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}
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@Test
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public void testFindPrimesTillThrowsExceptionForNonPositiveInput() {
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assertThrows(IllegalArgumentException.class, () -> SieveOfEratosthenes.findPrimesTill(0));
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}
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}
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