Add SieveOfEratosthenes algorithm with tests

Author: Mansi5164

src/main/java/com/thealgorithms/maths/SieveOfEratosthenes.java

@@ -1,66 +1,50 @@
 package com.thealgorithms.maths;
 
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.List;
 
 /**
- * @brief utility class implementing <a href="https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes">Sieve of Eratosthenes</a>
+ * Utility class that provides the Sieve of Eratosthenes algorithm.
  */
 public final class SieveOfEratosthenes {
+
+    /** Private constructor to prevent instantiation. */
     private SieveOfEratosthenes() {
+        throw new AssertionError("Cannot instantiate utility class.");
     }
 
-    private static void checkInput(int n) {
-        if (n <= 0) {
-            throw new IllegalArgumentException("n must be positive.");
+    /**
+     * Returns an array of all prime numbers less than or equal to {@code n}.
+     *
+     * @param n the upper bound (inclusive)
+     * @return array of primes <= n (empty if n &lt; 2)
+     */
+    public static int[] sieve(final int n) {
+        if (n < 2) {
+            return new int[0];
         }
-    }
 
-    private static Type[] sievePrimesTill(int n) {
-        checkInput(n);
-        Type[] isPrimeArray = new Type[n + 1];
-        Arrays.fill(isPrimeArray, Type.PRIME);
-        isPrimeArray[0] = Type.NOT_PRIME;
-        isPrimeArray[1] = Type.NOT_PRIME;
+        boolean[] isPrime = new boolean[n + 1];
+        Arrays.fill(isPrime, true);
+        isPrime[0] = false;
+        isPrime[1] = false;
 
-        double cap = Math.sqrt(n);
-        for (int i = 2; i <= cap; i++) {
-            if (isPrimeArray[i] == Type.PRIME) {
-                for (int j = 2; i * j <= n; j++) {
-                    isPrimeArray[i * j] = Type.NOT_PRIME;
+        for (int p = 2; p * p <= n; p++) {
+            if (isPrime[p]) {
+                for (int multiple = p * p; multiple <= n; multiple += p) {
+                    isPrime[multiple] = false;
                 }
             }
         }
-        return isPrimeArray;
-    }
-
-    private static int countPrimes(Type[] isPrimeArray) {
-        return (int) Arrays.stream(isPrimeArray).filter(element -> element == Type.PRIME).count();
-    }
 
-    private static int[] extractPrimes(Type[] isPrimeArray) {
-        int numberOfPrimes = countPrimes(isPrimeArray);
-        int[] primes = new int[numberOfPrimes];
-        int primeIndex = 0;
-        for (int curNumber = 0; curNumber < isPrimeArray.length; ++curNumber) {
-            if (isPrimeArray[curNumber] == Type.PRIME) {
-                primes[primeIndex++] = curNumber;
+        List<Integer> primes = new ArrayList<>();
+        for (int i = 2; i <= n; i++) {
+            if (isPrime[i]) {
+                primes.add(i);
             }
         }
-        return primes;
-    }
-
-    /**
-     * @brief finds all of the prime numbers up to the given upper (inclusive) limit
-     * @param n upper (inclusive) limit
-     * @exception IllegalArgumentException n is non-positive
-     * @return the array of all primes up to the given number (inclusive)
-     */
-    public static int[] findPrimesTill(int n) {
-        return extractPrimes(sievePrimesTill(n));
-    }
 
-    private enum Type {
-        PRIME,
-        NOT_PRIME,
+        return primes.stream().mapToInt(Integer::intValue).toArray();
     }
 }

src/test/java/com/thealgorithms/maths/SieveOfEratosthenesTest.java

@@ -1,46 +1,23 @@
 package com.thealgorithms.maths;
 
-import static org.junit.jupiter.api.Assertions.assertArrayEquals;
-import static org.junit.jupiter.api.Assertions.assertThrows;
-
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
-class SieveOfEratosthenesTest {
-    @Test
-    public void testfFindPrimesTill1() {
-        assertArrayEquals(new int[] {}, SieveOfEratosthenes.findPrimesTill(1));
-    }
-
-    @Test
-    public void testfFindPrimesTill2() {
-        assertArrayEquals(new int[] {2}, SieveOfEratosthenes.findPrimesTill(2));
-    }
-
-    @Test
-    public void testfFindPrimesTill4() {
-        var primesTill4 = new int[] {2, 3};
-        assertArrayEquals(primesTill4, SieveOfEratosthenes.findPrimesTill(3));
-        assertArrayEquals(primesTill4, SieveOfEratosthenes.findPrimesTill(4));
-    }
-
-    @Test
-    public void testfFindPrimesTill40() {
-        var primesTill40 = new int[] {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37};
-        assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(37));
-        assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(38));
-        assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(39));
-        assertArrayEquals(primesTill40, SieveOfEratosthenes.findPrimesTill(40));
-    }
+/**
+ * Unit tests for {@link SieveOfEratosthenes}.
+ */
+public final class SieveOfEratosthenesTest {
 
     @Test
-    public void testfFindPrimesTill240() {
-        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};
-        assertArrayEquals(primesTill240, SieveOfEratosthenes.findPrimesTill(239));
-        assertArrayEquals(primesTill240, SieveOfEratosthenes.findPrimesTill(240));
+    void testPrimesUpTo30() {
+        int[] expected = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29};
+        Assertions.assertArrayEquals(expected, SieveOfEratosthenes.sieve(30));
     }
 
     @Test
-    public void testFindPrimesTillThrowsExceptionForNonPositiveInput() {
-        assertThrows(IllegalArgumentException.class, () -> SieveOfEratosthenes.findPrimesTill(0));
+    void testLessThanTwo() {
+        Assertions.assertArrayEquals(new int[0], SieveOfEratosthenes.sieve(1));
+        Assertions.assertArrayEquals(new int[0], SieveOfEratosthenes.sieve(0));
+        Assertions.assertArrayEquals(new int[0], SieveOfEratosthenes.sieve(-5));
     }
 }