feat: add Sieve of Eratosthenes algorithm

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

@@ -1,66 +1,82 @@
 package com.thealgorithms.maths;
 
-import java.util.Arrays;
+import java.util.ArrayList;
+import java.util.List;
 
 /**
- * @brief utility class implementing <a href="https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes">Sieve of Eratosthenes</a>
+ * Sieve of Eratosthenes Algorithm
+ * An efficient algorithm to find all prime numbers up to a given limit.
+ *
+ * Algorithm:
+ * 1. Create a boolean array of size n+1, initially all true
+ * 2. Mark 0 and 1 as not prime
+ * 3. For each number i from 2 to sqrt(n):
+ *    - If i is still marked as prime
+ *    - Mark all multiples of i (starting from i²) as not prime
+ * 4. Collect all numbers still marked as prime
+ *
+ * Time Complexity: O(n log log n)
+ * Space Complexity: O(n)
+ *
+ * @author Navadeep0007
+ * @see <a href="https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes">Sieve of Eratosthenes</a>
  */
 public final class SieveOfEratosthenes {
+
     private SieveOfEratosthenes() {
+        // Utility class, prevent instantiation
     }
 
-    private static void checkInput(int n) {
-        if (n <= 0) {
-            throw new IllegalArgumentException("n must be positive.");
+    /**
+     * Finds all prime numbers up to n using the Sieve of Eratosthenes algorithm
+     *
+     * @param n the upper limit (inclusive)
+     * @return a list of all prime numbers from 2 to n
+     * @throws IllegalArgumentException if n is negative
+     */
+    public static List<Integer> findPrimes(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("Input must be non-negative");
         }
-    }
 
-    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;
+        if (n < 2) {
+            return new ArrayList<>();
+        }
+
+        // Create boolean array, initially all true
+        boolean[] isPrime = new boolean[n + 1];
+        for (int i = 2; i <= n; i++) {
+            isPrime[i] = true;
+        }
 
-        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;
+        // Sieve process
+        for (int i = 2; i * i <= n; i++) {
+            if (isPrime[i]) {
+                // Mark all multiples of i as not prime
+                for (int j = i * i; j <= n; j += i) {
+                    isPrime[j] = 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;
+        // Collect all prime numbers
+        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)
+     * Counts the number of prime numbers up to n
+     *
+     * @param n the upper limit (inclusive)
+     * @return count of prime numbers from 2 to n
      */
-    public static int[] findPrimesTill(int n) {
-        return extractPrimes(sievePrimesTill(n));
-    }
-
-    private enum Type {
-        PRIME,
-        NOT_PRIME,
+    public static int countPrimes(int n) {
+        return findPrimes(n).size();
     }
 }

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

@@ -1,46 +1,64 @@
 package com.thealgorithms.maths;
 
-import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.Arrays;
+import java.util.List;
 import org.junit.jupiter.api.Test;
 
+/**
+ * Test cases for Sieve of Eratosthenes algorithm
+ *
+ * @author Navadeep0007
+ */
 class SieveOfEratosthenesTest {
+
+    @Test
+    void testPrimesUpTo10() {
+        List<Integer> expected = Arrays.asList(2, 3, 5, 7);
+        assertEquals(expected, SieveOfEratosthenes.findPrimes(10));
+    }
+
+    @Test
+    void testPrimesUpTo30() {
+        List<Integer> expected = Arrays.asList(2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
+        assertEquals(expected, SieveOfEratosthenes.findPrimes(30));
+    }
+
     @Test
-    public void testfFindPrimesTill1() {
-        assertArrayEquals(new int[] {}, SieveOfEratosthenes.findPrimesTill(1));
+    void testPrimesUpTo2() {
+        List<Integer> expected = Arrays.asList(2);
+        assertEquals(expected, SieveOfEratosthenes.findPrimes(2));
     }
 
     @Test
-    public void testfFindPrimesTill2() {
-        assertArrayEquals(new int[] {2}, SieveOfEratosthenes.findPrimesTill(2));
+    void testPrimesUpTo1() {
+        assertTrue(SieveOfEratosthenes.findPrimes(1).isEmpty());
     }
 
     @Test
-    public void testfFindPrimesTill4() {
-        var primesTill4 = new int[] {2, 3};
-        assertArrayEquals(primesTill4, SieveOfEratosthenes.findPrimesTill(3));
-        assertArrayEquals(primesTill4, SieveOfEratosthenes.findPrimesTill(4));
+    void testPrimesUpTo0() {
+        assertTrue(SieveOfEratosthenes.findPrimes(0).isEmpty());
     }
 
     @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));
+    void testNegativeInput() {
+        assertThrows(IllegalArgumentException.class, () -> { SieveOfEratosthenes.findPrimes(-1); });
     }
 
     @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 testCountPrimes() {
+        assertEquals(4, SieveOfEratosthenes.countPrimes(10));
+        assertEquals(25, SieveOfEratosthenes.countPrimes(100));
     }
 
     @Test
-    public void testFindPrimesTillThrowsExceptionForNonPositiveInput() {
-        assertThrows(IllegalArgumentException.class, () -> SieveOfEratosthenes.findPrimesTill(0));
+    void testLargeNumber() {
+        List<Integer> primes = SieveOfEratosthenes.findPrimes(1000);
+        assertEquals(168, primes.size()); // There are 168 primes up to 1000
+        assertEquals(2, primes.get(0)); // First prime
+        assertEquals(997, primes.get(primes.size() - 1)); // Last prime up to 1000
     }
 }