diff --git a/src/main/java/com/thealgorithms/maths/SieveOfEratosthenes.java b/src/main/java/com/thealgorithms/maths/SieveOfEratosthenes.java
index f22d22e8c6af..5a15c4201a15 100644
--- a/src/main/java/com/thealgorithms/maths/SieveOfEratosthenes.java
+++ b/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 Sieve of Eratosthenes
+ * 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 Sieve of Eratosthenes
*/
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 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 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();
}
}
diff --git a/src/test/java/com/thealgorithms/maths/SieveOfEratosthenesTest.java b/src/test/java/com/thealgorithms/maths/SieveOfEratosthenesTest.java
index ebbd5df712fc..5d491a493ee7 100644
--- a/src/test/java/com/thealgorithms/maths/SieveOfEratosthenesTest.java
+++ b/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 expected = Arrays.asList(2, 3, 5, 7);
+ assertEquals(expected, SieveOfEratosthenes.findPrimes(10));
+ }
+
+ @Test
+ void testPrimesUpTo30() {
+ List 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 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 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
}
}