From a38e6e295e6fb82dbb68b1c25df9a9366aaa04ed Mon Sep 17 00:00:00 2001
From: Muhammed Almas <eralmas7@yahoo.com>
Date: Fri, 10 Nov 2017 01:24:36 +0530
Subject: [PATCH] BAEL-1296 Prime numbers till n. (#2956)

* BAEL-1296 Prime numbers till n.

* BAEL-1296 Renamed packge
---
 .../algorithms/prime/PrimeGenerator.java      | 59 +++++++++++++++++++
 .../algorithms/prime/PrimeGeneratorTest.java  | 28 +++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 algorithms/src/main/java/com/baeldung/algorithms/prime/PrimeGenerator.java
 create mode 100644 algorithms/src/test/java/com/baeldung/algorithms/prime/PrimeGeneratorTest.java

diff --git a/algorithms/src/main/java/com/baeldung/algorithms/prime/PrimeGenerator.java b/algorithms/src/main/java/com/baeldung/algorithms/prime/PrimeGenerator.java
new file mode 100644
index 0000000000..48d51a8848
--- /dev/null
+++ b/algorithms/src/main/java/com/baeldung/algorithms/prime/PrimeGenerator.java
@@ -0,0 +1,59 @@
+package com.baeldung.algorithms.prime;
+
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+public class PrimeGenerator {
+    public static List<Integer> sieveOfEratosthenes(int n) {
+        final boolean prime[] = new boolean[n + 1];
+        Arrays.fill(prime, true);
+
+        for (int p = 2; p * p <= n; p++) {
+            if (prime[p]) {
+                for (int i = p * 2; i <= n; i += p)
+                    prime[i] = false;
+            }
+        }
+
+        final List<Integer> primes = new LinkedList<>();
+        for (int i = 2; i <= n; i++) {
+            if (prime[i])
+                primes.add(i);
+        }
+        return primes;
+    }
+
+    public static List<Integer> primeNumbersBruteForce(int max) {
+        final List<Integer> primeNumbers = new LinkedList<Integer>();
+        for (int i = 2; i <= max; i++) {
+            if (isPrimeBruteForce(i)) {
+                primeNumbers.add(i);
+            }
+        }
+        return primeNumbers;
+    }
+
+    private static boolean isPrimeBruteForce(int x) {
+        for (int i = 2; i < x; i++) {
+            if (x % i == 0) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    public static List<Integer> primeNumbersTill(int max) {
+        return IntStream.rangeClosed(2, max)
+            .filter(x -> isPrime(x))
+            .boxed()
+            .collect(Collectors.toList());
+    }
+
+    private static boolean isPrime(int x) {
+        return IntStream.rangeClosed(2, (int) (Math.sqrt(x)))
+            .allMatch(n -> x % n != 0);
+    }
+}
diff --git a/algorithms/src/test/java/com/baeldung/algorithms/prime/PrimeGeneratorTest.java b/algorithms/src/test/java/com/baeldung/algorithms/prime/PrimeGeneratorTest.java
new file mode 100644
index 0000000000..4995e938b7
--- /dev/null
+++ b/algorithms/src/test/java/com/baeldung/algorithms/prime/PrimeGeneratorTest.java
@@ -0,0 +1,28 @@
+package com.baeldung.algorithms.prime;
+
+import static com.baeldung.algorithms.prime.PrimeGenerator.*;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class PrimeGeneratorTest {
+    @Test
+    public void whenBruteForced_returnsSuccessfully() {
+        final List<Integer> primeNumbers = primeNumbersBruteForce(20);
+        assertEquals(Arrays.asList(new Integer[] { 2, 3, 5, 7, 11, 13, 17, 19 }), primeNumbers);
+    }
+
+    @Test
+    public void whenOptimized_returnsSuccessfully() {
+        final List<Integer> primeNumbers = primeNumbersTill(20);
+        assertEquals(Arrays.asList(new Integer[] { 2, 3, 5, 7, 11, 13, 17, 19 }), primeNumbers);
+    }
+
+    @Test
+    public void whenSieveOfEratosthenes_returnsSuccessfully() {
+        final List<Integer> primeNumbers = sieveOfEratosthenes(20);
+        assertEquals(Arrays.asList(new Integer[] { 2, 3, 5, 7, 11, 13, 17, 19 }), primeNumbers);
+    }
+}