From cf5842a3abc18e1ab2f9d163218898c88015e340 Mon Sep 17 00:00:00 2001
From: vunamtien <vunamtien16@gmail.com>
Date: Thu, 15 Feb 2024 21:54:14 +0700
Subject: [PATCH] BAEL-7358-find-prime-under-given (#15888)

---
 .../primeundernumber/LargestPrimeFinder.java  | 43 +++++++++++++++++++
 .../LargestPrimeFinderUnitTest.java           | 32 ++++++++++++++
 2 files changed, 75 insertions(+)
 create mode 100644 algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinder.java
 create mode 100644 algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinderUnitTest.java

diff --git a/algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinder.java b/algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinder.java
new file mode 100644
index 0000000000..bf4e785f29
--- /dev/null
+++ b/algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinder.java
@@ -0,0 +1,43 @@
+package com.baeldung.algorithms.primeundernumber;
+
+import java.util.Arrays;
+
+public class LargestPrimeFinder {
+
+    public static int findByBruteForce(int n) {
+        for (int i = n - 1; i >= 2; i--) {
+            if (isPrime(i)) {
+                return i;
+            }
+        }
+        return -1; // Return -1 if no prime number is found
+    }
+
+    public static boolean isPrime(int number) {
+        for (int i = 2; i <= Math.sqrt(number); i++) {
+            if (number % i == 0) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    public static int findBySieveOfEratosthenes(int n) {
+        boolean[] isPrime = new boolean[n];
+        Arrays.fill(isPrime, true);
+        for (int p = 2; p*p < n; p++) {
+            if (isPrime[p]) {
+                for (int i = p * p; i < n; i += p) {
+                    isPrime[i] = false;
+                }
+            }
+        }
+
+        for (int i = n - 1; i >= 2; i--) {
+            if (isPrime[i]) {
+                return i;
+            }
+        }
+        return -1;
+    }
+}
diff --git a/algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinderUnitTest.java b/algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinderUnitTest.java
new file mode 100644
index 0000000000..f046431424
--- /dev/null
+++ b/algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/primeundernumber/LargestPrimeFinderUnitTest.java
@@ -0,0 +1,32 @@
+package com.baeldung.algorithms.primeundernumber;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class LargestPrimeFinderUnitTest {
+
+    @Test
+    public void givenNormalCases_whenFindPrime_ThenFoundResult() {
+        assertEquals(7, LargestPrimeFinder.findByBruteForce(10));
+        assertEquals(97, LargestPrimeFinder.findByBruteForce(100));
+        assertEquals(7, LargestPrimeFinder.findBySieveOfEratosthenes(10));
+        assertEquals(97, LargestPrimeFinder.findBySieveOfEratosthenes(100));
+    }
+
+    @Test
+    public void givenEdgeCases_whenFindPrime_ThenNotFoundResult() {
+        assertEquals(-1, LargestPrimeFinder.findByBruteForce(0));
+        assertEquals(-1, LargestPrimeFinder.findByBruteForce(1));
+        assertEquals(-1, LargestPrimeFinder.findByBruteForce(2));
+        assertEquals(-1, LargestPrimeFinder.findBySieveOfEratosthenes(0));
+        assertEquals(-1, LargestPrimeFinder.findBySieveOfEratosthenes(1));
+        assertEquals(-1, LargestPrimeFinder.findBySieveOfEratosthenes(2));
+    }
+
+    @Test
+    public void givenLargeInput_whenFindPrime_ThenFoundResult() {
+        assertEquals(99991, LargestPrimeFinder.findByBruteForce(100000));
+        assertEquals(99991, LargestPrimeFinder.findBySieveOfEratosthenes(100000));
+    }
+}