KTLN-692: Check if a number can be expressed as sum of two primes (#758)

Co-authored-by: Tapan Avasthi <[email protected]>

Author: tapankavasthi

kotlin-math-2/src/main/kotlin/com/baeldung/math/sumofprimes/SumOfPrimes.kt

@@ -0,0 +1,46 @@
+package com.baeldung.math.sumofprimes
+
+
+fun checkPrimeUsingBruteForce(number: Int): Boolean {
+    if (number <= 1) return false
+    for (i in 2 until number) {
+        if (number % i == 0) {
+            return false
+        }
+    }
+    return true
+}
+
+fun canBeExpressedAsSumOfTwoPrimesUsingBruteForceApproach(n: Int): Boolean {
+    for (i in 2..n / 2) {
+        if (checkPrimeUsingBruteForce(i) && checkPrimeUsingBruteForce(n - i)) {
+            return true
+        }
+    }
+    return false
+}
+
+fun sieveOfEratosthenes(n: Int): BooleanArray {
+    val primes = BooleanArray(n + 1) { true }
+    primes[0] = false
+    primes[1] = false
+    for (p in 2..n) {
+        if (primes[p]) {
+            for (i in p * p..n step p) {
+                primes[i] = false
+            }
+        }
+    }
+    return primes
+}
+
+fun canBeExpressedAsSumOfTwoPrimesUsingSieveOfEratosthenes(n: Int): Boolean {
+    if (n < 2) return false
+    val primes = sieveOfEratosthenes(n)
+    for (i in 2 until n) {
+        if (primes[i] && primes[n - i]) {
+            return true
+        }
+    }
+    return false
+}

kotlin-math-2/src/test/kotlin/com/baeldung/math/sumofprimes/SumOfTwoPrimesUnitTest.kt

@@ -0,0 +1,44 @@
+package com.baeldung.math.sumofprimes
+
+import kotlin.test.assertTrue
+import kotlin.test.assertFalse
+import org.junit.jupiter.api.Test
+
+class SumOfTwoPrimesUnitTest {
+
+    private val sumOfTwoPrimes = arrayOf(10, 13, 25, 28)
+    private val notSumOfTwoPrimes = arrayOf(11, 27, 51, 57)
+
+    @Test
+    fun `given number that can be expressed as sum of two primes, when check using brute force approach, then return true`() {
+        for (n in sumOfTwoPrimes) {
+            val result = canBeExpressedAsSumOfTwoPrimesUsingBruteForceApproach(n)
+            assertTrue(result)
+        }
+    }
+
+    @Test
+    fun `given number that cannot be expressed as sum of two primes, when check using brute force approach, then return false`() {
+        for (n in notSumOfTwoPrimes) {
+            val result = canBeExpressedAsSumOfTwoPrimesUsingBruteForceApproach(n)
+            assertFalse(result)
+        }
+    }
+
+    @Test
+    fun `given number that can be expressed as sum of two primes, when check using sieve of eratosthenes, then return true`() {
+        for (n in sumOfTwoPrimes) {
+            val result = canBeExpressedAsSumOfTwoPrimesUsingSieveOfEratosthenes(n)
+            assertTrue(result)
+        }
+    }
+
+    @Test
+    fun `given number that cannot be expressed as sum of two primes, when check using sieve of eratosthenes, then return false`() {
+        for (n in notSumOfTwoPrimes) {
+            val result = canBeExpressedAsSumOfTwoPrimesUsingSieveOfEratosthenes(n)
+            assertFalse(result)
+        }
+    }
+
+}