Solve 'Integers: Recreation One' kata

pom.xml

@@ -63,6 +63,14 @@
                 <artifactId>maven-failsafe-plugin</artifactId>
                 <version>3.2.5</version>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>17</source>
+                    <target>17</target>
+                </configuration>
+            </plugin>
         </plugins>
     </build>
 

src/main/kotlin/integersrecreationone/SumSquaredDivisors.kt

@@ -0,0 +1,54 @@
+package solution
+
+import java.util.stream.LongStream
+import java.util.stream.LongStream.range
+
+/**
+ * https://www.codewars.com/kata/55aa075506463dac6600010d/train/kotlin
+ */
+object SumSquaredDivisors {
+
+    fun listSquared(m: Long, n: Long): String {
+        return (m..n)
+            .asSequence()
+            .map { Pair(it, it.divisors()) }
+            .map { pair -> pair.mapSecond(LongStream::squareSum) }
+            .filter { it.second.isSquare() }
+            .map { it.toList() }
+            .toList()
+            .toString()
+    }
+}
+
+fun Long.divisors(): LongStream {
+    return range(1, this + 1)
+        .takeWhile { it * it <= this }
+        .filter { this % it == 0L }
+        .flatMap {
+            if (it * it == this)
+                LongStream.of(it)
+            else
+                LongStream.of(it, this / it)
+        }
+}
+
+fun LongStream.squareSum(): Long {
+    return this.map { it * it }.sum()
+}
+
+fun Long.sqrt(): Long {
+    return kotlin.math.sqrt(this.toDouble()).toLong()
+}
+
+fun Long.isSquare(): Boolean {
+    val sqrt = this.sqrt()
+    return sqrt * sqrt == this
+}
+
+fun Pair<Long, Long>.toList(): List<Long> {
+    return listOf(this.first, this.second)
+}
+
+fun <A, B, C> Pair<A, B>.mapSecond(f: (B) -> C): Pair<A, C> {
+    return Pair(this.first, f(this.second))
+}

src/test/kotlin/integersrecreationone/SumSquaredDivisorsTest.kt

@@ -0,0 +1,13 @@
+package solution
+
+import org.junit.Test
+import kotlin.test.assertEquals
+
+class SumSquaredDivisorsTest {
+
+    @Test
+    fun basicTests() {
+        assertEquals("[[1, 1], [42, 2500], [246, 84100]]", SumSquaredDivisors.listSquared(1, 250))
+        assertEquals("[[42, 2500], [246, 84100]]", SumSquaredDivisors.listSquared(42, 250))
+    }
+}