Reverse string in Kotlin

Question

In the question of how to reverse a string in Java, a comment mentioned that combining Unicode code points need to be taken into account.

The below code works as intended for all test cases I tried. Probably there are some edge cases in other scripts and languages I do not know. I'd like to learn about these, as well as any coding style issues.

package de.roland_illig.strrev
import com.ibm.icu.lang.UCharacter
import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test
/**
 * Returns the reversed string, keeping clusters of combining code points
 * (such as German umlauts or Arabic tashkīl) together.
 */
fun String.reverse(): String {
 fun isLamAlef(cluster: List<Int>, ch: Int) =
 cluster.isNotEmpty() && cluster.first() == 0x0644 && ch == 0x0627
 val clusters = mutableListOf<List<Int>>()
 val cluster = mutableListOf<Int>()
 this.codePoints().forEachOrdered { ch ->
 if (!(UCharacter.getCombiningClass(ch) != 0) && !isLamAlef(cluster, ch)) {
 if (cluster.isNotEmpty()) {
 clusters += cluster.toList()
 cluster.clear()
 }
 }
 cluster += ch
 }
 if (cluster.isNotEmpty()) {
 clusters += cluster.toList()
 cluster.clear()
 }
 return fromCodePoints(*clusters.reversed().flatten().toIntArray())
}
class StringReverseTest {
 @Test
 fun ascii() {
 assertThat("hello".reverse()).isEqualTo("olleh")
 }
 @Test
 fun surrogates() {
 val emoji = fromCodePoints(0x1F645)
 assertThat(emoji.reverse()).isEqualTo(emoji)
 }
 @Test
 fun combining() {
 val combinedUmlaut = fromCodePoints(0x0041, 0x0308)
 assertThat(combinedUmlaut.reverse()).isEqualTo(combinedUmlaut)
 }
 @Test
 fun arabic() {
 assertThat("أَهْلًا وَ سَهْلًا".reverse()).isEqualTo("لًاهْسَ وَ لًاهْأَ")
 }
 @Test
 fun combiningAtBeginning() {
 val combinedUmlaut = fromCodePoints(0x0308, 0x0041)
 assertThat(combinedUmlaut.reverse())
 .isEqualTo(fromCodePoints(0x0041, 0x0308))
 }
}
private fun fromCodePoints(vararg codePoints: Int): String =
 String(codePoints, 0, codePoints.size)

For completeness, here are the Gradle dependencies for build.gradle:

dependencies {
 compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
 compile group: 'com.ibm.icu', name: 'icu4j', version: '61.1'
 testCompile "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
 testCompile "org.junit.jupiter:junit-jupiter-api:5.0.2"
 testCompile "org.assertj:assertj-core:3.9.0"
}