Reverse string in Kotlin

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"
}