kotlin slice with default and negative index

slice.kt

internal fun <T> List<T>.slice(indices: Iterable<Int>, default: (index: Int) -> T): List<T> {
    fun measureSizeOrDefault(iterator: Iterable<Int>, defaultValue: Int): Int =
        when (iterator) {
            is IntProgression -> {
                iterator.run {
                    when {
                        step > 0 -> if (first > last) 0 else ((last - first) / step) + 1
                        step < 0 -> if (last > first) 0 else ((first - last) / step.absoluteValue) + 1
                        else -> error("step equals zero.")
                    }
                }
            }
            is Collection<Int> -> iterator.size
            else -> defaultValue
        }

    val size = measureSizeOrDefault(indices, 10)
    val target = ArrayList<T>(size)
    val availRange = 0..this.lastIndex
    indices.forEach {
        val pos = if (it < 0) this.size + it else it
        if (pos in availRange) {
            target.add(this[pos])
        } else {
            target.add(default.invoke(pos))
        }
    }
    return target
}

tests.kt

internal class UtilKtTest {

    @Test
    fun slice() {
        val list = listOf(0, 1, 2, 3, 4, 5, 6, 7, 8)
        assertEquals(list.slice(1..5) { 0 }, listOf(1, 2, 3, 4, 5))
        assertEquals(list.slice(0..1) { 0 }, listOf(0, 1))
        assertEquals(list.slice(1..0) { 0 }, emptyList<Int>())
        assertEquals(list.slice(1..1) { 0 }, listOf(1))
        assertEquals(list.slice(0..0) { 0 }, listOf(0))
        assertEquals(list.slice(0..20) { it }, (0..20).toList())

        assertEquals(list.slice(-5..-1) { 0 }, listOf(4, 5, 6, 7, 8))
        assertEquals(list.slice(-5..1) { 0 }, listOf(4, 5, 6, 7, 8, 0, 1))
        assertEquals(list.slice(0..5 step 2){0}, listOf(0,2,4))
        assertEquals(list.slice(0..6 step 2){0}, listOf(0,2,4,6))

        assertEquals(list.slice(5 downTo 0){0}, listOf(5,4,3,2,1,0))
        assertEquals(list.slice(3 downTo -3){0}, listOf(3,2,1,0,8,7,6))
        assertEquals(list.slice(3 downTo -3 step 2){0}, listOf(3,1,8,6))
    }
}