Software Development

Kotlin – tail recursion optimization

Kotlin compiler optimizes tail recursive calls with a few catches. Consider a rank function to search for the index of an element in a sorted array, implemented the following way using tail recursion and a test for it:

fun rank(k: Int, arr: Array<Int>): Int {
    tailrec fun rank(low: Int, high: Int): Int {
        if (low > high) {
            return -1
        val mid = (low + high) / 2

        return when {
            (k < arr[mid]) -> rank(low, mid)
            (k > arr[mid]) -> rank(mid + 1, high)
            else -> mid

    return rank(0, arr.size - 1)

fun rankTest() {
    val array = arrayOf(2, 4, 6, 9, 10, 11, 16, 17, 19, 20, 25)
    assertEquals(-1, rank(100, array))
    assertEquals(0, rank(2, array))
    assertEquals(2, rank(6, array))
    assertEquals(5, rank(11, array))
    assertEquals(10, rank(25, array))

IntelliJ provides an awesome feature to show the bytecode of any Kotlin code, along the lines of the following screenshot:

A Kotlin equivalent of the type of bytecode that the Kotlin compiler generates is the following:

fun rankIter(k: Int, arr: Array<Int>): Int {
    fun rankIter(low: Int, high: Int): Int {
        var lo = low
        var hi = high
        while (lo <= hi) {
            val mid = (lo + hi)/2

            if (k < arr[mid]) {
                hi = mid
            } else if (k > arr[mid]){
                lo = mid + 1
            } else {
                return mid

        return -1

    return rankIter(0, arr.size - 1)

the tail calls have been translated to a simple loop.

There are a few catches that I could see though:

1. The compiler has to be explicitly told which calls are tail-recursive using the “tailrec” modifier

2. Adding tailrec modifier to a non-tail-recursive function does not generate compiler errors, though a warning does appear during compilation step

Published on Java Code Geeks with permission by Biju Kunjummen, partner at our JCG program. See the original article here: Kotlin – tail recursion optimization

Opinions expressed by Java Code Geeks contributors are their own.

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