Kotlin Arrays: Lookahead without going out of bounds?

Question

I have a long array of Booleans and I want to extract from it the index positions of TRUE followed by FALSE patterns. For example, this array

arrayOf(true, false, true, false, true, false)

should produce

[0, 2, 4]

but this one

arrayOf(true, false, false, false, true, false)

should produce

[0, 4]

Here's a fun for that except, of course, the 3rd example will crash Java:

fun main() {
    println(dontCrash(arrayOf(true, false, true, false, true, false)))
    println(dontCrash(arrayOf(true, false, false, false, true, false)))
    println(dontCrash(arrayOf(true, false, false, false, true, true)))
}
fun dontCrash(a: Array<Boolean>): List<Int> {
    val goodGuys = mutableListOf<Int>()
    for ((ind, element) in a.withIndex())
        if (element && !a[ind+1]) goodGuys.add(ind)
    return goodGuys
}

[0, 2, 4]
[0, 4]
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 6 out of bounds for length 6

My question: Does Kotlin have some cute trick to deal this (step??) or do I need to add a .size() check?

Answer 1

windowed(2) or zipWithNext() lets you slide along pairs of adjacent values without going past the bounds, but it's only available for Iterables and Sequences, not arrays:

fun dontCrash(a: Array<Boolean>): List<Int> {
    a.asSequence()
        .zipWithNext()
        .mapIndexedNotNull { i, (a, b) -> i.takeIf { a && !b } }
        .toList()
}