fun <T> Iterable<T>.runLengths() = fold(mutableListOf<Pair<T, Int>>()) { acc, t ->
    acc.apply {
        if (acc.isEmpty() || acc.last().first != t)
            acc.add(t to 1)
        else
            acc[acc.size-1] = t to acc.last().second + 1
    }
}.toList()

listOf("a", "a", "b", "c", "c", "a", "a")
    .runLengths()

fun <T> Iterable<T>.runLengths() = buildList<Pair<T, Int>> {
    for (t in this@runLengths)
        if (isEmpty() || last().first != t) add(t to 1) else set(size - 1, t to last().second + 1)
}

listOf("a", "a", "b", "c", "c", "a", "a").runLengths()