RxRandom.kt

// Picks a random value from an arbitrary sequence
fun <T: Any> random(input: Observable<T>): Observable<T> {
    var size = 1
    return input
        .filter { Math.floor(Math.random() * size++) == 0.0 }
        .last()
}

// Test whether all items were chosen equally
fun test() {
    val results = IntArray(5)
    for (i in 1..10000) {
        random(Observable.just(1, 2, 3, 4, 5)).subscribe { results[it-1]++ }
    }
    println(results.joinToString())
}