The sequence is generated by following this pseudo-code:

A:  The first term is zero.
    Repeatedly apply:
        If the last term is *new* to the sequence so far then:
B:          The next term is zero.
        Otherwise:
C:          The next term is how far back this last term occured previously.



Example

Using A:

0
Using B:

0 0
Using C:

0 0 1
Using B:

0 0 1 0
Using C: (zero last occurred two steps back - before the one)

0 0 1 0 2
Using B:

0 0 1 0 2 0
Using C: (two last occurred two steps back - before the zero)

0 0 1 0 2 0 2 2
Using C: (two last occurred one step back)

0 0 1 0 2 0 2 2 1
Using C: (one last appeared six steps back)

0 0 1 0 2 0 2 2 1 6
...



Task

 Create a function/procedure/method/subroutine/... to generate the Van Eck sequence of numbers.
 Use it to display here, on this page:
 The first ten terms of the sequence.
 Terms 991 - to - 1000 of the sequence.


References

 Don't Know (the Van Eck Sequence) - Numberphile video.
 Wikipedia Article: Van Eck's Sequence.
  OEIS sequence: A181391.


fun main() {
    println("First 10 terms of Van Eck's sequence:")
    vanEck(1, 10)
    println("")
    println("Terms 991 to 1000 of Van Eck's sequence:")
    vanEck(991, 1000)
}
 
private fun vanEck(firstIndex: Int, lastIndex: Int) {
    val vanEckMap = mutableMapOf<Int, Int>()
    var last = 0
    if (firstIndex == 1) {
        println("VanEck[1] = 0")
    }
    for (n in 2..lastIndex) {
        val vanEck = if (vanEckMap.containsKey(last)) n - vanEckMap[last]!! else 0
        vanEckMap[last] = n
        last = vanEck
        if (n >= firstIndex) {
            println("VanEck[$n] = $vanEck")
        }
    }
}