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.


struct VanEckSequence: Sequence, IteratorProtocol {
    private var index = 0
    private var lastTerm = 0
    private var lastPos = Dictionary<Int, Int>()
 
    mutating func next() -> Int? {
        let result = lastTerm
        var nextTerm = 0
        if let v = lastPos[lastTerm] {
            nextTerm = index - v
        }
        lastPos[lastTerm] = index
        lastTerm = nextTerm
        index += 1
        return result
    }
}
 
let seq = VanEckSequence().prefix(1000)
 
print("First 10 terms of the Van Eck sequence:")
for n in seq.prefix(10) {
    print(n, terminator: " ")
}
print("\nTerms 991 to 1000 of the Van Eck sequence:")
for n in seq.dropFirst(990) {
    print(n, terminator: " ")
}
print()