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.


Imports System.Linq
Module Module1
    Dim h() As Integer
    Sub sho(i As Integer)
        Console.WriteLine(String.Join(" ", h.Skip(i).Take(10)))
    End Sub
    Sub Main()
        Dim a, b, c, d, f, g As Integer : g = 1000
        h = new Integer(g){} : a = 0 : b = 1 : For c = 2 To g
            f = h(b) : For d = a To 0 Step -1
                If f = h(d) Then h(c) = b - d: Exit For
            Next : a = b : b = c : Next : sho(0) : sho(990)
    End Sub
End Module 