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.


begin
integer array eck[1:1000];
integer i, j;
 
for i := 1 step 1 until 1000 do 
    eck[i] := 0;
 
for i := 1 step 1 until 999 do
begin
    j := i - 1;
    while j > 0 and eck[i] <> eck[j] do
        j := j - 1;
    if j <> 0 then
        eck[i+1] := i - j;
end;
 
for i := 1 step 1 until 10 do
    writeon(eck[i]);
write("");
for i := 991 step 1 until 1000 do
    writeon(eck[i]);
 
end