Van Eck sequence

editable example

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ghul

use IO.Std.write_line

use Collections.MAP

use Ghul.Pipes

van_eck() -> Pipe[int] is

let last_seen = MAP[int, int]()

let term mut = 0

let position mut = 0

do

yield term

let previous mut = 0

let next =

if last_seen.try_get_value(term, previous ref) then

position - previous

else

0

fi

last_seen[term] = position

term = next

position = position + 1

od

si

write_line("first ten: {van_eck() |> take(10) |> join(" ")}")

write_line(

"terms 991-1000: {van_eck() |> skip(990) |> take(10) |> join(" ")}")

first ten:      0 0 1 0 2 0 2 2 1 6
terms 991-1000: 4 7 30 25 67 225 488 0 10 136