do  --[[ find elements of the Van Eck Sequence - first term is 0, following
         terms are 0 if the previous was the first appearance of the element
            or how far back in the sequence the last element appeared otherwise
      ]]
    local fmt = require( "fmt" )                          -- RC formatting module

    local function VanEck( n : number ) : table -- returns the first n elements of the Van Eck sequence
        local result <const> = {}     for i = 1, n     do result[ i ] = 0 end
        local pos    <const> = {}     for i = 1, n + 1 do pos[    i ] = 0 end
        for i = 2, n do
            local j    <const> = i - 1
            local prev <const> = result[ j ] + 1; -- prev is indexed from 1, not 0
            if pos[ prev ] != 0 then
                -- not a new element
                result[ i ] = j - pos[ prev ]
            end
            pos[ prev ] = j
        end
        return result
    end

    local veSeq <const> = VanEck( 1000 )  -- construct the first 1000 terms of the sequence
    -- show the first and last 10 elements
    fmt.lprint( veSeq:slice( 1, 10 ) )
    fmt.lprint( veSeq:slice( # veSeq - 9, # veSeq ) )
end