defmodule Explorer.SkippedBlocks do
  @moduledoc """
    Fill in older blocks that were skipped during processing.
  """
  import Ecto.Query, only: [from: 2, limit: 2]
  alias Explorer.Block
  alias Explorer.Repo.NewRelic, as: Repo

  @missing "SELECT generate_series(?, 0, -1) AS missing_number"

  def first, do: first(1)
  def first(count) do
    blocks = from b in Block,
      right_join: missing_number in fragment(@missing, ^latest_block_number()),
      on: b.number == fragment("missing_number"),
      limit: ^count,
      where: is_nil(b.id),
      select: fragment("missing_number")
    Repo.all(blocks) |> Enum.map(&Integer.to_string/1)
  end

  def latest_block_number do
    block = Repo.one(Block |> Block.latest |> limit(1)) || Block.null
    block.number
  end
end