defmodule Mv.MembershipFees.CycleEndBackfill do
  @moduledoc """
  One-time backfill for the denormalized `cycle_end` column on
  `membership_fee_cycles`.

  For every cycle whose `cycle_end` is still `NULL`, sets it to
  `CalendarCycles.calculate_cycle_end(cycle_start, interval)` using the cycle's
  own fee-type interval. Idempotent: rows that already have a `cycle_end` are
  left untouched, so it is safe to re-run.

  Invoked from the `cycle_end` migration. Kept as a module function (rather than
  inline migration SQL) so the exact calendar arithmetic is reused from
  `CalendarCycles` and can be tested directly.
  """

  import Ecto.Query

  alias Mv.MembershipFees.CalendarCycles
  alias Mv.Repo

  @doc """
  Backfills `cycle_end` for all cycles missing it. Returns `{:ok, updated_count}`.
  """
  @spec run() :: {:ok, non_neg_integer()}
  def run do
    rows =
      from(c in "membership_fee_cycles",
        join: t in "membership_fee_types",
        on: c.membership_fee_type_id == t.id,
        where: is_nil(c.cycle_end),
        select: {c.id, c.cycle_start, t.interval}
      )
      |> Repo.all()

    count =
      Enum.reduce(rows, 0, fn {id, cycle_start, interval}, acc ->
        cycle_end = CalendarCycles.calculate_cycle_end(cycle_start, normalize_interval(interval))

        from(c in "membership_fee_cycles", where: c.id == ^id)
        |> Repo.update_all(set: [cycle_end: cycle_end])

        acc + 1
      end)

    {:ok, count}
  end

  # The raw query returns the enum column as a string.
  defp normalize_interval(interval) when is_atom(interval), do: interval

  defp normalize_interval(interval) when is_binary(interval),
    do: String.to_existing_atom(interval)
end