psql_tetris

defmodule PsqlTetris.Optimizer do
  @moduledoc """
  Pure ordering logic. Given a list of column descriptors, returns them in the order PostgreSQL would prefer for minimal row padding.

  A column descriptor is a map (or struct-shaped map) with at least:

    * `:type` : the Ecto migration type (atom or tuple)
    * `:opts` : keyword list passed to `add/3` (may be `[]`)

  Any other keys are preserved untouched.

  The sort is stable, so columns of the same alignment rank keep their author-defined order. Within a rank, `null: false` columns come first, which is a tiny CPU win during tuple deforming.
  """

  alias PsqlTetris.Types

  @spec optimize([map()]) :: [map()]
  def optimize(columns) when is_list(columns) do
    columns
    |> Enum.with_index()
    |> Enum.sort_by(fn {col, idx} ->
      rank = Types.rank(col.type, Map.get(col, :opts, []))
      not_null_first = if not_null?(col), do: 0, else: 1
      {rank, not_null_first, idx}
    end)
    |> Enum.map(fn {col, _idx} -> col end)
  end

  defp not_null?(%{opts: opts}) when is_list(opts), do: Keyword.get(opts, :null) == false
  defp not_null?(_), do: false
end