defmodule Mercury.QueryParams do
  @moduledoc """
  Normalizes the keyword lists / maps passed as list-endpoint filters into a
  clean keyword list suitable for `Req`'s `:params` option: `nil` values are
  dropped (matching the reference SDK's `omitempty` semantics) and atoms are
  stringified.
  """

  @doc false
  @spec build(params :: keyword() | map() | nil) :: keyword()
  def build(nil), do: []

  def build(params) when is_list(params) or is_map(params) do
    params
    |> Enum.to_list()
    |> Enum.reject(fn {_key, value} -> is_nil(value) end)
    |> Enum.map(fn {key, value} -> {key, normalize(value)} end)
  end

  defp normalize(nil), do: nil
  defp normalize(value) when is_list(value), do: Enum.map(value, &normalize/1)

  defp normalize(value) when is_atom(value) and value not in [true, false],
    do: Atom.to_string(value)

  defp normalize(value), do: value
end