defmodule ToweropsWeb.Api.ErrorHelpers do
  @moduledoc "Shared error translation helpers for API controllers."

  @doc """
  Translates changeset errors into a map of field names to error message lists.

  Interpolates message parameters (e.g. `%{count}`) safely, validating keys
  before atom conversion to prevent atom exhaustion attacks.
  """
  @spec translate_errors(Ecto.Changeset.t()) :: %{atom() => [String.t()]}
  def translate_errors(changeset) do
    Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} ->
      Regex.replace(~r"%{(\w+)}", msg, fn _, key ->
        safe_translate_key(key, opts)
      end)
    end)
  end

  defp safe_translate_key(key, opts) do
    if String.length(key) <= 50 and String.match?(key, ~r/^[a-z_]+$/) do
      opts |> Keyword.get(String.to_existing_atom(key), key) |> to_string()
    else
      key
    end
  end
end