defmodule UsCongress.BillDtd.Hierarchy do
  @moduledoc """
  Produce a hierarchical map of nodes,
  called "Measures".

  A measure can be:

  - a `String`
  - a `%{scheme: "", ...}`
  - a `%{label: "", params: %{}, measure: []}`

  During parsing,
  measures are commonly merged alongside preceding ones.
  The function `UsCongress.BillDtd.Shape.combine/2`
  helps manage the rules here.
  """
  @behaviour Saxy.Handler
  alias UsCongress.BillDtd.Shape

  def handle_event(:start_element, {name, attributes}, prior),
  do: {:ok, [
    %{ label: name, params: attributes |> Map.new, measure: [] }
  | prior ]}

  def handle_event(:characters, chars,
    [%{label: name, measure: measure} = current
  | prior] ), do:
    {:ok, [Shape.merge(current, chars) | prior]}

  def handle_event(:end_element, "header",
    [ header, %{scheme: "enum"} = enum
  | prior ] ), do:
    {:ok, [ Shape.combine(enum, Shape.scheme(header)) | prior]}

  def handle_event(:end_element, _name,
    [ %{ measure: [_body]} = current,
      %{measure: measure } = former
  | prior] ), do:
    {:ok, [former |> Shape.combine(Shape.scheme(current)) | prior]}

  def handle_event(:end_element, _name, [
    %{ measure: [_body]} = current,
    %{measure: measure} = basis
  | prior] ), do:
    {:ok, [ Shape.combine(basis, current) | prior ] }

  def handle_event(_, _, prior), do: {:ok, prior}
end