defmodule Transmap do
  @default_key :_default
  @spread_key :_spread

  @doc """
  Transforms the given map.

  `rule_map` is a map has elements `key => rule`.

  case {key, rule} do
    {:_default, default_rule} -> Change the default rule for the map from false to the given rule
    {:_spread, targets} -> Pop the maps for specific keys and merge it into the result map
    {key, true} -> Put the value for a specific key to the result map.
    {key, false} -> Don't put the value for a specific key to the result map.
    {key, rule_map} when is_map(rule_map) -> Transform the map for a specific key with the given rule
    {key, {new_key, rule}} -> Rename the key to new_key
    {key, new_key} -> Shorthand for {new_key, true}
  end

  ## Examples

  Basic transformation:

      iex> map = %{a: 1, b: 2, c: 3}
      iex> rule = %{a: true}
      iex> Transmap.transform(map, rule)
      %{a: 1}

      iex> map = %{a: %{b: 1, c: 2}, d: %{e: 3, f: 4}}
      iex> rule = %{a: %{c: true}, d: true}
      iex> Transmap.transform(map, rule)
      %{a: %{c: 2}, d: %{e: 3, f: 4}}

  Transfomation with _default:

      iex> map = %{a: 1, b: 2, c: %{d: 3, e: 4}}
      iex> rule = %{_default: true, b: false, c: %{_default: true}}
      iex> Transmap.transform(map, rule)
      %{a: 1, c: %{d: 3, e: 4}}

  Transformation with _spread:

      iex> map = %{a: %{b: 1, c: 2}, d: %{e: %{f: 3, g: 4}, h: 5}}
      iex> rule = %{_spread: [[:a], [:d, :e]], a: true, d: %{e: %{f: true}}}
      iex> Transmap.transform(map, rule)
      %{b: 1, c: 2, f: 3}

  Transformation with renaming:

      iex> map = %{a: 1, b: 2, c: %{d: 3, e: 4}, f: %{g: 5}}
      iex> rule = %{_spread: [[:f]], a: {"A", true}, b: "B", c: {:C, %{d: 6}}, f: %{g: "G"}}
      iex> Transmap.transform(map, rule)
      %{"A" => 1, "B" => 2, :C => %{6 => 3}, "G" => 5}
  """
  @spec transform(data :: any, rule :: any) :: any
  def transform(data, _) when not is_map(data), do: data
  def transform(data, true), do: data
  def transform(data, rule) when is_map(data) do
    {default, rule} = Map.pop(rule, @default_key, false)
    {spread, rule} = Map.pop(rule, @spread_key, [])
    {data, rule} = apply_spread(data, rule, spread)
    Enum.reduce(data, %{}, fn {key, value}, result ->
      {key, rule_value} = case Map.get(rule, key, default) do
        boolean when is_boolean(boolean) -> {key, boolean}
        {key, rule_value} -> {key, rule_value}
        rule_value when is_map(rule_value) -> {key, rule_value}
        new_key -> {new_key, true}
      end
      if rule_value != false do
        filtered = transform(value, rule_value)
        case filtered do
          map when is_map(map) and map_size(map) == 0 -> result
          other -> Map.put(result, key, other)
        end
      else
        result
      end
    end)
  end

  defp apply_spread(data, rule, spread) do
    Enum.reduce(spread, {data, rule}, fn keys, {data, rule} ->
      {data_map, data} = pop_in(data, keys)
      {rule_map, rule} = pop_in(rule, keys)
      if is_map(data_map) do
        data = Map.merge(data, data_map)
        rule = if is_map(rule_map) do
          Map.merge(rule, rule_map)
        else
          rule_map = for {key, _} <- data_map, into: %{}, do: {key, true}
          Map.merge(rule, rule_map)
        end
        {data, rule}
      else
        {data, rule}
      end
    end)
  end
end