defmodule Readmix do
  alias Readmix.BlockSpec
  alias Readmix.Context

  @moduledoc """
  Readmix is a tool for generating and maintaining documentation with dynamic
  content.

  It allows you to embed special tags in your markdown or other text files that
  will be processed and replaced with generated content.

  ## Basic Usage

  ```elixir
  # Create a new Readmix instance
  rdmx = Readmix.new([])

  # Update a file containing Readmix blocks
  Readmix.update_file(rdmx, "README.md")
  ```

  ## Block Format

  Readmix blocks are defined in HTML comments with a special `rdmx` prefix:

  ```
  <!-- rdmx my_namespace:my_action param1:"value1" param2:$my_var -->
  This content will be replaced by the generator.
  <!-- rdmx my_namespace:my_action -->
  ```

  ## Configuration

  You can configure Readmix with custom generators, variables, and context
  modules:

  ```elixir
  Readmix.new(
    generators: %{my_namespace: MyGeneratorModule},
    vars: %{my_var: "hello"},
    contexts: [MyContext, Readmix.Contexts.Defaults]
  )
  ```
  """

  defstruct [:resolver, :vars, :backup_fun]

  @type t :: %__MODULE__{
          resolver: function(),
          backup_fun: function(),
          vars: %{optional(atom) => term}
        }

  def new(opts) do
    %__MODULE__{
      resolver: opt_resolver(opts),
      vars: opt_vars(opts),
      backup_fun: opt_backup(opts)
    }
  end

  @actions_schema NimbleOptions.new!(
                    *: [
                      type: :keyword_list,
                      keys: [
                        as: [type: :atom],
                        params: [
                          type: :keyword_list,
                          required: true
                          # TODO await https://github.com/dashbitco/nimble_options/issues/141
                          # keys: NimbleOptions.options_schema()
                        ],
                        doc: [type: :string]
                      ]
                    ]
                  )

  defp opt_resolver(opts) do
    generators =
      case opts[:generators] do
        nil ->
          Readmix.default_generators()

        mods ->
          mods
          |> Map.new()
          |> Map.put_new(:rdmx, Readmix.Generators.BuiltIn)
      end

    Map.new(generators, fn {ns, mod} ->
      actions =
        mod.actions()
        |> NimbleOptions.validate!(@actions_schema)
        |> Map.new(fn {key, spec} ->
          params = NimbleOptions.new!(Keyword.fetch!(spec, :params))
          {key, params}
        end)

      {ns, {mod, actions}}
    end)
  end

  defp opt_vars(opts) do
    external_vars =
      case opts[:vars] do
        vars when is_map(vars) -> vars
        nil -> %{}
        other -> raise "invalid option :vars, expected a map, got: #{inspect(other)}"
      end

    contexts =
      case opts[:contexts] do
        nil -> Readmix.default_contexts()
        list when is_list(list) -> list
      end

    # Modules defined first in the list have precedence, subsequent modules do
    # not overwrite their vars. The external vars have precedence over all
    # modules.
    Enum.reduce(contexts, external_vars, fn mod, acc ->
      case mod.get_vars() do
        map when is_map(map) ->
          Map.merge(mod.get_vars(), acc)

        other ->
          raise "invalid return value from #{inspect(mod)}.get_vars/0, expected a map, got: #{inspect(other)}"
      end
    end)
  end

  defp opt_backup(opts) do
    case opts[:backup?] do
      true ->
        backup_root_dir =
          case opts[:backup_dir] do
            nil -> raise ArgumentError, "option :backup_dir is required when :backup? is true"
            dir -> dir
          end

        call_time =
          case opts[:backup_datetime] do
            %DateTime{} = dt -> dt
            nil -> DateTime.utc_now()
          end

        make_backup_callback(backup_root_dir, call_time)

      _ ->
        _no_backup = fn _, _ -> :ok end
    end
  end

  defp make_backup_callback(backup_root_dir, call_time) do
    stamp = Calendar.strftime(call_time, "%x-%H-%M-%S-%f")
    backup_dir = Path.join(backup_root_dir, "readmix-backup-#{stamp}")
    File.mkdir_p!(backup_dir)

    fn orginal_path, content ->
      sub_path =
        case orginal_path do
          "/" <> sub_abs_path -> sub_abs_path
          rel_path -> rel_path
        end

      target_path = Path.join(backup_dir, sub_path)

      if File.exists?(target_path) do
        raise "cannot backup #{orginal_path} to #{target_path}, file exists"
      end

      with :ok <- File.mkdir_p!(Path.dirname(target_path)),
           :ok <- File.write(target_path, content) do
        CliMate.CLI.writeln("Wrote backup of #{orginal_path} in #{target_path}")
        :ok
      else
        {:error, _} = err -> err
      end
    end
  end

  @default_generators %{rdmx: Readmix.Generators.BuiltIn}

  @doc """
  Returns the mapping from namespace to generator used by default by Readmix if
  no `:resolver` or `:generators` option is given to `Readmix.new/1`.

  Those defaults are pulled with `Application.fetch_env(:readmix, :generators)`,
  which is a configuration value that you can define in `config/dev.exs` or
  `config/runtime.exs`.

  If not configuration is defined, the default value is
  `#{inspect(@default_generators)}`.
  """
  def default_generators do
    case Application.fetch_env(:readmix, :generators) do
      {:ok, generators} when is_map(generators) -> generators
      :error -> @default_generators
    end
  end

  @default_contexts [Readmix.Contexts.Defaults]

  @doc """
  Returns the list of modules used to define the context of execution. For now
  this only defines which variables are available for usage in blocks arguments.

  Those defaults are pulled with `Application.fetch_env(:readmix, :contexts)`,
  which is a configuration value that you can define in `config/dev.exs` or
  `config/runtime.exs`.

  If not configuration is defined, the default value is
  `#{inspect(@default_contexts)}`.
  """
  def default_contexts do
    case Application.fetch_env(:readmix, :contexts) do
      {:ok, contexts} when is_list(contexts) -> contexts
      :error -> @default_contexts
    end
  end

  def update_file(rdmx, path) do
    with {:ok, content} <- read_source(path),
         {:ok, iodata} <- transform_string(rdmx, content, source_path: path),
         :ok <- call_backup(rdmx, path, content) do
      File.write(path, iodata)
    else
      {:error, reason} -> {:error, convert_error(reason, path)}
    end
  end

  defp read_source(path) do
    case File.read(path) do
      {:ok, content} -> {:ok, content}
      {:error, reason} -> {:error, {:file_error, path, reason}}
    end
  end

  defp call_backup(rdmx, path, content) do
    rdmx.backup_fun.(path, content)
  end

  def transform_string(rdmx, string, opts \\ [])

  def transform_string(rdmx, string, opts) do
    with {:ok, iodata} <- transform_string_to_iodata(rdmx, string, opts) do
      {:ok, IO.iodata_to_binary(iodata)}
    end
  end

  def transform_string_to_iodata(rdmx, string, opts \\ [])

  def transform_string_to_iodata(rdmx, string, opts) when is_binary(string) do
    with {:ok, blocks} <- parse_string(string, opts[:source_path]) do
      blocks_to_iodata(rdmx, blocks)
    end
  end

  def parse_string(string, source_path \\ nil)

  def parse_string(string, nil) do
    parse_string(string, "nofile")
  end

  def parse_string(string, source_path) do
    Readmix.Parser.parse_string(string, source_path)
  end

  def blocks_to_iodata(rdmx, blocks) do
    result =
      Enum.reduce_while(blocks, {:ok, []}, fn block, {:ok, acc} ->
        case block_to_iodata(rdmx, block) do
          {:ok, new_block} -> {:cont, {:ok, [new_block | acc]}}
          {:error, _} = err -> {:halt, err}
        end
      end)

    case result do
      {:ok, rev} -> {:ok, :lists.reverse(rev)}
      {:error, _} = err -> err
    end
  end

  defp block_to_iodata(_rdmx, {:text, iodata}), do: {:ok, iodata}

  defp block_to_iodata(rdmx, {:generated, block}) do
    %{
      generator: {ns, action, params},
      raw_header: raw_header,
      raw_footer: raw_footer,
      content: sub_blocks
    } = block

    # We do not use the :as param of an action. This is for the macro. We will
    # always call the generate/3 callback.
    try do
      with {:ok, {mod, params}} <- resolve_call(rdmx, ns, action, params),
           {:ok, iodata} <- call_transformer(rdmx, mod, action, params, sub_blocks) do
        {:ok, [raw_header, iodata, raw_footer]}
      else
        {:error, reason} -> {:error, convert_error(reason, block)}
      end
    catch
      :throw, {:undef_var, _} = e -> {:error, convert_error(e, block)}
    end
  end

  defp resolve_call(rdmx, ns, action, params_in) do
    with {:ok, {mod, actions}} <- resolve_mod(rdmx, ns),
         errctx = {ns, mod, action},
         {:ok, params_schema} <- resolve_fun(actions, action, errctx),
         {:ok, params} <- swap_variables(rdmx, params_in),
         {:ok, params} <- validate_params(params, params_schema, errctx) do
      {:ok, {mod, params}}
    end
  end

  defp resolve_mod(rdmx, ns) do
    case Map.fetch(rdmx.resolver, ns) do
      {:ok, {mod, actions}} when is_atom(mod) ->
        {:ok, {mod, actions}}

      {:ok, other} ->
        raise "invalid mapped generator: #{inspect(other)}"

      :error ->
        {:error, {:unresolved_generator, ns}}
    end
  end

  defp resolve_fun(actions, action, errctx) do
    case Map.fetch(actions, action) do
      {:ok, %NimbleOptions{}} = found ->
        found

      :error ->
        {:error, {:unknown_action, {action, errctx}}}
    end
  end

  defp swap_variables(rdmx, params) do
    %{vars: vars} = rdmx

    swapped =
      Enum.map(params, fn
        {k, {:var, var}} -> {k, Readmix.Generator.expect_variable(vars, var)}
        {k, v} -> {k, v}
      end)

    {:ok, swapped}
  end

  defp validate_params(params, params_schema, errctx) do
    case NimbleOptions.validate(params, params_schema) do
      {:ok, _} = fine ->
        fine

      {:error, validation_error} ->
        {:error, {:params_validation_error, {validation_error, errctx}}}
    end
  end

  # {:ok, mod} -> {:ok, mod}
  # :error when ns == :rdmx -> {:ok, Readmix.Generators.BuiltIn}
  # :error -> {:error, {:unresolved_generator, ns}}
  # other -> raise "invalid resolver return value: #{inspect(other)}"
  defp call_transformer(rdmx, mod, action, params, previous_content) do
    context = %Context{previous_content: previous_content, readmix: rdmx}

    case mod.generate(action, params, context) do
      {:ok, iodata} -> {:ok, iodata}
      {:error, reason} -> {:error, {:generator_error, {mod, action, params, reason}}}
      other -> {:error, {:invalid_generator_return, {mod, action, params, other}}}
    end
  end

  def format_error(%Readmix.Parser.ParseError{} = e) do
    Readmix.Parser.ParseError.message(e)
  end

  def format_error(%Readmix.Error{} = e) do
    Readmix.Error.message(e)
  end

  defp convert_error(reason, %BlockSpec{} = block) do
    convert_error(reason, block.file, block.loc)
  end

  defp convert_error(reason, path) when is_binary(path) do
    convert_error(reason, path, nil)
  end

  defp convert_error(%Readmix.Error{} = e, _path, _loc) do
    e
  end

  defp convert_error(%Readmix.Parser.ParseError{} = e, _path, _loc) do
    e
  end

  defp convert_error(reason, path, loc) do
    Readmix.Error.convert(reason, path, loc)
  end
end