defmodule Mix.Tasks.NebulaGraphEx.Gen.Graph do
  @shortdoc "Generates a NebulaGraphEx graph module for your application"

  @moduledoc """
  The recommended way to set up `nebula_graph_ex` in your application.

  Generates a `NebulaGraphEx.Graph` module scoped to your OTP app, adds a
  default config block to `config/config.exs`, and registers the module in
  your application's supervision tree.

  ## Usage

      mix nebula_graph_ex.gen.graph MyApp.Graph

  The module name becomes both the Elixir module and the registered connection
  pool name. Running the generator:

  1. Creates `lib/my_app/graph.ex` — the graph module
  2. Inserts a default config block into `config/config.exs`
  3. Injects the module as the first child in `lib/my_app/application.ex`
  4. Prints a `config/runtime.exs` snippet for secrets

  All three file modifications are idempotent — re-running the generator on an
  already-configured app skips each step that is already in place.

  The file path for the graph module is derived automatically from the module
  name. Use `--path` to override it if your project layout differs from the
  standard convention:

      mix nebula_graph_ex.gen.graph MyApp.Graph --path lib/my_app/connections/graph.ex

  ## Multiple connections

  Run the generator once per graph module:

      mix nebula_graph_ex.gen.graph MyApp.PrimaryGraph
      mix nebula_graph_ex.gen.graph MyApp.AnalyticsGraph

  Each module manages its own connection pool and reads its config from
  `Application.get_env(:my_app, MyApp.PrimaryGraph)` independently.
  """

  use Mix.Task

  @switches [path: :string]

  @impl Mix.Task
  def run(args) do
    {opts, positional, _invalid} = OptionParser.parse(args, strict: @switches)

    module_name =
      case positional do
        [name | _] ->
          name

        [] ->
          Mix.raise("Expected a module name. Usage: mix nebula_graph_ex.gen.graph MyApp.Graph")
      end

    otp_app = Mix.Project.config()[:app]
    file_path = Keyword.get_lazy(opts, :path, fn -> module_to_path(module_name) end)

    generate_module(module_name, otp_app, file_path)
    inject_config(module_name, otp_app)
    inject_supervision(module_name, otp_app)
    print_runtime_instructions(module_name, otp_app)
  end

  # ─── File path derivation ──────────────────────────────────────────────────

  defp module_to_path(module_name) do
    parts =
      module_name
      |> String.split(".")
      |> Enum.map(&Macro.underscore/1)

    "lib/#{Enum.join(parts, "/")}.ex"
  end

  # ─── Graph module ──────────────────────────────────────────────────────────

  defp generate_module(module_name, otp_app, file_path) do
    dir = Path.dirname(file_path)

    unless File.exists?(dir) do
      File.mkdir_p!(dir)
      Mix.shell().info([:green, "* creating ", :reset, dir <> "/"])
    end

    if File.exists?(file_path) do
      Mix.shell().info([:yellow, "* skipping ", :reset, file_path, " (already exists)"])
    else
      File.write!(file_path, module_content(module_name, otp_app))
      Mix.shell().info([:green, "* creating ", :reset, file_path])
    end
  end

  defp module_content(module_name, otp_app) do
    """
    defmodule #{module_name} do
      use NebulaGraphEx.Graph, otp_app: :#{otp_app}
    end
    """
  end

  # ─── config/config.exs ────────────────────────────────────────────────────

  defp inject_config(module_name, otp_app) do
    path = "config/config.exs"
    marker = "config :#{otp_app}, #{module_name}"

    cond do
      not File.exists?(path) ->
        Mix.shell().info([:yellow, "* skipping ", :reset, path, " (file not found)"])

      File.read!(path) |> String.contains?(marker) ->
        Mix.shell().info([:yellow, "* skipping ", :reset, path, " (config already present)"])

      true ->
        snippet = """

        config :#{otp_app}, #{module_name},
          hostname: "localhost",
          port: 9669,
          username: "root",
          pool_size: 10
        """

        updated = insert_config_snippet(File.read!(path), snippet)
        File.write!(path, updated)
        Mix.shell().info([:green, "* updating ", :reset, path])
    end
  end

  defp insert_config_snippet(content, snippet) do
    case String.split(content, "\n", trim: false) do
      [] ->
        String.trim_trailing(snippet) <> "\n"

      lines ->
        case import_config_index(lines) do
          nil ->
            String.trim_trailing(content) <> "\n" <> String.trim_trailing(snippet) <> "\n"

          index ->
            insert_at = comment_block_start(lines, index)

            {head, tail} = Enum.split(lines, insert_at)

            Enum.join(
              head ++ [String.trim_trailing(snippet), ""] ++ tail,
              "\n"
            )
        end
    end
  end

  defp import_config_index(lines) do
    Enum.find_index(lines, &String.match?(&1, ~r/^\s*import_config\s+.+$/))
  end

  defp comment_block_start(lines, import_index) do
    lines
    |> Enum.take(import_index)
    |> Enum.with_index()
    |> Enum.reverse()
    |> Enum.reduce_while(import_index, fn {line, index}, acc ->
      cond do
        String.trim(line) == "" ->
          {:cont, index}

        String.match?(line, ~r/^\s*#/) ->
          {:cont, index}

        true ->
          {:halt, acc}
      end
    end)
  end

  # ─── lib/<otp_app>/application.ex ─────────────────────────────────────────

  defp inject_supervision(module_name, otp_app) do
    path = "lib/#{otp_app}/application.ex"

    cond do
      not File.exists?(path) ->
        Mix.shell().info([:yellow, "* skipping ", :reset, path, " (file not found)"])

      File.read!(path) |> String.contains?(module_name) ->
        Mix.shell().info([
          :yellow,
          "* skipping ",
          :reset,
          path,
          " (#{module_name} already present)"
        ])

      true ->
        content = File.read!(path)

        case insert_child(content, module_name) do
          {:ok, updated} ->
            File.write!(path, updated)
            Mix.shell().info([:green, "* updating ", :reset, path])

          :error ->
            Mix.shell().info([
              :yellow,
              "* skipping ",
              :reset,
              path,
              " (could not locate children list — add #{module_name} manually)"
            ])
        end
    end
  end

  # Injects module_name as the first entry in the `children = [...]` list.
  # Handles both inline (`children = []`) and multiline (`children = [\n`) forms.
  defp insert_child(content, module_name) do
    lines = String.split(content, "\n")

    {lines_out, done} =
      Enum.flat_map_reduce(lines, false, fn line, done ->
        if done do
          {[line], true}
        else
          case Regex.run(~r/^(\s*).*children\s*=\s*\[/, line) do
            [_, indent] ->
              child_indent = indent <> "  "

              if String.contains?(line, "]") do
                # Inline empty list: children = []
                new_line =
                  String.replace(line, "[]", "[\n#{child_indent}#{module_name}\n#{indent}]")

                {[new_line], true}
              else
                # Multiline: children = [
                {[line, "#{child_indent}#{module_name},"], true}
              end

            nil ->
              {[line], false}
          end
        end
      end)

    if done, do: {:ok, Enum.join(lines_out, "\n")}, else: :error
  end

  # ─── Runtime secrets reminder ──────────────────────────────────────────────

  defp print_runtime_instructions(module_name, otp_app) do
    Mix.shell().info("""

    To configure secrets at runtime, add the following to config/runtime.exs:

        import Config

        config :#{otp_app}, #{module_name},
          hostname: System.get_env("NEBULA_HOST", "localhost"),
          password: fn -> System.fetch_env!("NEBULA_PASS") end

    Passing :password as a function ensures it is never captured in compiled
    modules or logged in crash reports.
    """)
  end
end