defmodule Mix.Tasks.Compile.ModuleIndexer do
  @moduledoc """
  Custom Mix compiler that generates NFTables.ExprIndex module.

  This compiler runs after the standard Elixir compiler and:
  1. Queries all compiled modules in the :nftables application
  2. Filters for modules matching NFTables.Expr.* prefix
  3. Generates and compiles NFTables.ExprIndex with all() function

  The generated module is used by the `use NFTables` macro to
  dynamically import all expression building modules.
  """

  use Mix.Task.Compiler

  @recursive true
  @manifest ".compile.module_indexer"

  @doc """
  Runs the module indexer compiler.

  Returns:
  - `{:ok, []}` on success with changes
  - `{:noop, []}` when no changes detected
  """
  def run(_args) do
    # Discover all NFTables.Expr.* modules
    expr_modules = discover_expr_modules()

    # Check if module list changed (compare hash)
    if modules_changed?(expr_modules) do
      # Generate and compile ExprIndex module
      generate_expr_index(expr_modules)

      # Write manifest with hash
      write_manifest(expr_modules)

      Mix.shell().info("Generated NFTables.ExprIndex with #{length(expr_modules)} modules")
      {:ok, []}
    else
      # No changes, skip regeneration
      {:noop, []}
    end
  end

  @doc """
  Cleans up generated files.
  """
  def clean do
    File.rm(@manifest)
    File.rm("lib/nftables_expr_index.ex")
    :ok
  end

  # Private functions

  defp discover_expr_modules do
    # Read modules from the .app file generated by elixir compiler
    app_file = Mix.Project.app_path() <> "/ebin/#{Mix.Project.config()[:app]}.app"

    if File.exists?(app_file) do
      # Read and parse the .app file
      {:ok, [{:application, _app, properties}]} = :file.consult(String.to_charlist(app_file))

      # Extract modules list
      modules = Keyword.get(properties, :modules, [])

      # Filter for NFTables.Expr.* modules
      modules
      |> Enum.filter(&expr_module?/1)
      |> Enum.sort()
    else
      # .app file doesn't exist yet (very first compile)
      []
    end
  end

  defp expr_module?(module) do
    module_string = to_string(module)
    # Match NFTables.Expr.* but exclude NFTables.Expr itself and internal modules
    String.starts_with?(module_string, "Elixir.NFTables.Expr.") and
      module_string != "Elixir.NFTables.Expr" and
      not String.ends_with?(module_string, ".Structs")
  end

  defp modules_changed?(current_modules) do
    current_hash = compute_hash(current_modules)

    case read_manifest() do
      {:ok, previous_hash} ->
        current_hash != previous_hash

      :error ->
        # No manifest exists, first run
        true
    end
  end

  defp compute_hash(modules) do
    :erlang.phash2(modules)
  end

  defp generate_expr_index(modules) do
    # Generate module code as a string
    module_code = """
    defmodule NFTables.ExprIndex do
      @moduledoc \"\"\"
      Auto-generated module index for NFTables.Expr modules.

      This module is generated at compile time by Mix.Tasks.Compile.ModuleIndexer.
      DO NOT EDIT - regenerated on each compilation when module list changes.

      Contains #{length(modules)} modules.
      \"\"\"

      @doc \"\"\"
      Returns list of all NFTables.Expr submodules.
      \"\"\"
      def all do
        #{inspect(modules)}
      end
    end
    """

    # Write to file
    output_path = "lib/nftables_expr_index.ex"
    File.write!(output_path, module_code)

    :ok
  end

  defp write_manifest(modules) do
    hash = compute_hash(modules)
    content = :erlang.term_to_binary(hash)
    File.write!(@manifest, content)
  end

  defp read_manifest do
    if File.exists?(@manifest) do
      content = File.read!(@manifest)
      hash = :erlang.binary_to_term(content)
      {:ok, hash}
    else
      :error
    end
  end
end