defmodule Defrag do
  @moduledoc """
  Provides a convenient macro for creating Ecto fragments with interpolation syntax.

  This module offers the `defrag/1` macro that transforms string interpolation syntax
  into Ecto fragment queries, making them more readable and maintainable.
  """

  @pattern ~r/\#\{([^\}]+)\}/

  @doc """
  Converts string interpolation syntax into an Ecto fragment.

  This macro allows you to write Ecto fragments using familiar string interpolation
  syntax (`\#{...}`). It automatically converts the interpolation into proper
  Ecto fragment parameters.

  ## Examples

      import Defrag

      # Instead of writing:
      fragment("date_part('year', ?)", some_date)

      # You can write:
      defrag("date_part('year', \#{some_date})")

      # Multiple interpolations are supported:
      defrag("BETWEEN \#{start_date} AND \#{end_date}")
      # Becomes: fragment("BETWEEN ? AND ?", start_date, end_date)

  ## Usage in Ecto Queries

      import Ecto.Query
      import Defrag

      def users_born_in_year(query, year) do
        from u in query,
        where: defrag("date_part('year', \#{u.birth_date}) = \#{^year}")
      end

  ## Notes

  - The macro converts each interpolation (`\#{...}`) into a parameter placeholder (`?`)
  - Variables used in interpolation must be in scope where the macro is called
  - The resulting fragment is safe against SQL injection as it uses Ecto's parameterization
  """
  defmacro defrag(ast) do
    string = Macro.to_string(ast)
    fragment_string = Regex.replace(@pattern, string, "?")
    vars = Enum.map(Regex.scan(@pattern, string), fn [_, var] -> var end)

    Code.string_to_quoted!("fragment(#{Enum.join([fragment_string | vars], ", ")})")
  end
end