defmodule SimpleStruct do
  @moduledoc """
  `SimpleStruct` is a library to create structs in a concise way.

  This is _not_ meant as a replacement of Elixir's `defstruct`. In fact, it uses
  it. It's meant to be used as a way to make it easier to define several structs
  inside a module.

  And if your struct needs to "graduate" at some point, you can easily extract
  it and expand it.

  ## Example

  Take an example where you want to define a bunch of events in your
  application.

  Ideally, you want each event to be a struct. That is our first-class data
  structure in Elixir, and it allows things like safety when constructing it and
  better pattern matching.

  You have 2 options:

  1. You have an explosion of files, each with a simple `defmodule` +
  `defstruct` in it, OR
  2. You have a really large module with tons of nested modules!

  Both seem overkill for different reasons. So, maybe you take secret option (3)
  and you don't make them into structs! :(

  With `SimpleStruct`, it's simple.

  Just `import SimpleStruct` and define all the structs you want in one place:

  ```elixir
  defmodule MyApp.Events do
    import SimpleStruct

    defdata UserLoggedIn, [:user_id, action: "logged in"]
    defdata OrderPlaced, [:user_id, :order_id, required: true]
    defdata OrderCancelled, [:user_id, :order_id, required: [:order_id]]
  end
  ```
  """

  @doc """
  Creates a module and defines a struct inside it with then given fields.

  ## Options

  - `required`: defaults to empty. You can pass a list of keys that are required
  or `true` to require all keys.

  ## Examples

  ```elixir
  defmodule MyApp.Events do
    import SimpleStruct

    defdata UserLoggedIn, [:user_id, action: "logged in"]
    defdata OrderPlaced, [:user_id, :order_id, required: true]
    defdata OrderCancelled, [:user_id, :order_id, required: [:order_id]]
  end
  ```

  That will create three structs: `%Myapp.Events.UserLoggedIn{}`,
  `%Myapp.Events.OrderPlaced{}`, and `%Myapp.Events.OrderCancelled{}`.

  - `UserLoggedIn` will have a default `action` of `"logged in"`,
  - `OrderPlaced` will enforce that both `:user_id` and `:order_id` keys are
  provided when creating the struct, and
  - `OrderCancelled` will only enforce `:order_id`

  In addition, all modules will have a `new/1` helper function that can be used
  to create the structs by passing keyword lists.

  ```elixir
  UserLoggedIn.new(user_id: 1)
  # => %Myapp.Events.UserLoggedIn{user_id: 1, action: "logged in"}
  ```

  In other words, it's the equivalent of doing this:

  ```elixir
  defmodule MyApp.Events do
    defmodule UserLoggedIn do
      defstruct [:user_id, action: "logged in"]

      def new(attrs) do
        struct!(__MODULE__, attrs)
      end
    end

    defmodule OrderPlaced  do
      @enforce_keys [:user_id, :order_id]
      defstruct [:user_id, :order_id]

      def new(attrs) do
        struct!(__MODULE__, attrs)
      end
    end

    defmodule OrderCancelled do
      @enforce_keys [:order_id]
      defstruct [:user_id, :order_id]

      def new(attrs) do
        struct!(__MODULE__, attrs)
      end
    end
  end
  ```
  """
  defmacro defdata(name, fields, opts \\ []) do
    required_keys =
      case Keyword.get(opts, :required) do
        true -> fields
        keys when is_list(keys) -> keys
        nil -> []
      end

    quote do
      defmodule unquote(name) do
        @enforce_keys unquote(required_keys)
        defstruct unquote(fields)

        def new(opts \\ []) when is_list(opts) do
          struct!(unquote(name), opts)
        end
      end
    end
  end
end