WIP: [schema] Adding Schema module

lib/instream/schema.ex

@@ -0,0 +1,193 @@
+defmodule Instream.Schema do
+  @moduledoc """
+  Defines a schema (series struct).
+
+  ## Usage
+
+      defmodule MySchema
+        use Instream.Schema
+
+        schema do
+          database    "my_database_optional"
+          measurement "cpu_load"
+
+          tag :host, default: "www"
+          tag :core
+
+          field :value, default: 100
+          field :value_desc
+        end
+      end
+
+  ## Metadata
+
+  The metadata of a schema (i.e. the measurement) can
+  be retrieved using the `__meta__/1` method.
+
+  ## Struct
+
+  Every schema will be registered as a struct.
+  Following the above usage example you will get the following struct:
+
+      %MySchema{
+        value: 100,
+        value_desc: nil,
+        host: "www",
+        core: nil,
+        timestamp: nil
+      }
+
+  `:timestamp` is expected to be a unix nanosecond timestamp.
+  """
+
+  defmacro __using__(_opts) do
+    quote do
+      @after_compile unquote(__MODULE__)
+
+      import unquote(__MODULE__), only: [ schema: 1 ]
+    end
+  end
+
+  defmacro __after_compile__(%{ module: module }, _bytecode) do
+    quote do
+      Instream.Series.Validator.proper_series?(unquote(module))
+    end
+  end
+
+
+  @doc """
+  Defines the schema.
+  """
+  defmacro schema(do: block) do
+    quote do
+      @behaviour unquote(__MODULE__)
+
+      @database    nil
+      @measurement nil
+
+      Module.register_attribute(__MODULE__, :fields_raw, accumulate: true)
+      Module.register_attribute(__MODULE__, :tags_raw, accumulate: true)
+
+      try do
+        # scoped import
+        import unquote(__MODULE__)
+        unquote(block)
+      after
+        :ok
+      end
+
+      @fields_names  @fields_raw |> Keyword.keys() |> Enum.sort()
+      @fields_struct @fields_raw |> Enum.sort( &unquote(__MODULE__).__sort_fields__/2 )
+
+      @tags_names  @tags_raw |> Keyword.keys() |> Enum.sort()
+      @tags_struct @tags_raw |> Enum.sort( &unquote(__MODULE__).__sort_fields__/2 )
+
+      def __meta__(:database),    do: @database
+      def __meta__(:measurement), do: @measurement
+      def __meta__(:fields),      do: @fields_names
+      def __meta__(:tags),        do: @tags_names
+
+      Module.eval_quoted __ENV__, [
+        unquote(__MODULE__).__struct_fields__(@fields_struct),
+        unquote(__MODULE__).__struct_tags__(@tags_struct),
+        unquote(__MODULE__).__struct__(@fields_struct ++ @tags_struct),
+      ]
+    end
+  end
+
+
+  @doc """
+  Provides metadata access for a schema.
+
+  ## Available information
+
+  - `:database`    - the database where the schema is stored (optional)
+  - `:fields`      - the fields in the schema
+  - `:measurement` - the measurement of the schema
+  - `:tags`        - the available tags defining the schema
+  """
+  @callback __meta__(atom) :: any
+
+  @doc """
+  Defines the database for the schema.
+  """
+  defmacro database(name) do
+    quote do
+      unquote(__MODULE__).__attribute__(__MODULE__, :database, unquote(name))
+    end
+  end
+
+  @doc """
+  Defines a field in the schema.
+  """
+  defmacro field(name, opts \\ []) do
+    quote do
+      unquote(__MODULE__).__attribute__(
+        __MODULE__, :fields_raw,
+        { unquote(name), unquote(opts[:default]) }
+      )
+    end
+  end
+
+  @doc """
+  Defines the measurement of the schema.
+  """
+  defmacro measurement(name) do
+    quote do
+      unquote(__MODULE__).__attribute__(__MODULE__, :measurement, unquote(name))
+    end
+  end
+
+  @doc """
+  Defines a tag in the schema.
+  """
+  defmacro tag(name, opts \\ []) do
+    quote do
+      unquote(__MODULE__).__attribute__(
+        __MODULE__, :tags_raw,
+        { unquote(name), unquote(opts[:default]) }
+      )
+    end
+  end
+
+
+  @doc false
+  def __attribute__(mod, name, value) do
+    Module.put_attribute(mod, name, value)
+  end
+
+
+  @doc false
+  def __sort_fields__({ left, _ }, { right, _ }), do: left > right
+
+
+  @doc false
+  def __struct__(fields) do
+    quote do
+      defstruct unquote(Macro.escape(fields)) ++ [ timestamp: nil ]
+    end
+  end
+
+  @doc false
+  def __struct_fields__(fields) do
+    quote do
+      defmodule Fields do
+        @type t :: %__MODULE__{}
+
+        defstruct unquote(Macro.escape(fields))
+      end
+    end
+  end
+
+  @doc false
+  def __struct_tags__(tags) do
+    quote do
+      defmodule Tags do
+        @type t :: %__MODULE__{}
+
+        defstruct unquote(Macro.escape(tags))
+      end
+    end
+  end
+
+end