global_variable

defmodule GVA do
  @moduledoc """
  Global VAriable utilities.

  All commands are prefixed with the letter `g` to prevent any clash with future `Kernel` functions.
  To use it in your script be sure to run at least Elixir `1.12` then import functions like the following:

      Mix.install([:gva])
      import GVA

  An ETS table is used underneath meaning you can use any funky tricks to manipulate it data, pattern match and so on.

  No variable table is created by default, you have to do it by yourself.

  ## Examples

      Mix.install([:gva])
      import GVA

      gnew :var
      gput :var, :answer, 42
      gget :var, :answer # 42
      gget_and_update :var, :answer, fn x -> 1 + x end # 43
  """

  @doc """
  Create a new table to hold your variables.
  A table can hold as much variables (and associated data) as your RAM allows you.
  """
  defmacro gnew(name) do
    quote do
      :ets.new(unquote(name), [:set, :protected, :named_table])
    end
  end

  @doc """
  Put a value in a variable table.
  """
  defmacro gput(table, key, value) do
    quote do
      :ets.insert(unquote(table), {unquote(key), unquote(value)})
    end
  end

  @doc """
  Get an variable's value from a table by it's key.
  """
  defmacro gget(table, key) do
    quote do
      [{_key, value}] = :ets.lookup(unquote(table), unquote(key))
      value
    end
  end

  @doc """
  Get and update a variable from a table by a function.
  Operation is probably not atomic and won't save you from race condition.
  (are you sure to write a script if you have these requirements?)

  `function` must be a function with an arity of 1.
  """
  defmacro gget_and_update(table, key, function) do
    quote do
      unquote(table)
      |> gget(unquote(key))
      |> then(unquote(function))
      |> then(&gput(unquote(table), unquote(key), &1))
    end
  end
end