defmodule Meter.Utils do
  @moduledoc """
  A module with utility functions to generate parameters to send to Google Analytycs.

  The function param_generator/6  is the default function in Meter.track/2 and Meter.track_error/3
  """

  @doc """
  Generate a list of 2-element tuple, the first element is the name of the parameter (cd1, cd2, cd ...) and the second is the value.

  This function is used in to the default param_generator function providing the values from the application configuration. It could be used in a custom param_generator. It maintain the order as defined in the configuration.

```
  iex> Meter.Utils.custom_dimensions([], [key: "value"])
  []

  iex> Meter.Utils.custom_dimensions([:key], [])
  []

  iex> Meter.Utils.custom_dimensions([:key], [key: "value"])
  [{"cd1", "value"}]

  iex> Meter.Utils.custom_dimensions([:key, :key2], [key: "value"])
  [{"cd1", "value"}, {"cd2", nil}]

  iex> Meter.Utils.custom_dimensions([:key, :key2], [key: "value", key3: 12])
  [{"cd1", "value"}, {"cd2", nil}]

```
  """
  @spec custom_dimensions(list(atom), Keyword.t) :: Keyword.t
  def custom_dimensions([], _), do: []
  def custom_dimensions(_, []), do: []
  def custom_dimensions(dimensions, kwargs) do
    1..length(dimensions)
    |> Enum.map(fn i -> "cd#{i}" end)
    |> Enum.zip(dimensions)
    |> Enum.map(fn {cdi,argname} -> {cdi, kwargs[argname]} end)
  end

  @doc """
  Default param_generator function. Generate all parameters necessary to send a tracking request to google analytics

  ```
  iex>Meter.Utils.param_generator(:fn, [arg1: 3, arg2: 5], "UA-123", [cid: :arg2], [:arg2, :arg1])
  [v: 1,
  tid: "UA-123",
  cid: 5,
  ds: "server",
  t: "pageview",
  dt: "fn",
  dp: "/fn"] ++
  [{"cd1", 5},{"cd2", 3}]

  iex>Meter.Utils.param_generator(:fn, [arg1: 3, arg2: 5], "UA-123", [cid: :arg2], [:arg2, :arg1], %RuntimeError{message: "an error"})
  [v: 1,
  tid: "UA-123",
  cid: 5,
  ds: "server",
  t: "pageview",
  dt: "fn",
  dp: "/fn"] ++
  [{"cd1", 5},{"cd2", 3}] ++ [exf: 1, exd: "%RuntimeError{message: \\"an error\\"}"]

  ```
  """
  def param_generator(function_name, kwargs, tid, mapping, custom_dimensions, error \\ nil) do
    [v: 1,
     tid: tid,
     cid: kwargs[mapping[:cid]] ,
     ds:  mapping[:ds] || "server",
     t:   mapping[:t]  || "pageview",
     dt: "#{function_name}",
     dp: "/#{function_name}"
    ]
    ++ custom_dimensions(custom_dimensions, kwargs)
    ++ error_params(error)
  end

  @doc """
  Default param_generator function. Generate all parameters necessary to send a tracking request to google analytics

  ```
  iex>Meter.Utils.error_params(%RuntimeError{message: "an error"})
  [exf: 1, exd: "%RuntimeError{message: \\"an error\\"}"]

  iex>Meter.Utils.error_params(nil)
  []

  """
  @spec error_params(map) :: Keyword.t
  def error_params(nil), do: []
  def error_params(error), do: [exf: 1, exd: inspect(error)]

  defp __param_generator(function_name, kwargs, tid, mapping, custom_dimensions) do
    [v: 1,
     tid: tid,
     cid: kwargs[mapping[:cid]] ,
     ds:  mapping[:ds] || "server",
     t:   mapping[:t]  || "pageview",
     dt: "#{function_name}",
     dp: "/#{function_name}"
    ] ++
      custom_dimensions(custom_dimensions, kwargs)
  end
end