defmodule HoneylixirTracing do
  @moduledoc """
  Used to trace units of work and send the information to Honeycomb.

  ## Installation

  Adding the library to your mix.exs as a dependency should suffice:

  ```
  def deps() do
    [
      {:honeylixir_tracing, "~> 0.2.0"}
    ]
  end
  ```

  This is the main entrypoint for this package, used to trace units of work by
  wrapping them in a function and report the duration as well as its relation
  to parent work.

  ## Configuration

  Most of the configuration for this package depends on configuration set for
  the `Honeylixir` project, the underlying library used for sending the data. The
  absolute minimum configuration required is to set the `team_writekey` and `dataset`
  fields:

  ```
  config :honeylixir,
    dataset: "your-dataset-name",
    team_writekey: "your-writekey"
  ```

  ## Usage

  Basic usage is to wrap any unit of work in a `HoneylixirTracing.span` call. Let's
  say you had the following module in your application already:

  ```
  defmodule TestModule do
    def cool_work(arg1, arg2) do
      arg1 + arg2
    end
  end
  ```

  If you wanted to trace this function, you could do:

  ```
  defmodule TestModule do
    def cool_work(arg1, arg2) do
      span("TestModule.cool_work/2", %{"arg1" => arg1, "arg2" => arg2}, fn ->
        arg1 + arg2
      end)
    end
  end
  ```

  Another option is to wrap the business work in a private function and invoke that
  in the span function:

  ```
  defmodule TestModule do
    def cool_work(arg1, arg2) do
      span("TestModule.cool_work/2", %{"arg1" => arg1, "arg2" => arg2}, fn ->
        do_cool_work(arg1, arg2)
      end)
    end

    defp do_cool_work(arg1, arg) do
      arg1 + arg2
    end
  end
  ```

  In both cases, the return value remains the same. The result of any `span` (`span/2`, `span/3`, `span/4`) calls
  is the result of whatever function is passed in as the work.

  ### Cross-Process traces

  Given this is Elixir running on Erlang, it's quite possible a GenServer or some other
  Process-based design will appear in your system. If this is the case, there are a couple of
  rough recommendations on how to ensure predictable tracing data:

  * For synchronous work, add a final argument of `ctx`, which is a `t:HoneylixirTracing.Propagation.t/0`
    struct, to the callback. This should not be *accepted* by the Client API but instead
    built for the user directly and passed to the Server. In the callback, use that as the
    first argument to a `HoneylixirTracing.span/4` call which wraps your work.
  * For asynchronous work, do *not* pass a context in and start a span from it.
    Asynchronous work is akin to background work done by a web application, meaning that
    one would consider them linked spans rather than child spans. You can use the
    underlying `Honeylixir` library to send these events along. Utility functions
    may be provided in the future to help with this.

  A small example for doing this within an application for synchronous work can be
  found in the `cross_process_example` project in the `examples` directory.

  ### Adding data to the current span

  If you want to add fields to your spans after initialization or invocation, you can
  use `add_field_data/1` to add data. `add_field_data/1` accepts a Map of strings
  to any encodable entity (just like `span/2` and the underlying `Honeylixir.Event`)
  and modifies the currently active span with the information. If no span is active,
  this function does nothing.

  ```
  defmodule TestModule do
    def some_work() do
      span("TestModule.some_work/0", fn ->
        result = CoolModule.do_something_else()

        HoneylixirTracing.add_field_data(%{"cool_mod.result" => result})
      end)
    end
  end
  ```
  """

  alias Honeylixir.Event
  alias HoneylixirTracing.Span

  @typedoc """
  Create and send a span to Honeycomb.
  """
  @type span_return :: any()

  @typedoc """
  A 0 arity function used as the work to be measured by the span.
  """
  @type work_function :: (() -> any())

  @doc """
  Create and send a span to Honeycomb.
  """
  @spec span(String.t(), work_function()) :: span_return()
  def span(span_name, work) when is_binary(span_name) and is_function(work, 0),
    do: span(span_name, %{}, work)

  @doc """
  Create and send a span to Honeycomb by propogating tracing context.

  Accepts a `t:HoneylixirTracing.Propagation.t/0` for continuing work from another Process's trace.
  """
  @spec span(
          HoneylixirTracing.Propagation.t(),
          String.t(),
          Honeylixir.Event.fields_map(),
          work_function()
        ) :: span_return()
  def span(%HoneylixirTracing.Propagation{} = propagation, span_name, %{} = fields, work)
      when is_binary(span_name) and is_function(work, 0) do
    Span.setup(propagation, span_name, fields)
    |> do_span(work)
  end

  @doc """
  Create and send a span to Honeycomb by optionally propogating tracing context.

  This form, `span/3`, has two possible calling signatures: the first is a non-propogated
  span with initial fields; the second accepts a propogated trace but no initial fields.
  """
  @spec span(HoneylixirTracing.Propagation.t(), String.t(), work_function()) :: span_return()
  @spec span(String.t(), Honeylixir.Event.fields_map(), work_function()) :: span_return()
  def span(propagation_or_name, name_or_fields, work)

  def span(span_name, %{} = fields, work) when is_binary(span_name) and is_function(work, 0) do
    Span.setup(span_name, fields) |> do_span(work)
  end

  def span(%HoneylixirTracing.Propagation{} = prop, span_name, work)
      when is_binary(span_name) and is_function(work, 0) do
    Span.setup(prop, span_name, %{})
    |> do_span(work)
  end

  defp do_span(%HoneylixirTracing.Span{} = span, work) do
    HoneylixirTracing.Context.set_current_span(span)

    try do
      work.()
    after
      latest_span = HoneylixirTracing.Context.clear_current_span()

      # Account for something going wrong and the current span being missing
      # somehow. We'll assume horrible things happened and not try to send a
      # possibly broken and outdated span from above.
      if latest_span do
        latest_span
        |> Span.prepare_to_send()
        |> Event.send()
      end
    end
  end

  @doc """
  Adds field data to the current span.

  This function does nothing if there is no currently active span. Any duplicate field
  names will have their contents replaced. Returns an `:ok` if a span was updated
  successfully, `nil` otherwise.
  """
  @spec add_field_data(Honeylixir.Event.fields_map()) :: :ok | nil
  def add_field_data(fields) when is_map(fields) do
    if current_span = HoneylixirTracing.Context.current_span() do
      HoneylixirTracing.Span.add_field_data(current_span, fields)
      |> HoneylixirTracing.Context.set_current_span()
    end
  end

  @doc """
  Provides a `t:Honeylixir.Propagation.t/0` for sharing tracing data between processes.

  If there is no span currently active, this will return `nil`.
  """
  @spec current_propagation_context() :: HoneylixirTracing.Propagation.t() | nil
  def current_propagation_context() do
    HoneylixirTracing.Context.current_span()
    |> HoneylixirTracing.Propagation.from_span()
  end
end