# frozen_string_literal: true

require "singleton"
require "json"

module PlainApm
  class Agent
    include Singleton

    def self.collect(event)
      instance.collect(event)
    end

    def self.start
      instance.start
    end

    def collect(event)
      return unless @config.enabled

      ##
      # Context contains the trace ID (which comes from either
      # HTTP_X_REQUEST_ID header, the deserialized job,
      # or is generated by the trace_id middleware).
      # It can also carry user inserted app data.
      if defined?(PlainApm::Extensions::Context)
        event.merge!(PlainApm::Extensions::Context.current)
      end

      event.merge!(
        "version" => PlainApm::VERSION,
        "collected_at" => Time.now.utc.to_f,
        "pid" => Process.pid,
        "thread_id" => Thread.current.object_id.to_s,
      )

      @events << event 
    end

    def start
      return unless @publisher.nil?
      return unless @config.enabled

      # TODO: sized queue w/ a timeout.
      @events = Thread::Queue.new

      # TODO: Multiple threads
      @publisher = Thread.new { publisher_loop }

      install_hooks

      # TODO: add a cleaner shutdown.
      at_exit { shutdown }
    end

    private

    def initialize
      # TODO: validate config
      @config = Config.new

      super
    end

    def install_hooks
      [
        Hooks::Deploy,
        Hooks::ActionMailer,
        Hooks::ActionPack,
        Hooks::ActionView,
        Hooks::ActiveJob,
        Hooks::ActiveRecord,
        Hooks::ErrorReporter
      ].map(&:new).each(&:install)
    end

    def shutdown
      return if @publisher.nil?

      @events << nil
      @publisher.join
    end

    ##
    # Run a background thread that pops events from the queue and posts them to
    # the target server.
    def publisher_loop
      # Have the thread keep it's own connection.
      transport = Transport.new(
        endpoint: @config.endpoint,
        app_key: @config.app_key
      )

      loop do
        event = @events.pop

        break if event.nil?

        # TODO: event serialization
        _response, _error, _retriable = transport.deliver(JSON.generate(event))

        # TODO: retries / drops
      end
    end
  end
end