# encoding: utf-8

require 'one_apm/probe'
require 'one_apm/support/method_tracer/helpers'

module OneApm
  module Support
    # This module contains class methods added to support installing custom
    # metric tracers and executing for individual metrics.
    #
    # == Examples
    #
    # When the agent initializes, it extends Module with these methods.
    # However if you want to use the API in code that might get loaded
    # before the agent is initialized you will need to require
    # this file:
    #
    #     require 'one_apm/support/method_tracer'
    #     class A
    #       include OneApm::Support::MethodTracer
    #       def process
    #         ...
    #       end
    #       add_method_tracer :process
    #     end
    #
    # To instrument a class method:
    #
    #     require 'one_apm/support/method_tracer'
    #     class An
    #       def self.process
    #         ...
    #       end
    #       class << self
    #         include OneApm::Support::MethodTracer
    #         add_method_tracer :process
    #       end
    #     end
    #
    # @api public
    #

    module MethodTracer
      def self.included clazz
        clazz.extend ClassMethods
      end

      def self.extended clazz
        clazz.extend ClassMethods
      end

      # Trace a given block with stats and keep track of the caller.
      # See OneApm::Support::MethodTracer::ClassMethods#add_method_tracer for a description of the arguments.
      # +metric_names+ is either a single name or an array of metric names.
      # If more than one metric is passed, the +produce_metric+ option only applies to the first.  The
      # others are always recorded.  Only the first metric is pushed onto the scope stack.
      #
      # Generally you pass an array of metric names if you want to record the metric under additional
      # categories, but generally this *should never ever be done*.  Most of the time you can aggregate
      # on the server.
      #
      # @api public
      #
      def trace_execution_scoped(metric_names, options={})
        OneApm::Support::MethodTracer::Helpers.trace_execution_scoped(metric_names, options) do
          # Using an implicit block avoids object allocation for a &block param
          yield
        end
      end

      # Trace a given block with stats assigned to the given metric_name.  It does not
      # provide scoped measurements, meaning whatever is being traced will not 'blame the
      # Controller'--that is to say appear in the breakdown chart.
      # This is code is inlined in #add_method_tracer.
      # * <tt>metric_names</tt> is a single name or an array of names of metrics
      #
      # @api public
      #
      def trace_execution_unscoped(metric_names, options={})
        return yield unless OneApm::Manager.tl_is_execution_traced?
        t0 = Time.now
        begin
          yield
        ensure
          duration = (Time.now - t0).to_f              # for some reason this is 3 usec faster than Time - Time
          OneApm::Manager.agent.stats_engine.tl_record_unscoped_metrics(metric_names, duration)
        end
      end

      # Defines methods used at the class level, for adding instrumentation
      # @api public
      module ClassMethods
        # contains methods refactored out of the #add_method_tracer method
        module AddMethodTracer
          ALLOWED_KEYS = [:force, :metric, :push_scope, :code_header, :code_footer].freeze

          DEPRECATED_KEYS = [:force, :scoped_metric_only, :deduct_call_time_from_parent].freeze

          # raises an error when the
          # OneApm::Support::MethodTracer::ClassMethods#add_method_tracer
          # method is called with improper keys. This aids in
          # debugging new instrumentation by failing fast
          def check_for_illegal_keys!(method_name, options)
            unrecognized_keys = options.keys - ALLOWED_KEYS
            deprecated_keys   = options.keys & DEPRECATED_KEYS

            if unrecognized_keys.any?
              raise "Unrecognized options when adding method tracer to #{method_name}: " +
                    unrecognized_keys.join(', ')
            end

            if deprecated_keys.any?
              OneApm::Manager.logger.warn("Deprecated options when adding method tracer to #{method_name}: "+
                deprecated_keys.join(', '))
            end
          end

          # validity checking - add_method_tracer must receive either
          # push scope or metric, or else it would record no
          # data. Raises an error if this is the case
          def check_for_push_scope_and_metric(options)
            unless options[:push_scope] || options[:metric]
              raise "Can't add a tracer where push_scope is false and metric is false"
            end
          end

          DEFAULT_SETTINGS = {:push_scope => true, :metric => true, :code_header => "", :code_footer => "" }.freeze

          # Checks the provided options to make sure that they make
          # sense. Raises an error if the options are incorrect to
          # assist with debugging, so that errors occur at class
          # construction time rather than instrumentation run time
          def validate_options(method_name, options)
            unless options.is_a?(Hash)
              raise TypeError.new("Error adding method tracer to #{method_name}: provided options must be a Hash")
            end
            check_for_illegal_keys!(method_name, options)
            options = DEFAULT_SETTINGS.merge(options)
            check_for_push_scope_and_metric(options)
            options
          end

          # Default to the class where the method is defined.
          #
          # Example:
          #  Foo.default_metric_name_code('bar') #=> "Custom/#{Foo.name}/bar"
          def default_metric_name_code(method_name)
            "Custom/#{self.name}/#{method_name.to_s}"
          end

          # Checks to see if the method we are attempting to trace
          # actually exists or not. #add_method_tracer can't do
          # anything if the method doesn't exist.
          def oneapm_method_exists?(method_name)
            exists = method_defined?(method_name) || private_method_defined?(method_name)
            OneApm::Manager.logger.error("Did not trace #{self.name}##{method_name} because that method does not exist") unless exists
            exists
          end

          # Checks to see if we have already traced a method with a
          # given metric by checking to see if the traced method
          # exists. Warns the user if methods are being double-traced
          # to help with debugging custom instrumentation.
          def traced_method_exists?(method_name, metric_name_code)
            exists = method_defined?(_traced_method_name(method_name, metric_name_code))
            OneApm::Manager.logger.error("Attempt to trace a method twice with the same metric: Method = #{method_name}, Metric Name = #{metric_name_code}") if exists
            exists
          end

          # Returns a code snippet to be eval'd that skips tracing
          # when the agent is not tracing execution. turns
          # instrumentation into effectively one method call overhead
          # when the agent is disabled
          def assemble_code_header(method_name, metric_name_code, options)
            header = "return #{_untraced_method_name(method_name, metric_name_code)}(*args, &block) unless OneApm::Manager.tl_is_execution_traced?\n"
            header += options[:code_header].to_s
            header
          end

          # returns an eval-able string that contains the traced
          # method code used if the agent is not creating a scope for
          # use in scoped metrics.
          def method_without_push_scope(method_name, metric_name_code, options)
            "def #{_traced_method_name(method_name, metric_name_code)}(*args, &block)
              #{assemble_code_header(method_name, metric_name_code, options)}
              t0 = Time.now
              begin
                #{_untraced_method_name(method_name, metric_name_code)}(*args, &block)\n
              ensure
                duration = (Time.now - t0).to_f
                OneApm::Manager.record_metric(\"#{metric_name_code}\", duration)
                #{options[:code_footer]}
              end
            end"
          end

          # returns an eval-able string that contains the tracing code
          # for a fully traced metric including scoping
          def method_with_push_scope(method_name, metric_name_code, options)
            "def #{_traced_method_name(method_name, metric_name_code)}(*args, &block)
              #{options[:code_header]}
              result = ::OneApm::Support::MethodTracer::Helpers.trace_execution_scoped(\"#{metric_name_code}\",
                        :metric => #{options[:metric]}) do
                #{_untraced_method_name(method_name, metric_name_code)}(*args, &block)
              end
              #{options[:code_footer]}
              result
            end"
          end

          # Decides which code snippet we should be eval'ing in this
          # context, based on the options.
          def code_to_eval(method_name, metric_name_code, options)
            options = validate_options(method_name, options)
            if options[:push_scope]
              method_with_push_scope(method_name, metric_name_code, options)
            else
              method_without_push_scope(method_name, metric_name_code, options)
            end
          end
        end
        include AddMethodTracer

        # Add a method tracer to the specified method.
        #
        # By default, this will cause invocations of the traced method to be
        # recorded in transaction traces, and in a metric named after the class
        # and method. It will also make the method show up in transaction-level
        # breakdown charts and tables.
        #
        # === Overriding the metric name
        #
        # +metric_name_code+ is a string that is eval'd to get the name of the
        # metric associated with the call, so if you want to use interpolation
        # evaluated at call time, then single quote the value like this:
        #
        #     add_method_tracer :foo, 'Custom/#{self.class.name}/foo'
        #
        # This would name the metric according to the class of the runtime
        # intance, as opposed to the class where +foo+ is defined.
        #
        # If not provided, the metric name will be <tt>Custom/ClassName/method_name</tt>.
        #
        # @param [Symbol] method_name
        #   the name of the method to trace
        #
        # @param [String] metric_name_code
        #   the metric name to record calls to the traced method under. This may
        #   be either a static string, or Ruby code to be evaluated at call-time
        #   in order to determine the metric name dynamically.
        #
        # @param [Hash] options
        #   additional options controlling how the method is traced.
        #
        #   @option options [Boolean] :push_scope (true)
        #     If false, the traced method will not appear in transaction traces or
        #     breakdown charts, and it will only be visible in custom dashboards.
        #
        #   @option options [Boolean] :metric (true)
        #     If false, the traced method will only appear in transaction traces,
        #     but no metrics will be recorded for it.
        #
        #   @option options [String] :code_header ('')
        #     Ruby code to be inserted and run before the tracer begins timing.
        #
        #   @option options [String] :code_footer ('')
        #     Ruby code to be inserted and run after the tracer stops timing.
        #
        # @example
        #
        #   add_method_tracer :foo
        #
        #   # With a custom metric name
        #   add_method_tracer :foo, 'Custom/#{self.class.name}/foo'
        #
        #   # Instrument foo only for custom dashboards (not in transaction
        #   # traces or breakdown charts)
        #   add_method_tracer :foo, 'Custom/foo', :push_scope => false
        #
        #   # Instrument foo in transaction traces only
        #   add_method_tracer :foo, 'Custom/foo', :metric => false
        #
        # @api public
        #
        def add_method_tracer(method_name, metric_name_code = nil, options = {})
          return unless oneapm_method_exists?(method_name)
          metric_name_code ||= default_metric_name_code(method_name)
          return if traced_method_exists?(method_name, metric_name_code)

          traced_method = code_to_eval(method_name, metric_name_code, options)

          visibility = OneApm::Helper.instance_method_visibility self, method_name

          class_eval traced_method, __FILE__, __LINE__
          alias_method _untraced_method_name(method_name, metric_name_code), method_name
          alias_method method_name, _traced_method_name(method_name, metric_name_code)
          send visibility, method_name
          send visibility, _traced_method_name(method_name, metric_name_code)
          OneApm::Manager.logger.debug("Traced method: class = #{self.name},"+
                    "method = #{method_name}, "+
                    "metric = '#{metric_name_code}'")
        end

        # For tests only because tracers must be removed in reverse-order
        # from when they were added, or else other tracers that were added to the same method
        # may get removed as well.
        def remove_method_tracer(method_name, metric_name_code) # :nodoc:
          return unless OneApm::Manager.config[:agent_enabled]
          if method_defined? "#{_traced_method_name(method_name, metric_name_code)}"
            alias_method method_name, "#{_untraced_method_name(method_name, metric_name_code)}"
            undef_method "#{_traced_method_name(method_name, metric_name_code)}"
            OneApm::Manager.logger.debug("removed method tracer #{method_name} #{metric_name_code}\n")
          else
            raise "No tracer for '#{metric_name_code}' on method '#{method_name}'"
          end
        end
        private

        # given a method and a metric, this method returns the
        # untraced alias of the method name
        def _untraced_method_name(method_name, metric_name)
          "#{_sanitize_name(method_name)}_without_trace_#{_sanitize_name(metric_name)}"
        end

        # given a method and a metric, this method returns the traced
        # alias of the method name
        def _traced_method_name(method_name, metric_name)
          "#{_sanitize_name(method_name)}_with_trace_#{_sanitize_name(metric_name)}"
        end

        # makes sure that method names do not contain characters that
        # might break the interpreter, for example ! or ? characters
        # that are not allowed in the middle of method names
        def _sanitize_name(name)
          name.to_s.tr_s('^a-zA-Z0-9', '_')
        end
      end

      # @!parse extend ClassMethods
    end
  end
end