require 'prometheus/client/mmaped_dict'
require 'json'

module Prometheus
  module Client
    # A float protected by a mutex backed by a per-process mmaped file.
    class MmapedValue
      @@files = {}
      @@files_lock = Mutex.new
      @@pid = Process.pid

      def initialize(type, metric_name, name, labels, multiprocess_mode = '')
        @pid = Process.pid
        @file_prefix = type.to_s
        @metric_name = metric_name
        @name = name
        @labels = labels
        if type == :gauge
          @file_prefix += '_' + multiprocess_mode.to_s
        end

        self.class.reset_on_pid_change if self.class.pid_changed?

        @mutex = Mutex.new
        initialize_file
      end

      def increment(amount=1)
        @mutex.synchronize do
          @value += amount
          write_value(@key, @value)
          @value
        end
      end

      def set(value)
        @mutex.synchronize do
          @value = value
          write_value(@key, @value)
          @value
        end
      end

      def get
        @mutex.synchronize do
          return @value
        end
      end

      def self.reset_on_pid_change
        @@files_lock.synchronize do
          if pid_changed?
            @@pid = Process.pid
            @@files = {}
          end
        end
      end

      def self.reinitialize_on_pid_change
        reset_on_pid_change
        ObjectSpace.each_object(MmapedValue, &:reinitialize)
      end

      def self.pid_changed?
        @@pid != Process.pid
      end

      def self.multiprocess
        true
      end

      def reinitialize
        if @pid != Process.pid
          @pid = Process.pid
          initialize_file
        end
      end

      def initialize_file
        @@files_lock.synchronize do
          unless @@files.has_key?(@file_prefix)
            unless @file.nil?
              puts @file
              @file.close
            end

            filename = File.join(Prometheus::Client.configuration.multiprocess_files_dir, "#{@file_prefix}_#{@@pid}.db")
            @@files[@file_prefix] = MmapedDict.new(filename)
          end
        end

        @mutex.synchronize do
          @file = @@files[@file_prefix]
          labelnames = []
          labelvalues = []
          @labels.each do |k, v|
            labelnames << k
            labelvalues << v
          end

          @key = [@metric_name, @name, labelnames, labelvalues].to_json
          @value = read_value(@key)
        end
      end

      private

      def write_value(key, val)
        @file.write_value(key, val)
      rescue StandardError => e
        Prometheus::Client.logger.warn("writing value to #{@file.path} failed with #{e}")
      end

      def read_value(key)
        @file.read_value(key)
      rescue StandardError => e
        Prometheus::Client.logger.warn("reading value from #{@file.path} failed with #{e}")
      end
    end
  end
end