require 'prometheus/client/helper/entry_parser'
require 'prometheus/client/helper/file_locker'
require 'mmap'

module Prometheus
  module Client
    module Helper
      class MmapedFile < Mmap
        include EntryParser
        attr_reader :filepath, :size

        def initialize(filepath, mode = 'r', protection = Mmap::MAP_SHARED, options = {})
          @filepath = filepath

          File.open(filepath, 'a+b') do |file|
            file.truncate(initial_mmap_file_size) if file.size < MINIMUM_SIZE
            @size = file.size
          end

          super(filepath, mode, protection, options)
        end

        def used=(value)
          self[0..3] = [value].pack('l')
        end

        def try_add_entry(data, value)
          self.used = START_POSITION if used.zero?

          # Pad to be 8-byte aligned.
          padded = data + (' ' * (8 - (data.length + 4) % 8))
          entry = [data.length, padded, value].pack("lA#{padded.length}d")
          used_ = used

          return false if (used_ + entry.length) > @size

          self[used_..used_ + entry.length] = entry
          self.used = used_ + entry.length

          true
        end

        def close
          munmap
        end

        private

        def initial_mmap_file_size
          Prometheus::Client.configuration.initial_mmap_file_size
        end

        public

        class << self
          def open(filepath)
            MmapedFile.new(filepath, 'rw', Mmap::MAP_SHARED)
          end

          def ensure_exclusive_file(file_prefix = 'mmaped_file')
            (0..Float::INFINITY).lazy
              .map { |f_num| "#{file_prefix}_#{Prometheus::Client.pid}-#{f_num}.db" }
              .map { |filename| File.join(Prometheus::Client.configuration.multiprocess_files_dir, filename) }
              .find { |path| Helper::FileLocker.lock_to_process(path) }
          end

          def open_exclusive_file(file_prefix = 'mmaped_file')
            filename = Helper::MmapedFile.ensure_exclusive_file(file_prefix)
            open(filename)
          end
        end
      end
    end
  end
end