require "large_object_store/version"
require "zlib"

module LargeObjectStore

  def self.wrap(store)
    RailsWrapper.new(store)
  end

  class RailsWrapper
    attr_reader :store

    MAX_OBJECT_SIZE = 1024**2
    ITEM_HEADER_SIZE = 100

    def initialize(store)
      @store = store
    end

    def write(key, value, options = {})
      value = Marshal.dump(value)
      value = Zlib::Deflate.deflate(value) if options.delete(:compress)

      # calculate slice size; note that key length is a factor because
      # the key is stored on the same slab page as the value
      slice_size = MAX_OBJECT_SIZE - ITEM_HEADER_SIZE - key.bytesize

      # store number of pages
      pages = (value.size / slice_size.to_f).ceil

      if pages == 1
        @store.write("#{key}_0", value, options)
      else
        # store object
        page = 1
        loop do
          slice = value.slice!(0, slice_size)
          break if slice.size == 0

          return false unless @store.write("#{key}_#{page}", slice, options.merge(raw: true))
          page += 1
        end

        @store.write("#{key}_0", pages, options)
      end
    end

    def read(key)
      # read pages
      pages = @store.read("#{key}_0")
      return if pages.nil?

      data = if pages.is_a?(Fixnum)
        # read sliced data
        keys = Array.new(pages).each_with_index.map{|_,i| "#{key}_#{i+1}" }
        slices = @store.read_multi(*keys).values
        return nil if slices.compact.size < pages
        slices.join("")
      else
        pages
      end

      if data.getbyte(0) == 0x78 && [0x01,0x9C,0xDA].include?(data.getbyte(1))
        data = Zlib::Inflate.inflate(data)
      end

      begin
        Marshal.load(data)
      rescue Exception => e
        Rails.logger.error "Cannot read large_object_store key #{key} : #{e.message} #{e.backtrace.inspect}"
        nil
      end
    end

    def fetch(key, options={})
      value = read(key)
      return value unless value.nil?
      value = yield
      write(key, value, options)
      value
    end

    def delete(key)
      @store.delete("#{key}_0")
    end
  end
end