module MiniSql
  module Sqlite
    class DeserializerCache

      DEFAULT_MAX_SIZE = 500

      def initialize(max_size = nil)
        @cache = {}
        @max_size = max_size || DEFAULT_MAX_SIZE
      end

      def materialize(result)

        key = result.columns

        # trivial fast LRU implementation
        materializer = @cache.delete(key)
        if materializer
          @cache[key] = materializer
        else
          materializer = @cache[key] = new_row_matrializer(result)
          @cache.shift if @cache.length > @max_size
        end

        r = []
        # quicker loop
        while !result.eof?
          data = result.next
          if data
            r << materializer.materialize(data)
          end
        end
        r
      end

      private

      def new_row_matrializer(result)
        fields = result.columns

        Class.new do
          attr_accessor(*fields)

          # AM serializer support
          alias :read_attribute_for_serialization :send

          def to_h
            r = {}
            instance_variables.each do |f|
              r[f.to_s.sub('@','').to_sym] = instance_variable_get(f)
            end
            r
          end

          instance_eval <<~RUBY
            def materialize(data)
              r = self.new
              #{col=-1; fields.map{|f| "r.#{f} = data[#{col+=1}]"}.join("; ")}
              r
            end
          RUBY
        end
      end
    end
  end
end