require_relative("../utils")
require_relative("count")

module Wukong
  class Processor

    # Groups sorted input records and emits each group with a count.
    #
    # Allows you to use several ways of extracting the key that
    # defines the group.
    #
    # **Note:** The input records must be previously sorted by the
    # same key used for grouping in order to ensure that groups are
    # not split up.
    #
    # @example Group simple string values on the command-line.
    #
    #   $ cat input
    #   apple
    #   cat
    #   banana
    #   apple
    #   ...
    #   $ cat input | wu-local sort | wu-local group
    #   apple	4
    #   banana	2
    #   cat	5
    #   ...
    #
    # @example Group using a nested key within a JSON string on the command-line
    #
    #   $ cat input
    #   {"id": 1, "word": "apple" }
    #   {"id": 2, "word": "cat"   }
    #   {"id": 3, "word": "banana"}
    #   ...
    #   $ cat input | wu-local sort --on==word | wu-local group --by=word
    #   apple	4
    #   banana	2
    #   cat	5
    #   ...
    #
    # A group fits nicely at the end of a dataflow.  Since it requires
    # a sort, it is blocking.
    #
    # @example Using a group at the end of a dataflow
    #
    #   Wukong.dataflow(:makes_groups) do
    #     ... | sort(on: 'field') | group(by: 'field')
    #   end
    #
    # @see Sort
    class Group < Count

      include DynamicGet
      field :by, Whatever

      # Get the key which defines the group for this `record`.
      #
      # @param [Object] record
      # @return [Object]
      def get_key(record)
        get(self.by, record)
      end

      # Reset the size counter for new group.
      #
      # @param [Object] record
      def start record
        self.size = 0
      end
      
      # Yields the current group along with its size
      #
      # @yield [key, size]
      # @yieldparam [Object] key the key defining the group
      # @yieldparam [Integer] size the size of the group
      def finalize
        yield [key, size].map(&:to_s).join("\t")
      end

      register
    end
  end
end