aggregator.rb

# Public: A fancy class for doing complicated group-bys.
# Construct it with a strategy object that responds to #key_for, #set, and
# #default_accumulator.
class Aggregator
  attr_reader :strategy

  # Public: Create a new Aggregator.
  #
  # strategy - An aggregation strategy object that responds to the following:
  #   #key_for(item) - takes an item from the collection and returns the key to use for it.
  #   #set(acc, value, key, collection) - takes the accumulator, key, and item
  #     and sets it appropriately. `collection` is the entire collection being aggregated.
  def initialize(strategy)
    @strategy = strategy
  end

  # Public: Aggregate an enumerable collection.
  #   collection - the collection to aggregate. Must respond to #each.
  #   accummulator - (optional) an accumulator object to use (defaults to the default
  #     accumulator defined in the strategy)
  #
  # Returns the resulting accumulator after #set has been called for each item
  # in the `collection`.
  def aggregate(collection, accumulator = strategy.default_accumulator)
    collection.each do |value|
      strategy.set(accumulator, value, strategy.key_for(value), collection)
    end

    accumulator
  end
end