# encoding: utf-8
module Mongoid
  module Contextual
    module Aggregable
      # Contains behaviour for aggregating values in Mongo.
      module Mongo

        # Get all the aggregate values for the provided field.
        #
        # @example Get all the aggregate values.
        #   aggregable.aggregates(:likes)
        #
        # @param [ String, Symbol ] field The field name.
        #
        # @return [ Hash ] count is a number of documents with the provided field. If there're none, then count is 0 and max, min, sum, avg are nil.
        #   {
        #     "count" => 2.0,
        #     "max" => 1000.0,
        #     "min" => 500.0,
        #     "sum" => 1500.0,
        #     "avg" => 750.0
        #   }
        #
        # @since 3.0.0
        def aggregates(field)
          result = collection.find.aggregate(pipeline(field)).to_a
          if result.empty?
            { "count" => 0, "sum" => nil, "avg" => nil, "min" => nil, "max" => nil }
          else
            result.first
          end
        end

        # Get the average value of the provided field.
        #
        # @example Get the average of a single field.
        #   aggregable.avg(:likes)
        #
        # @param [ Symbol ] field The field to average.
        #
        # @return [ Float ] The average.
        #
        # @since 3.0.0
        def avg(field)
          aggregates(field)["avg"]
        end

        # Get the max value of the provided field. If provided a block, will
        # return the Document with the greatest value for the field, in
        # accordance with Ruby's enumerable API.
        #
        # @example Get the max of a single field.
        #   aggregable.max(:likes)
        #
        # @example Get the document with the max value.
        #   aggregable.max do |a, b|
        #     a.likes <=> b.likes
        #   end
        #
        # @param [ Symbol ] field The field to max.
        #
        # @return [ Float, Document ] The max value or document with the max
        #   value.
        #
        # @since 3.0.0
        def max(field = nil)
          block_given? ? super() : aggregates(field)["max"]
        end

        # Get the min value of the provided field. If provided a block, will
        # return the Document with the smallest value for the field, in
        # accordance with Ruby's enumerable API.
        #
        # @example Get the min of a single field.
        #   aggregable.min(:likes)
        #
        # @example Get the document with the min value.
        #   aggregable.min do |a, b|
        #     a.likes <=> b.likes
        #   end
        #
        # @param [ Symbol ] field The field to min.
        #
        # @return [ Float, Document ] The min value or document with the min
        #   value.
        #
        # @since 3.0.0
        def min(field = nil)
          block_given? ? super() : aggregates(field)["min"]
        end

        # Get the sum value of the provided field. If provided a block, will
        # return the sum in accordance with Ruby's enumerable API.
        #
        # @example Get the sum of a single field.
        #   aggregable.sum(:likes)
        #
        # @example Get the sum for the provided block.
        #   aggregable.sum(&:likes)
        #
        # @param [ Symbol ] field The field to sum.
        #
        # @return [ Float ] The sum value.
        #
        # @since 3.0.0
        def sum(field = nil)
          block_given? ? super() : aggregates(field)["sum"] || 0
        end

        private

        # Get the aggregation pipeline for provided field.
        #
        # @api private
        #
        # @example Get the pipeline.
        #   aggregable.pipeline(:likes)
        #
        # @param [ String, Symbol ] field The name of the field.
        #
        # @return [ Array ] The array of pipeline operators.
        #
        # @since 3.1.0
        def pipeline(field)
          db_field = "$#{database_field_name(field)}"
          pipeline = []
          pipeline << { "$match" => criteria.nin(field => nil).selector }
          pipeline << { "$sort" => criteria.options[:sort] } if criteria.options[:sort]
          pipeline << { "$skip" => criteria.options[:skip] } if criteria.options[:skip]
          pipeline << { "$limit" => criteria.options[:limit] } if criteria.options[:limit]
          pipeline << {
            "$group"  => {
              "_id"   => field.to_s,
              "count" => { "$sum" => 1 },
              "max"   => { "$max" => db_field },
              "min"   => { "$min" => db_field },
              "sum"   => { "$sum" => db_field },
              "avg"   => { "$avg" => db_field }
            }
          }
        end
      end
    end
  end
end