module Saulabs

  module Reportable

    # The +ReportCache+ class is a regular +ActiveRecord+ model and represents cached results for single {Saulabs::Reportable::ReportingPeriod}s.
    # +ReportCache+ instances are identified by the combination of +model_name+, +report_name+, +grouping+, +aggregation+ and +reporting_period+.
    #
    class ReportCache < ActiveRecord::Base

      set_table_name :reportable_cache

      self.skip_time_zone_conversion_for_attributes = [:reporting_period]

      # Clears the cache for the specified +klass+ and +report+
      #
      # @param [Class] klass
      #   the model the report to clear the cache for works on
      # @param [Symbol] report
      #   the name of the report to clear the cache for
      #
      # @example Clearing the cache for a report
      #
      #   class User < ActiveRecord::Base
      #     reportable :registrations
      #   end
      #
      #   Saulabs::Reportable::ReportCache.clear_for(User, :registrations)
      #
      def self.clear_for(klass, report)
        self.delete_all(:conditions => {
          :model_name  => klass.name,
          :report_name => report.to_s
        })
      end

      # Processes the report using the respective cache.
      #
      # @param [Saulabe::Reportable::Report] report
      #   the report to process
      # @param [Hash] options
      #   options for the report
      #
      # @option options [Symbol] :grouping (:day)
      #   the period records are grouped in (+:hour+, +:day+, +:week+, +:month+); <b>Beware that <tt>reportable</tt> treats weeks as starting on monday!</b>
      # @option options [Fixnum] :limit (100)
      #   the number of reporting periods to get (see +:grouping+)
      # @option options [Hash] :conditions ({})
      #   conditions like in +ActiveRecord::Base#find+; only records that match these conditions are reported;
      # @option options [Boolean] :live_data (false)
      #   specifies whether data for the current reporting period is to be read; <b>if +:live_data+ is +true+, you will experience a performance hit since the request cannot be satisfied from the cache alone</b>
      # @option options [DateTime, Boolean] :end_date (false)
      #   when specified, the report will only include data for the +:limit+ reporting periods until this date.
      #
      # @return [Array<Array<DateTime, Float>>]
      #   the result of the report as pairs of {DateTime}s and {Float}s
      #
      def self.process(report, options, &block)
        raise ArgumentError.new('A block must be given') unless block_given?
        self.transaction do
          cached_data = read_cached_data(report, options)
          new_data = read_new_data(cached_data, options, &block)
          prepare_result(new_data, cached_data, report, options)
        end
      end

      private

        def self.prepare_result(new_data, cached_data, report, options)
          new_data = new_data.map { |data| [ReportingPeriod.from_db_string(options[:grouping], data[0]), data[1]] }
          cached_data.map! { |cached| [ReportingPeriod.new(options[:grouping], cached.reporting_period), cached.value] }
          current_reporting_period = ReportingPeriod.new(options[:grouping])
          reporting_period = get_first_reporting_period(options)
          result = []
          while reporting_period < (options[:end_date] ? ReportingPeriod.new(options[:grouping], options[:end_date]).next : current_reporting_period)
            if cached = cached_data.find { |cached| reporting_period == cached[0] }
              result << [cached[0].date_time, cached[1]]
            else
              new_cached = build_cached_data(report, options[:grouping], options[:conditions], reporting_period, find_value(new_data, reporting_period))
              new_cached.save!
              result << [reporting_period.date_time, new_cached.value]
            end
            reporting_period = reporting_period.next
          end
          if options[:live_data]
            result << [current_reporting_period.date_time, find_value(new_data, current_reporting_period)]
          end
          result
        end

        def self.find_value(data, reporting_period)
          data = data.detect { |d| d[0] == reporting_period }
          data ? data[1] : 0.0
        end

        def self.build_cached_data(report, grouping, conditions, reporting_period, value)
          self.new(
            :model_name       => report.klass.to_s,
            :report_name      => report.name.to_s,
            :grouping         => grouping.identifier.to_s,
            :aggregation      => report.aggregation.to_s,
            :conditions       => conditions.to_s,
            :reporting_period => reporting_period.date_time,
            :value            => value
          )
        end

        def self.read_cached_data(report, options)
          conditions = [
            %w(model_name report_name grouping aggregation conditions).map do |column_name|
              "#{self.connection.quote_column_name(column_name)} = ?"
            end.join(' AND '),
            report.klass.to_s,
            report.name.to_s,
            options[:grouping].identifier.to_s,
            report.aggregation.to_s,
            options[:conditions].to_s
          ]
          first_reporting_period = get_first_reporting_period(options)
          last_reporting_period = get_last_reporting_period(options)
          if last_reporting_period
            conditions.first << ' AND reporting_period BETWEEN ? AND ?'
            conditions << first_reporting_period.date_time
            conditions << last_reporting_period.date_time
          else
            conditions.first << ' AND reporting_period >= ?'
            conditions << first_reporting_period.date_time
          end
          self.all(
            :conditions => conditions,
            :limit      => options[:limit],
            :order      => 'reporting_period ASC'
          )
        end

        def self.read_new_data(cached_data, options, &block)
          if !options[:live_data] && cached_data.length == options[:limit]
            []
          else
            first_reporting_period_to_read = if cached_data.length < options[:limit]
              get_first_reporting_period(options)
            else
              ReportingPeriod.new(options[:grouping], cached_data.last.reporting_period).next
            end
            last_reporting_period_to_read = options[:end_date] ? ReportingPeriod.new(options[:grouping], options[:end_date]).last_date_time : nil
            yield(first_reporting_period_to_read.date_time, last_reporting_period_to_read)
          end
        end

        def self.get_first_reporting_period(options)
          if options[:end_date]
            ReportingPeriod.first(options[:grouping], options[:limit] - 1, options[:end_date])
          else
            ReportingPeriod.first(options[:grouping], options[:limit])
          end
        end

        def self.get_last_reporting_period(options)
          return ReportingPeriod.new(options[:grouping], options[:end_date]) if options[:end_date]
        end

    end

  end

end