Maybe/app/models/trend.rb

class Trend
    attr_reader :current, :previous

    def initialize(current, previous)
        @current = current
        @previous = previous
    end

    def direction
        return "flat" unless @previous
        return "up" if @current > @previous
        return "down" if @current < @previous
        "flat"
    end

    def amount
      return 0 if @previous.nil?
      @current - @previous
    end

    def percent
      return 0 if @previous.nil?
      return Float::INFINITY if @previous == 0
      ((@current - @previous).abs / @previous.to_f * 100).round(1)
    end
end