require 'money'
require 'open-uri'
require 'multi_json'

class Money
  module Bank
    class GoogleCurrencyRailsCache < Money::Bank::VariableExchange

      SERVICE_HOST = "www.google.com"
      SERVICE_PATH = "/finance/converter"

      ##
      # Clears all rates stored in @rates
      #
      # @example
      #   @bank = GoogleCurrencyRailsCache.new  #=> <Money::Bank::GoogleCurrency...>
      #   @bank.get_rate(:USD, :EUR)  #=> 0.776337241
      #   @bank.flush_rates           #=> nil
      def flush_rates
        rates = Rails.cache.read("exchange_rate/all_rates")
        unless rates.nil?
          rates.each do |rate_key|
            Rails.cache.delete "exchange_rate/#{rate_key}"
          end
        end
        Rails.cache.delete("exchange_rate/all_rates")
      end

      ##
      # Clears the specified rate stored in @rates.
      #
      # @param [String, Symbol, Currency] from Currency to convert from (used
      #   for key into @rates).
      # @param [String, Symbol, Currency] to Currency to convert to (used for
      #   key into @rates).
      #
      # @return [Boolean] Indicating successful removal.
      #
      # @example
      #   @bank = GoogleCurrencyRailsCache.new    #=> <Money::Bank::GoogleCurrency...>
      #   @bank.get_rate(:USD, :EUR)    #=> 0.776337241
      #   @bank.flush_rate(:USD, :EUR)  #=> 0.776337241
      def flush_rate(from, to)
        rate_key = rate_key_for(from, to)
        Rails.cache.delete("exchange_rate/#{rate_key}")
        remove_from_stored_rates(rate_key)
      end

      ##
      # Returns the requested rate.
      #
      # @param [String, Symbol, Currency] from Currency to convert from
      # @param [String, Symbol, Currency] to Currency to convert to
      #
      # @return [Float] The requested rate.
      #
      # @example
      #   @bank = GoogleCurrencyRailsCache.new  #=> <Money::Bank::GoogleCurrency...>
      #   @bank.get_rate(:USD, :EUR)  #=> 0.776337241
      def get_rate(from, to)
          Rails.cache.fetch("exchange_rate/#{rate_key_for(from, to)}", :expires_in => 12.hours ) {
            fetch_rate(from, to)
          }
      end
      
      ##
      # Lists all rates currency cached.
      #
      # @return [Array] An array of rates.
      #
      def cached_rates
        Rails.cache.read("exchange_rate/all_rates")
      end

      private

      ##
      # Queries for the requested rate and returns it.
      #
      # @param [String, Symbol, Currency] from Currency to convert from
      # @param [String, Symbol, Currency] to Currency to convert to
      #
      # @return [BigDecimal] The requested rate.
      def fetch_rate(from, to)
        from, to = Currency.wrap(from), Currency.wrap(to)

        data = build_uri(from, to).read
        extract_rate(data)
      end

      ##
      # Build a URI for the given arguments.
      #
      # @param [Currency] from The currency to convert from.
      # @param [Currency] to The currency to convert to.
      #
      # @return [URI::HTTP]
      def build_uri(from, to)
        uri = URI::HTTP.build(
          :host  => SERVICE_HOST,
          :path  => SERVICE_PATH,
          :query => "a=1&from=#{from.iso_code}&to=#{to.iso_code}"
        )
      end

      def extract_rate(data)
        case data
        when /<span class=bld>(\d+\.?\d*) [A-Z]{3}<\/span>/
          BigDecimal($1)
        when /Could not convert\./
          raise UnknownRate
        else
          raise GoogleCurrencyFetchError
        end
      end
      
      ##
      # Removes the specified rate from the global list of stored rates
      #
      # @param [String] rate The rate to remove
      #
      # @return [Boolean]
      def remove_from_stored_rates(rate)
        rates = Rails.cache.read("exchange_rate/all_rates")
        unless rates.nil?
          rates.delete(rate)
          Rails.cache.write("exchange_rate/all_rates", rates)
        end
      end
      
      ##
      # Add the specified rate to the global list of stored rates
      #
      # @param [String] rate The rate to add
      #
      # @return [Boolean]
      def add_to_stored_rates(rate)
        rates = Rails.cache.read("exchange_rate/all_rates")
        if rates.nil?
          rates = []
        end
        rates << rate
        Rails.cache.write("exchange_rate/all_rates", rates)
      end      

    end
  end
end