class ExchangeRates

at(date, from, to) click to toggle source

ExchangeRates.at gives an exchange rate between two currencies at a particular date. Raises exceptions if the dates are out of range of the file or if the currencies are unknown.

# File lib/exchange_rates.rb, line 11
def self.at(date, from, to)
  parse_rates
  if rates = @@rates[date]
    unless from_rate = rates[from]
      if from == @@base_currency
        from_rate = 1.0
      else
        raise "Unknown 'from' currency"
      end
    end
    unless to_rate = rates[to]
      if to == @@base_currency
        to_rate = 1.0
      else
        raise "Unknown 'to' currency"
      end
    end
    to_rate / from_rate
  else
    raise "Date out of known dates."
  end
end

available_currencies() click to toggle source

Outputs an array of supported currencies.

# File lib/exchange_rates.rb, line 106
def self.available_currencies
  parse_rates
  @@rates.first[1].keys + [@@base_currency]
end

available_dates() click to toggle source

Outputs an array of dates included in the current data set.

# File lib/exchange_rates.rb, line 100
def self.available_dates
  parse_rates
  @@rates.keys
end

convert(amount, opts = {}) click to toggle source

Converts an amount of money between currencies. The options for this method are:

from - from currency, defaults to base_currency (typically EUR)
to   - to currency, defaults to  base_currency (typically EUR)
date - date at which to perform conversion, defaults to latest available date in dateset

# File lib/exchange_rates.rb, line 39
def self.convert(amount, opts = {})
  parse_rates
  options = {from: @@base_currency, to: @@base_currency, date: @@rates.keys.sort.reverse.first}.merge(opts)
  amount.to_f * at(options[:date], options[:from], options[:to])
end

fetch_rates!() click to toggle source

Goes and makes a request to download latest ECB data

# File lib/exchange_rates.rb, line 83
def self.fetch_rates!
  rates = Net::HTTP.get URI.parse 'http://www.ecb.europa.eu/stats/eurofxref/eurofxref-hist-90d.xml'
  File.write(ENV['EXCHANGE_RATE_FILE'] || './exchange-rates.xml', rates)
  rates
end

over_time(from, to) click to toggle source

Calculates exchange rates between two currencies over all available dates.

# File lib/exchange_rates.rb, line 46
def self.over_time(from, to)
  parse_rates
  results = {}
  @@rates.each do |date, rates|
    results[date] = self.at(date, from, to)
  end
  results
end

parse_rates() click to toggle source

Reads and parses the XML data feed providing the underlying data source. The data source is currently assumed to be in the format of the European Central Bank feed accesible at <www.ecb.europa.eu/stats/eurofxref/eurofxrefhist90d.xml> which provides a 90 day history of exchange rates. It is assumed that this file is saved locally, best through a cron tab (see README). If a different source is to be used, call the ‘set_rates` method.

# File lib/exchange_rates.rb, line 61
def self.parse_rates
  @@rates ||= nil
  return @@rates if @@rates
  @@base_currency = 'EUR'
  rate_file = ENV['EXCHANGE_RATE_FILE'] || './exchange-rates.xml'
  if File.exist?(rate_file)
    doc = Nokogiri::XML(File.read(rate_file))
  else
    doc = Nokogiri::XML(fetch_rates!)
  end
  @@rates = {}
  doc.css('Cube>Cube[time]').each do |day|
    time = Date.parse day.attr('time')
    @@rates[time] = {}
    day.css('Cube').each{ |c|
      @@rates[time][c.attr('currency')] = c.attr('rate').to_f
    }
  end
  @@rates
end

set_rates(rates, base = 'EUR') click to toggle source

Set custom rates data. The data should be a hash of hashes, where the keys in the outer hash are Date objects and the keys in the inner hashes are three letter currency codes. The values are floats showing the exchange rate between a currency and the base currency. This second argument should be the base currency to which all values are relative to.

# File lib/exchange_rates.rb, line 94
def self.set_rates(rates, base = 'EUR')
  @@rates = rates
  @@base_currency = base
end

Constants

Public Class Methods