# inspired by: https://github.com/rack/rack-contrib/blob/master/lib/rack/contrib/locale.rb

class LocaleMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    old_locale = I18n.locale

    begin
      locale = accept_locale(env) || I18n.default_locale
      env['rack.locale'] = I18n.locale = locale.to_s
      status, headers, body = @app.call(env)
      headers['Content-Language'] = I18n.locale.to_s
      [status, headers, body]
    ensure
      I18n.locale = old_locale
    end
  end

  private

  # http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
  def accept_locale(env)
    accept_langs = env["HTTP_ACCEPT_LANGUAGE"]
    return if accept_langs.blank?

    languages_and_qvalues = accept_langs.split(",").map { |l|
      l += ';q=1.0' unless l =~ /;q=\d+(?:\.\d+)?$/
      l.split(';q=')
    }

    # gets tuple of supported locale with highest q-value
    lang = [nil, 0]
    languages_and_qvalues.each do |(locale, qvalue)|
      next if locale == '*'
      # locale could be en-GB and we only use en
      locale = locale.split('-').first.try(:to_sym)
      # check if locale is supported
      next unless I18n.available_locales.include?(locale)
      # return the new locale if its priority is higher
      lang = [locale, qvalue] if qvalue.to_f >= lang[1].to_f
    end
    lang.first
  end
end
~