Comparison bundles up into Rails helpers the logic for rendering visually informative comparisons of numbers. For example, say you were comparing the sales figures from one quarter to the same quarter in a previous year. You might want to show the percentage change, accompanied by an arrow or icon and color-coded to indicate positive or negative growth. This plugin provides helpers that abstract the logic of deciding what to show into a handful of simple methods and leveraging I18n.

The library has three components: the Comparator class for performing the actual math, the Presenter class for decorating the comparator with view-friendly output, and a helper module for using the presenter with the view.

The #compare helper takes the two numbers to be compared and yields the presenter to a block.

#difference_as_currency provides the difference between the two numbers, literally value - other, formatted as currency. Formatting is handled by ActiveSupport::NumberHelper#number_to_currency.

#percentage provides the percentage difference between the two numbers. Under the hood it uses ActiveSupport#NumberHelper#number_to_percentage to format the percentage. Options are passed through to that method.

#unsigned_percentage returns the absolute value of the percentage. Use this if you prefer to use other cues, such as colors or icons, to indicate positive or negative changes.

#arrow returns an HTML character entity for an arrow (up, down, or, for no change, an empty string).

<%= compare value, other do |cmp| %>
  <td><%= number_to_currency value %></td>
  <td><%= number_to_currency other %></td>
  <td><%= cmp.difference_as_currency %></td>
  <td>
    <%= cmp.arrow %>
    <%= cmp.percentage precision: 1 %>
  </td>
<% end %>

Comparison uses I18n to configure the output of some of the Presenter methods. Default implementations are provided where it makes sense. You can provide your own implementations by adding translations to your application.

en:
  comparison:
    dom_classes:
      positive: 'comparison positive'
      negative: 'comparison negative'
      nochange: 'comparison nochange'
    inline_style:
      positive: 'color: #3c763d; background-color: #dff0d8;'
      negative: 'color: #a94442; background-color: #f2dede;'
      nochange: 'color: #777777;'
    icons:
      positive_html: '<span class="glyphicon glyphicon-arrow-up"></span>'
      negative_html: '<span class="glyphicon glyphicon-arrow-down"></span>'
      nochange_html: '<span class="glyphicon glyphicon-minus"></span>'
    arrows:
      positive_html: '&uarr;'
      negative_html: '&darr;'
      nochange_html: ''
    infinity_html: '&infin;'

If the dom_classes keys are not present, translation falls back on classes. If the inline_style keys are not present, translation falls back on style and css. In both cases, those fallback keys are deprecated. Expect support for those keys to eventually be dropped.

Add this line to your application's Gemfile:

gem 'comparison'

And then execute:

$ bundle

Include the Comparison helpers by adding the following to your application_helper.rb:

# app/helpers/application_helper.rb
module ApplicationHelper
  include Comparison::ApplicationHelper
end

Open an GitHub issue for problems and suggestions. This library is in its infancy, so use it at your own risk.

The gem is available as open source under the terms of the MIT License.