Add a gemojione:sprite Rake task

This task will generate a standard and Retina sprite of all of the
current Gemojione Emojis, with the accompanying SCSS map.

It will not appear in `rake -T` output, and the dependent gems are not
included in the Gemfile by default, because this task will only be
needed occasionally.

[ci skip]

lib/tasks/gemojione.rake

@@ -0,0 +1,121 @@
+# This task will generate a standard and Retina sprite of all of the current
+# Gemojione Emojis, with the accompanying SCSS map.
+#
+# It will not appear in `rake -T` output, and the dependent gems are not
+# included in the Gemfile by default, because this task will only be needed
+# occasionally, such as when new Emojis are added to Gemojione.
+
+begin
+  require 'sprite_factory'
+  require 'rmagick'
+rescue LoadError
+  # noop
+end
+
+namespace :gemojione do
+  task sprite: :environment do
+    check_requirements!
+
+    SIZE   = 20
+    RETINA = SIZE * 2
+
+    Dir.mktmpdir do |tmpdir|
+      # Copy the Gemojione assets to the temporary folder for resizing
+      FileUtils.cp_r(Gemojione.index.images_path, tmpdir)
+
+      Dir.chdir(tmpdir) do
+        Dir["**/*.png"].each do |png|
+          resize!(File.join(tmpdir, png), SIZE)
+        end
+      end
+
+      style_path = Rails.root.join(*%w(app assets stylesheets pages emojis.scss))
+
+      # Combine the resized assets into a packed sprite and re-generate the SCSS
+      SpriteFactory.cssurl = "image-url('$IMAGE')"
+      SpriteFactory.run!(File.join(tmpdir, 'images'), {
+        output_style: style_path,
+        output_image: "app/assets/images/emoji.png",
+        selector:     '.emoji-',
+        style:        :scss,
+        nocomments:   true,
+        pngcrush:     true,
+        layout:       :packed
+      })
+
+      # SpriteFactory's SCSS is a bit too verbose for our purposes here, so
+      # let's simplify it
+      system(%Q(sed -i '' "s/width: #{SIZE}px; height: #{SIZE}px; background: image-url('emoji.png')/background-position:/" #{style_path}))
+      system(%Q(sed -i '' "s/ no-repeat//" #{style_path}))
+
+      # Append a generic rule that applies to all Emojis
+      File.open(style_path, 'a') do |f|
+        f.puts
+        f.puts <<-CSS.strip_heredoc
+        .emoji-icon {
+          background-image: image-url('emoji.png');
+          background-repeat: no-repeat;
+          height: #{SIZE}px;
+          width: #{SIZE}px;
+
+          @media only screen and (-webkit-min-device-pixel-ratio: 2),
+                 only screen and (min--moz-device-pixel-ratio: 2),
+                 only screen and (-o-min-device-pixel-ratio: 2/1),
+                 only screen and (min-device-pixel-ratio: 2),
+                 only screen and (min-resolution: 192dpi),
+                 only screen and (min-resolution: 2dppx) {
+            background-image: image-url('emoji@2x.png');
+            background-size: 840px 820px;
+          }
+        }
+        CSS
+      end
+    end
+
+    # Now do it again but for Retina
+    Dir.mktmpdir do |tmpdir|
+      # Copy the Gemojione assets to the temporary folder for resizing
+      FileUtils.cp_r(Gemojione.index.images_path, tmpdir)
+
+      Dir.chdir(tmpdir) do
+        Dir["**/*.png"].each do |png|
+          resize!(File.join(tmpdir, png), RETINA)
+        end
+      end
+
+      # Combine the resized assets into a packed sprite and re-generate the SCSS
+      SpriteFactory.run!(File.join(tmpdir, 'images'), {
+        output_image: "app/assets/images/emoji@2x.png",
+        style:        false,
+        nocomments:   true,
+        pngcrush:     true,
+        layout:       :packed
+      })
+    end
+  end
+
+  def check_requirements!
+    return if defined?(SpriteFactory) && defined?(Magick)
+
+    puts <<-MSG.strip_heredoc
+      This task is disabled by default and should only be run when the Gemojione
+      gem is updated with new Emojis.
+
+      To enable this task, *temporarily* add the following lines to Gemfile and
+      re-bundle:
+
+      gem 'sprite-factory'
+      gem 'rmagick'
+    MSG
+
+    exit 1
+  end
+
+  def resize!(image_path, size)
+    # Resize the image in-place, save it, and free the object
+    image = Magick::Image.read(image_path).first
+    image.resize!(size, size)
+    image.write(image_path) { self.quality = 100 }
+    image.destroy!
+  end
+end