Preserve chronological order in gapless gallery rows

_plugins/gallery_items.rb

@@ -1,6 +1,53 @@
 require 'nokogiri'
 
 module GalleryItems
+  JPEG_START_OF_FRAME = [0xC0, 0xC1, 0xC2, 0xC3, 0xC5, 0xC6, 0xC7,
+                         0xC9, 0xCA, 0xCB, 0xCD, 0xCE, 0xCF].freeze
+
+  # Read just the dimensions from a local JPEG. The rendered gallery needs
+  # aspect ratios to size each chronological pair without cropping either photo.
+  def jpeg_dimensions(path)
+    File.open(path, 'rb') do |file|
+      return nil unless file.read(2) == "\xFF\xD8".b
+
+      while (prefix = file.read(1))
+        next unless prefix.getbyte(0) == 0xFF
+
+        marker = file.read(1)&.getbyte(0)
+        break unless marker
+        next if marker == 0xFF || marker == 0x01 || (0xD0..0xD7).cover?(marker)
+        break if marker == 0xD9 || marker == 0xDA
+
+        size = file.read(2)&.unpack1('n')
+        break unless size && size >= 2
+        if JPEG_START_OF_FRAME.include?(marker)
+          dimensions = file.read(5)
+          return nil unless dimensions&.length == 5
+
+          height, width = dimensions.byteslice(1, 4).unpack('n2')
+          return [width, height] if width.positive? && height.positive?
+          return nil
+        end
+        file.seek(size - 2, IO::SEEK_CUR)
+      end
+    end
+    nil
+  rescue Errno::ENOENT, EOFError
+    nil
+  end
+
+  def gallery_image_ratio(content)
+    image = Nokogiri::HTML::DocumentFragment.parse(content).at_css('img')
+    source = image&.[]('src')
+    return 1.5 unless source&.start_with?('/img/arts/')
+
+    path = File.join(File.expand_path('..', __dir__), source.delete_prefix('/'))
+    width, height = jpeg_dimensions(path)
+    return 1.5 unless width && height
+
+    (width.to_f / height).round(4)
+  end
+
   # Uploaded batches share one source document, but each photo is a gallery item.
   # Keeping this at render time also fixes batches that have already been published.
   def gallery_items(content)