1 files changed, 99 insertions, 2 deletions
diff --git a/lib/paperclip/gif_transcoder.rb b/lib/paperclip/gif_transcoder.rb
index cbab6fd99..64f12f963 100644
--- a/lib/paperclip/gif_transcoder.rb
+++ b/lib/paperclip/gif_transcoder.rb
@@ -1,5 +1,103 @@
 # frozen_string_literal: true
 
+class GifReader
+  attr_reader :animated
+
+  EXTENSION_LABELS = [0xf9, 0x01, 0xff].freeze
+  GIF_HEADERS      = %w(GIF87a GIF89a).freeze
+
+  class GifReaderException; end
+
+  class UnknownImageType < GifReaderException; end
+
+  class CannotParseImage < GifReaderException; end
+
+  def self.animated?(path)
+    new(path).animated
+  rescue GifReaderException
+    false
+  end
+
+  def initialize(path, max_frames = 2)
+    @path      = path
+    @nb_frames = 0
+
+    File.open(path, 'rb') do |s|
+      raise UnknownImageType unless GIF_HEADERS.include?(s.read(6))
+
+      # Skip to "packed byte"
+      s.seek(4, IO::SEEK_CUR)
+
+      # "Packed byte" gives us the size of the GIF color table
+      packed_byte, = s.read(1).unpack('C')
+
+      # Skip background color and aspect ratio
+      s.seek(2, IO::SEEK_CUR)
+
+      if packed_byte & 0x80 != 0
+        # GIF uses a global color table, skip it
+        s.seek(3 * (1 << ((packed_byte & 0x07) + 1)), IO::SEEK_CUR)
+      end
+
+      # Now read data
+      while @nb_frames < max_frames
+        separator = s.read(1)
+
+        case separator
+        when ',' # Image block
+          @nb_frames += 1
+
+          # Skip to "packed byte"
+          s.seek(8, IO::SEEK_CUR)
+          packed_byte, = s.read(1).unpack('C')
+
+          if packed_byte & 0x80 != 0
+            # Image uses a local color table, skip it
+            s.seek(3 * (1 << ((packed_byte & 0x07) + 1)), IO::SEEK_CUR)
+          end
+
+          # Skip lzw min code size
+          raise InvalidValue unless s.read(1).unpack('C')[0] >= 2
+
+          # Skip image data sub-blocks
+          skip_sub_blocks!(s)
+        when '!' # Extension block
+          skip_extension_block!(s)
+        when ';' # Trailer
+          break
+        else
+          raise CannotParseImage
+        end
+      end
+    end
+
+    @animated = @nb_frames > 1
+  end
+
+  private
+
+  def skip_extension_block!(file)
+    if EXTENSION_LABELS.include?(file.read(1).unpack('C')[0])
+      block_size, = file.read(1).unpack('C')
+      file.seek(block_size, IO::SEEK_CUR)
+    end
+
+    # Read until extension block end marker
+    skip_sub_blocks!(file)
+  end
+
+  # Skip sub-blocks up until block end marker
+  def skip_sub_blocks!(file)
+    loop do
+      size, = file.read(1).unpack('C')
+
+      break if size.zero?
+
+      file.seek(size, IO::SEEK_CUR)
+    end
+  end
+end
+
 module Paperclip
   # This transcoder is only to be used for the MediaAttachment model
   # to convert animated gifs to webm
@@ -19,8 +117,7 @@ module Paperclip
     private
 
     def needs_convert?
-      num_frames = identify('-format %n :file', file: file.path).to_i
-      options[:style] == :original && num_frames > 1
+      options[:style] == :original && GifReader.animated?(file.path)
     end
   end
 end