Add automatic APNG detection (#2237)

app/logical/apng_inspector.rb

@@ -0,0 +1,70 @@
+class APNGInspector
+  attr_reader :frames
+  
+  PNG_MAGIC_NUMBER = ["89504E470D0A1A0A"].pack('H*')
+  
+  def initialize(file_path)
+    @file_path = file_path
+    @corrupted = false
+    @animated = false
+  end
+  
+  #Calls associated block for each PNG chunk
+  #parameters passed are |chunk_name, chunk_length, file_descriptor|
+  #returns true if file is read succesfully from start to IEND, false otherwise. 
+  def each_chunk
+    iend_reached = false
+    File.open(@file_path, 'rb') do |file|
+      return false if file.read(8) != PNG_MAGIC_NUMBER
+      
+      while chunkheader = file.read(8)
+        return false if chunkheader.to_s.length != 8
+
+        chunk_len, chunk_name = chunkheader.unpack("Na4")
+        current_pos = file.tell
+        yield chunk_name, chunk_len, file
+        if chunk_name == "IEND"
+          iend_reached = true
+          break
+        end
+        file.seek(current_pos+chunk_len+4, IO::SEEK_SET)
+      end
+    end
+    return iend_reached
+  end
+  
+  def inspect!
+    actl_corrupted = false
+    
+    read_success = each_chunk do |name, len, file|
+      if name == 'acTL'
+        if len != 8 then
+          actl_corrupted = true
+        else
+          framedata = file.read(4)
+          if framedata.to_s.length != 4
+            actl_corrupted = true
+          else
+            framecount = framedata.unpack("N")[0]
+            if framecount < 1 
+              actl_corrupted = true
+            else
+              @animated = true
+              @frames = framecount
+            end
+          end
+        end
+      end
+    end
+    @corrupted = !read_success || actl_corrupted 
+    return !@corrupted
+  end
+  
+  def corrupted?
+    @corrupted
+  end
+  
+  def animated?
+    !@corrupted && @animated && @frames > 1
+  end
+end

app/models/post.rb

@@ -163,6 +163,16 @@ class Post < ActiveRecord::Base
         return false
       end
     end
+    
+    def is_animated_png?
+      if file_ext =~ /png/i
+        apng = APNGInspector.new(file_path)
+        apng.inspect!
+        return apng.animated?
+      else
+        return false
+      end
+    end
 
     def is_flash?
       file_ext =~ /swf/i
@@ -607,7 +617,7 @@ class Post < ActiveRecord::Base
     def add_automatic_tags(tags)
       return tags if !Danbooru.config.enable_dimension_autotagging
 
-      tags -= %w(incredibly_absurdres absurdres highres lowres huge_filesize animated_gif flash webm mp4)
+      tags -= %w(incredibly_absurdres absurdres highres lowres huge_filesize animated_gif animated_png flash webm mp4)
 
       if has_dimensions?
         if image_width >= 10_000 || image_height >= 10_000
@@ -639,6 +649,10 @@ class Post < ActiveRecord::Base
       if is_animated_gif?
         tags << "animated_gif"
       end
+      
+      if is_animated_png?
+        tags << "animated_png"
+      end
 
       if is_flash?
         tags << "flash"