Fix decoding error with trailing comment extension blocks

The GIF files that can be found on the Internet come in a wide variety
of forms. Some strictly adhere to the original specification, others do
not and differ in the actual sequence of blocks or their number.

For this reason, my decoder has a kind of "virtual" FrameBlock, which
can contain all possible blocks in different order that occur in a GIF
animation.

- Graphic Control Extension
- Image Description
- Local Color Table
- Image Data Block
- Plain Text Extension
- Application Extension
- Comment Extension

The TableBasedImage block, which is a chain of ImageDescriptor, (Local
Color Table) and ImageData, is used as a feature for terminating a
FrameBlock.

So far I have only seen GIF files that follow this scheme. However,
there was one example where one (or more) comment extensions were added
before the end. Here the decoding process failed.

This fix also adds "global comments" that are not part of my FrameBlock
and are appended to the GifDataStream afterwards.

src/Decoders/GifDataStreamDecoder.php

@@ -4,7 +4,9 @@
 
 namespace Intervention\Gif\Decoders;
 
+use Intervention\Gif\AbstractExtension;
 use Intervention\Gif\Blocks\ColorTable;
+use Intervention\Gif\Blocks\CommentExtension;
 use Intervention\Gif\Blocks\FrameBlock;
 use Intervention\Gif\Blocks\Header;
 use Intervention\Gif\Blocks\LogicalScreenDescriptor;
@@ -38,7 +40,16 @@ public function decode(): GifDataStream
         }
 
         while ($this->viewNextByte() != Trailer::MARKER) {
-            $gif->addFrame(FrameBlock::decode($this->handle));
+            match ($this->viewNextBytes(2)) {
+                // trailing "global" comment blocks which are not part of "FrameBlock"
+                AbstractExtension::MARKER . CommentExtension::LABEL
+                => $gif->addComment(
+                    CommentExtension::decode($this->handle)
+                ),
+                default => $gif->addFrame(
+                    FrameBlock::decode($this->handle)
+                ),
+            };
         }
 
         return $gif;

src/Encoders/GifDataStreamEncoder.php

@@ -30,6 +30,7 @@ public function encode(): string
             $this->source->getLogicalScreenDescriptor()->encode(),
             $this->maybeEncodeGlobalColorTable(),
             $this->encodeFrames(),
+            $this->encodeComments(),
             $this->source->getTrailer()->encode(),
         ]);
     }
@@ -54,4 +55,16 @@ protected function encodeFrames(): string
             return $frame->encode();
         }, $this->source->getFrames()));
     }
+
+    /**
+     * Encode comment extension blocks of source
+     *
+     * @return string
+     */
+    protected function encodeComments(): string
+    {
+        return implode('', array_map(function ($commentExtension) {
+            return $commentExtension->encode();
+        }, $this->source->getComments()));
+    }
 }

src/GifDataStream.php

@@ -5,6 +5,7 @@
 namespace Intervention\Gif;
 
 use Intervention\Gif\Blocks\ColorTable;
+use Intervention\Gif\Blocks\CommentExtension;
 use Intervention\Gif\Blocks\FrameBlock;
 use Intervention\Gif\Blocks\Header;
 use Intervention\Gif\Blocks\LogicalScreenDescriptor;
@@ -20,7 +21,8 @@ public function __construct(
         protected Header $header = new Header(),
         protected LogicalScreenDescriptor $logicalScreenDescriptor = new LogicalScreenDescriptor(),
         protected ?ColorTable $globalColorTable = null,
-        protected array $frames = []
+        protected array $frames = [],
+        protected array $comments = []
     ) {
     }
 
@@ -122,6 +124,16 @@ public function getFrames(): array
         return $this->frames;
     }
 
+    /**
+     * Return array of "global" comments
+     *
+     * @return array
+     */
+    public function getComments(): array
+    {
+        return $this->comments;
+    }
+
     /**
      * Return first frame
      *
@@ -149,6 +161,19 @@ public function addFrame(FrameBlock $frame): self
         return $this;
     }
 
+    /**
+     * Add frame
+     *
+     * @param FrameBlock $frame
+     * @return GifDataStream
+     */
+    public function addComment(CommentExtension $comment): self
+    {
+        $this->comments[] = $comment;
+
+        return $this;
+    }
+
     /**
      * Set the current data
      *

tests/GifDataStreamTest.php

@@ -153,4 +153,16 @@ public function testDecode()
         }, $gif->getFrames()));
         $this->assertEquals([0, 0, 0, 0, 0, 0, 0, 0], $sizes);
     }
+
+    public function testDecodeTrailingComment(): void
+    {
+        $gif = GifDataStream::decode(
+            $this->getTestHandle(
+                file_get_contents(__DIR__ . '/images/animation_trailing_comment.gif')
+            ),
+        );
+
+        $this->assertInstanceOf(GifDataStream::class, $gif);
+        $this->assertCount(1, $gif->getComments());
+    }
 }