Support GIF files without 0x3B trailer byte

packages/image_size_getter/lib/src/decoder/impl/gif_decoder.dart

@@ -41,6 +41,19 @@ class GifDecoder extends BaseDecoder with MutilFileHeaderAndFooterValidator {
   MutilFileHeaderAndFooter get headerAndFooter => _GifInfo();
 }
 
+/// Internal class that defines GIF file format validation rules.
+///
+/// This class supports both standard and non-standard GIF files:
+/// - Standard GIF files end with the 0x3B trailer byte
+/// - Non-standard GIF files may omit the 0x3B trailer
+///
+/// The relaxed footer validation (accepting files without 0x3B) is intentional
+/// because:
+/// 1. Some valid GIF files produced by certain encoders don't include the trailer
+/// 2. The GIF size information is stored in the header (bytes 6-10), not the footer
+/// 3. The primary purpose of this library is to extract image dimensions, not
+///    to perform comprehensive file validation
+/// 4. The GIF header is still validated, ensuring basic format correctness
 class _GifInfo with MutilFileHeaderAndFooter {
   static const start89a = [
     0x47,
@@ -60,9 +73,14 @@ class _GifInfo with MutilFileHeaderAndFooter {
   ];
 
   static const end = [0x3B];
+
+  // Allow GIF files without the standard 0x3B trailer.
+  // Some GIF encoders create valid GIF files without the trailer byte.
+  // We provide both options: with trailer and without (empty list).
+  static const emptyEnd = <int>[];
 
   @override
-  List<List<int>> get mutipleEndBytesList => [end];
+  List<List<int>> get mutipleEndBytesList => [end, emptyEnd];
 
   @override
   List<List<int>> get mutipleStartBytesList => [

packages/image_size_getter/test/non_standard_gif_test.dart

@@ -0,0 +1,66 @@
+import 'dart:io';
+
+import 'package:image_size_getter/file_input.dart';
+import 'package:image_size_getter/image_size_getter.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Test non-standard GIF (without 0x3B ending)', () {
+    test('Test GIF without 0x3B trailer', () {
+      final gif = File('../../example/asset/non_standard_ending.gif');
+
+      const GifDecoder decoder = GifDecoder();
+      final input = FileInput(gif);
+
+      // The file should still be valid even without 0x3B ending
+      expect(decoder.isValid(input), isTrue, 
+        reason: 'GIF should be valid even without 0x3B trailer');
+
+      // Size should still be readable
+      expect(decoder.getSize(input), Size(200, 150));
+    });
+
+    test('Test standard GIF with 0x3B trailer for comparison', () {
+      final gif = File('../../example/asset/87a.gif');
+
+      const GifDecoder decoder = GifDecoder();
+      final input = FileInput(gif);
+
+      expect(decoder.isValid(input), isTrue);
+      expect(decoder.getSize(input), Size(200, 150));
+    });
+
+    test('Test GIF89a format with and without trailer', () {
+      final gif89a = File('../../example/asset/dialog.gif');
+
+      const GifDecoder decoder = GifDecoder();
+      final input = FileInput(gif89a);
+
+      // Standard GIF89a should still work
+      expect(decoder.isValid(input), isTrue);
+      expect(decoder.getSize(input), Size(688, 1326));
+    });
+
+    test('Test that invalid headers are still rejected', () {
+      // Create a file with invalid header in a temporary directory
+      final tempDir = Directory.systemTemp.createTempSync('gif_test_');
+      final tempFile = File('${tempDir.path}/temp_invalid.gif');
+
+      try {
+        tempFile.writeAsBytesSync([0x47, 0x49, 0x46, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]);
+
+        const GifDecoder decoder = GifDecoder();
+        final input = FileInput(tempFile);
+
+        // Invalid GIF header should be rejected
+        expect(decoder.isValid(input), isFalse, 
+          reason: 'File with invalid GIF header should be rejected');
+      } finally {
+        // Clean up temp directory and all its contents
+        if (tempDir.existsSync()) {
+          tempDir.deleteSync(recursive: true);
+        }
+      }
+    });
+  });
+}