Update APNGInspector class and add tests (fixes #2237)

test/unit/apng_inspector_test.rb

@@ -0,0 +1,64 @@
+require "test_helper"
+
+class DTextTest < ActiveSupport::TestCase
+  def inspect(filename)
+    apng = APNGInspector.new("#{Rails.root}/test/files/apng/#{filename}")
+    apng.inspect!
+    apng
+  end
+  context "APNG inspector" do
+    should "correctly parse normal APNG file" do
+      apng = inspect('normal_apng.png')
+      assert_equal(3, apng.frames)
+      assert_equal(true, apng.animated?)
+      assert_equal(false, apng.corrupted?)
+    end
+
+    should "recognize 1-frame APNG as animated" do
+      apng = inspect('single_frame.png')
+      assert_equal(1, apng.frames)
+      assert_equal(true, apng.animated?)
+      assert_equal(false, apng.corrupted?)
+    end
+
+    should "correctly parse normal PNG file" do
+      apng = inspect('not_apng.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(false, apng.corrupted?)
+    end
+
+    should "handle empty file" do
+      apng = inspect('empty.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+    should "handle corrupted files" do
+      apng = inspect('iend_missing.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+      apng = inspect('misaligned_chunks.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+      apng = inspect('broken.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+    should "handle incorrect acTL chunk" do
+      apng = inspect('actl_wronglen.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+      apng = inspect('actl_zero_frames.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+    should "handle non-png files" do
+      apng = inspect('jpg.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+  end
+end