Decode unsynchronized ID3v2 frames efficiently.

It makes a great difference when decoding huge unsynchronized ID3v2 frames.

NEWS

@@ -3,6 +3,7 @@ TagLib 1.11 (Jan 30, 2016)
 
  * Better handling of PCM WAV files with a 'fact' chunk.
  * Better handling of corrupted APE tags.
+ * Efficient decoding of unsynchronized ID3v2 frames.
  * Fixed text encoding when saving certain frames in ID3v2.3 tags.
  * Several smaller bug fixes and performance improvements.
 

taglib/mpeg/id3v2/id3v2synchdata.cpp

@@ -74,11 +74,29 @@ ByteVector SynchData::fromUInt(unsigned int value)
 
 ByteVector SynchData::decode(const ByteVector &data)
 {
+  // We have this optimized method instead of using ByteVector::replace(),
+  // since it makes a great difference when decoding huge unsynchronized frames.
+
+  if(data.size() < 2)
+    return data;
+
   ByteVector result = data;
 
-  ByteVector pattern(2, char(0));
-  pattern[0] = '\xFF';
-  pattern[1] = '\x00';
+  char *begin = result.data();
+  char *end = begin + result.size();
 
-  return result.replace(pattern, '\xFF');
+  char *dst = begin;
+  const char *src = begin;
+
+  do {
+    *dst++ = *src++;
+
+    if(*(src - 1) == '\xff' && *src == '\x00')
+      src++;
+
+  } while (src < end);
+
+  result.resize(static_cast<unsigned int>(dst - begin));
+
+  return result;
 }

tests/test_synchdata.cpp

@@ -40,6 +40,7 @@ class TestID3v2SynchData : public CppUnit::TestFixture
   CPPUNIT_TEST(testToUIntBrokenAndTooLarge);
   CPPUNIT_TEST(testDecode1);
   CPPUNIT_TEST(testDecode2);
+  CPPUNIT_TEST(testDecode3);
   CPPUNIT_TEST_SUITE_END();
 
 public:
@@ -104,6 +105,14 @@ public:
     CPPUNIT_ASSERT_EQUAL(ByteVector("\xff\x44", 2), a);
   }
 
+  void testDecode3()
+  {
+    ByteVector a("\xff\xff\x00", 3);
+    a = ID3v2::SynchData::decode(a);
+    CPPUNIT_ASSERT_EQUAL((unsigned int)2, a.size());
+    CPPUNIT_ASSERT_EQUAL(ByteVector("\xff\xff", 2), a);
+  }
+
 };
 
 CPPUNIT_TEST_SUITE_REGISTRATION(TestID3v2SynchData);