Properly support parsing MP3 files without headers. (#264)

* Properly support parsing MP3 files with no headers.

* Properly parse MP3 files without headers.

pedalboard/juce_overrides/juce_PatchedMP3AudioFormat.cpp

@@ -2125,11 +2125,10 @@ struct PatchedMP3Stream {
     auto oldPos = stream.getPosition();
     int offset = -3;
     uint32 header = 0;
-    bool isFirstScanAtThisPosition = true;
+    bool isParsingFirstFrame = oldPos == 0;
 
     for (;;) {
       auto streamPos = stream.getPosition();
-      bool isParsingFirstFrame = streamPos == 0;
 
       if (stream.isExhausted() || streamPos > oldPos + 32768) {
         offset = -1;
@@ -2156,15 +2155,22 @@ struct PatchedMP3Stream {
           break;
       }
 
-      if (isFirstScanAtThisPosition && isParsingFirstFrame) {
+      if (isParsingFirstFrame && offset == 0) {
         // If we're parsing the first frame of a file/stream, that frame must be
-        // at the start of the stream. This prevents accidentally parsing .mp4
-        // files (among others) as MP3 files that start with junk.
+        // at the start of the stream (i.e.: the first four bytes must be a
+        // valid MP3 header).
+        //
+        // If the first four bytes were a valid MP3 header, the above `if (...
+        // isValidHeader)` check would have exited the `for` loop, and we'd
+        // never get here.
+        //
+        // This prevents accidentally parsing .mp4 files (among others) as MP3
+        // files that start with junk.
+        offset = -1;
         break;
       }
 
       ++offset;
-      isFirstScanAtThisPosition = false;
     }
 
     if (offset >= 0) {

tests/test_io.py

@@ -1052,6 +1052,13 @@ def test_real_mp3_parsing_with_lyrics3():
         assert f.read(f.frames).shape[1] == f.frames
 
 
+def test_real_mp3_parsing_with_no_header():
+    filename = os.path.join(os.path.dirname(__file__), "audio", "correct", "no_header.mp3")
+    with pedalboard.io.AudioFile(filename) as f:
+        assert f.frames >= 40000 and f.frames <= 45000
+        assert f.read(f.frames).shape[1] == f.frames
+
+
 @pytest.mark.parametrize("samplerate", [44100, 32000])
 @pytest.mark.parametrize("chunk_size", [1, 2, 16])
 @pytest.mark.parametrize("target_samplerate", [44100, 32000, 22050, 1234.56])