fix: calculate ID3v2 duration at the microsecond (#54)

We had an issue where the library was calculating the duration, as described in issue #49. Files with a duration under 1 second were incorrect.

The formula was rounding the values to seconds. This is problematic because it can compute microseconds and therefore achieve better duration precision.

Author: ClementBeal

lib/src/parsers/id3v2.dart

@@ -259,18 +259,29 @@ class ID3v2Parser extends TagParser {
         if (xingFrameFlag == 1) {
           final numberOfFrames =
               getUint32(possibleXingHeader.sublist(i + 8, i + 12));
-          metadata.duration = Duration(
-              seconds: numberOfFrames *
-                  (_getSamplePerFrame(mpegVersion, mpegLayer) ?? 0) ~/
-                  metadata.samplerate!);
+          final samplesPerFrame =
+              _getSamplePerFrame(mpegVersion, mpegLayer) ?? 0;
+          final sampleRate = metadata.samplerate;
+
+          if (sampleRate != null && sampleRate > 0 && samplesPerFrame > 0) {
+            final totalSamples = numberOfFrames * samplesPerFrame;
+            final durationInSeconds = totalSamples / sampleRate;
+
+            final durationInMicroseconds =
+                (durationInSeconds * 1000000).toInt();
+            metadata.duration = Duration(microseconds: durationInMicroseconds);
+          }
         }
       } else {
         // it's a CBR file (Constant Bit Rate)
         if (metadata.bitrate != null && metadata.bitrate! > 0) {
           final fileSizeWithoutMetadata = reader.lengthSync() - size;
-          metadata.duration = Duration(
-              seconds:
-                  (8 * fileSizeWithoutMetadata / metadata.bitrate!).round());
+          final durationInSeconds =
+              (8 * fileSizeWithoutMetadata) / metadata.bitrate!;
+
+          // Convert to microseconds
+          final durationInMicroseconds = (durationInSeconds * 1000000).toInt();
+          metadata.duration = Duration(microseconds: durationInMicroseconds);
         }
       }
     }

test/mp3/generated_under_one_second.mp3

@@ -15,7 +15,7 @@ void main() {
     expect(result.sampleRate, equals(44100));
     expect(result.title, equals("Title"));
     expect(result.trackNumber, equals(1));
-    expect(result.duration, equals(Duration(seconds: 1)));
+    expect(result.duration!.inMilliseconds, closeTo(1130, 10));
     expect(result.totalDisc, equals(1));
     expect(result.lyrics, equals("Lyrics"));
     expect(result.trackTotal, equals(10));
@@ -43,7 +43,8 @@ void main() {
     expect(result.title, "How to Fly");
     expect(result.artist, "Sticky Fingers");
     expect(result.year, DateTime(2013));
-    expect(result.duration, Duration(minutes: 3, seconds: 22));
+    expect(
+        result.duration, Duration(minutes: 3, seconds: 22, milliseconds: 240));
     expect(result.sampleRate, 44100);
   });
 
@@ -60,4 +61,12 @@ void main() {
     expect(result.year, DateTime(2013));
     expect(result.sampleRate, 44100);
   });
+
+  test("Round duration to microseconds", () {
+    final track = File("./test/mp3/generated_under_one_second.mp3");
+    final result = readMetadata(track, getImage: false);
+    expect(result.pictures.length, 0);
+    expect(result.duration, isNotNull);
+    expect(result.duration!.inMilliseconds, closeTo(310, 5));
+  });
 }