mp4: More robust parsing and updated mapping of rtng atom

src/mp4/ilst/atom.rs

@@ -216,33 +216,44 @@ pub enum AtomData {
 }
 
 /// The parental advisory rating
+///
+/// See also:
+/// <https://docs.mp3tag.de/mapping/#itunesadvisory>
+/// <https://exiftool.org/TagNames/QuickTime.html>
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum AdvisoryRating {
-	/// A rating of 0
+	/// *Inoffensive*/*None* (0)
 	Inoffensive,
-	/// A rating of 2
-	Clean,
-	/// A rating of (1 || > 2)
+	/// *Explicit* (1 or 4)
+	///
+	/// In the past Apple used the value 4 for explicit content
+	/// that has later been replaced by 1. Both values are considered
+	/// as valid when reading but only the newer value 1 is written.
 	Explicit,
+	/// *Clean*/*Edited* (2)
+	Clean,
 }
 
 impl AdvisoryRating {
 	/// Returns the rating as it appears in the `rtng` atom
 	pub fn as_u8(&self) -> u8 {
 		match self {
 			AdvisoryRating::Inoffensive => 0,
+			AdvisoryRating::Explicit => 1,
 			AdvisoryRating::Clean => 2,
-			AdvisoryRating::Explicit => 4,
 		}
 	}
 }
 
-impl From<u8> for AdvisoryRating {
-	fn from(input: u8) -> Self {
+impl TryFrom<u8> for AdvisoryRating {
+	type Error = u8;
+
+	fn try_from(input: u8) -> Result<Self, Self::Error> {
 		match input {
-			0 => Self::Inoffensive,
-			2 => Self::Clean,
-			_ => Self::Explicit,
+			0 => Ok(Self::Inoffensive),
+			1 | 4 => Ok(Self::Explicit),
+			2 => Ok(Self::Clean),
+			value => Err(value),
 		}
 	}
 }

src/mp4/ilst/mod.rs

@@ -152,17 +152,15 @@ impl Ilst {
 
 	/// Returns the parental advisory rating according to the `rtng` atom
 	pub fn advisory_rating(&self) -> Option<AdvisoryRating> {
-		if let Some(atom) = self.atom(&ADVISORY_RATING) {
-			let rating = match atom.data().next() {
-				Some(AtomData::SignedInteger(si)) => *si as u8,
-				Some(AtomData::Unknown { data: c, .. }) if !c.is_empty() => c[0],
-				_ => return None,
-			};
-
-			return Some(AdvisoryRating::from(rating));
-		}
-
-		None
+		self.atom(&ADVISORY_RATING)
+			.into_iter()
+			.flat_map(Atom::data)
+			.filter_map(|data| match data {
+				AtomData::SignedInteger(si) => u8::try_from(*si).ok(),
+				AtomData::Unknown { data, .. } => data.first().copied(),
+				_ => None,
+			})
+			.find_map(|rating| AdvisoryRating::try_from(rating).ok())
 	}
 
 	/// Sets the advisory rating