MP4: Use all available free atoms

closes #346

CHANGELOG.md

@@ -10,6 +10,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Truncate**: `impl<T: Truncate> Truncate for &mut T` ([PR](https://github.com/Serial-ATA/lofty-rs/pull/384))
 - **Length**: `impl<T: Length> Truncate for &T` ([PR](https://github.com/Serial-ATA/lofty-rs/pull/384))
 
+### Changed
+- **MP4**: All surrounding `free` atoms will be used when writing `ilst` tags ([issue](https://github.com/Serial-ATA/lofty-rs/issues/346)) ([PR](https://github.com/Serial-ATA/lofty-rs/pull/386))
+  - Previously, only the `free` atoms immediately surrounding the `ilst` atom were used.
+
 ## [0.19.0] - 2024-04-21
 
 ### Added

lofty/src/mp4/ilst/write.rs

@@ -256,21 +256,35 @@ fn save_to_existing(
 
 			// Check for one directly before the `ilst` atom
 			if ilst_idx > 0 {
-				let previous_atom = &tree[ilst_idx - 1];
-
-				if previous_atom.ident == AtomIdent::Fourcc(*b"free") {
-					range_start = previous_atom.start;
-					available_space += previous_atom.len;
+				let mut i = ilst_idx;
+				while i != 0 {
+					i -= 1;
+					let atom = &tree[i];
+					if atom.ident != AtomIdent::Fourcc(*b"free") {
+						break;
+					}
+
+					available_space += atom.len;
+					range_start = atom.start;
 				}
+
+				log::trace!("Found {} preceding `free` atoms", ilst_idx - i)
 			}
 
 			// And after
 			if ilst_idx != tree.len() - 1 {
-				let next_atom = &tree[ilst_idx + 1];
-
-				if next_atom.ident == AtomIdent::Fourcc(*b"free") {
-					available_space += next_atom.len;
+				let mut i = ilst_idx + 1;
+				while i < tree.len() {
+					let atom = &tree[i];
+					if atom.ident != AtomIdent::Fourcc(*b"free") {
+						break;
+					}
+
+					available_space += atom.len;
+					i += 1;
 				}
+
+				log::trace!("Found {} succeeding `free` atoms", (i - 1) - ilst_idx)
 			}
 
 			let ilst_len = ilst.len() as u64;