feat(core): season tracked rule over revealed episodes

crates/arr-core/src/tracking.rs

@@ -29,11 +29,25 @@ pub fn apply_auto_track(series: &Series, seasons: &mut [RefreshedSeason]) {
     }
 }
 
+/// Applies a season's tracking rule (`DESIGN.md` §4.1) to its
+/// already-revealed episodes.
+///
+/// Turning tracking on marks every revealed episode wanted; future reveals
+/// follow while the season stays tracked. Turning it off withdraws nothing:
+/// leaf intent is never removed implicitly.
+pub fn apply_tracked(tracked: bool, episodes: &mut [Episode]) {
+    if tracked {
+        for episode in episodes {
+            episode.wanted = true;
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::time::SystemTime;
 
-    use super::{apply_auto_track, RefreshedSeason};
+    use super::{apply_auto_track, apply_tracked, RefreshedSeason};
     use crate::{
         Episode, EpisodeId, Language, MediaState, RootId, Season, SeasonId, Series, SeriesId,
         TitleOverrides,
@@ -131,4 +145,55 @@ mod tests {
         assert!(!refresh[0].season.tracked);
         assert!(refresh[0].episodes.iter().all(|episode| !episode.wanted));
     }
+
+    #[test]
+    fn tracked_on_marks_revealed_episodes_wanted() {
+        let cases: [&[bool]; 4] = [
+            &[],
+            &[false],
+            &[true, false, false],
+            &[false, true, false, true],
+        ];
+        for wanted in cases {
+            let mut episodes = wanted
+                .iter()
+                .enumerate()
+                .map(|(index, wanted)| {
+                    episode(1, u16::try_from(index + 1).expect("episode number fits"), *wanted)
+                })
+                .collect::<Vec<_>>();
+
+            apply_tracked(true, &mut episodes);
+
+            assert!(
+                episodes.iter().all(|episode| episode.wanted),
+                "tracked on must mark every revealed episode wanted, input {wanted:?}"
+            );
+        }
+    }
+
+    #[test]
+    fn tracked_off_withdraws_nothing() {
+        let cases: [&[bool]; 4] = [
+            &[],
+            &[false],
+            &[true, false, false],
+            &[false, true, false, true],
+        ];
+        for wanted in cases {
+            let mut episodes = wanted
+                .iter()
+                .enumerate()
+                .map(|(index, wanted)| {
+                    episode(1, u16::try_from(index + 1).expect("episode number fits"), *wanted)
+                })
+                .collect::<Vec<_>>();
+            let before = episodes.iter().map(|episode| episode.wanted).collect::<Vec<_>>();
+
+            apply_tracked(false, &mut episodes);
+
+            let after = episodes.iter().map(|episode| episode.wanted).collect::<Vec<_>>();
+            assert_eq!(before, after, "tracked off must not touch intent");
+        }
+    }
 }