feat: retain bounded preview caches across selections and views

src/thumbnail.rs

@@ -18,6 +18,8 @@ use ratatui_image::{
 
 use crate::youtube::ThumbnailJob;
 
+const CACHE_SIZE: usize = 8;
+
 struct ThumbnailImage {
     id: String,
     protocol: ThreadProtocol,
@@ -31,8 +33,8 @@ pub struct Thumbnail {
     fetch: Option<ThumbnailJob>,
     decoder: Option<Receiver<Result<ThumbnailImage, String>>>,
     image: Option<ThumbnailImage>,
-    // ponytail: one completed thumbnail plus an in-flight replacement, not a video cache.
-    cached: Option<ThumbnailImage>,
+    // ponytail: eight-entry linear lookup; use a keyed LRU if the buffer grows.
+    cached: std::collections::VecDeque<ThumbnailImage>,
     error: Option<String>,
 }
 
@@ -44,7 +46,7 @@ impl Thumbnail {
             fetch: None,
             decoder: None,
             image: None,
-            cached: None,
+            cached: std::collections::VecDeque::with_capacity(CACHE_SIZE),
             error: None,
         }
     }
@@ -62,10 +64,16 @@ impl Thumbnail {
         if self.selected.as_deref() == Some(id) {
             return;
         }
+        // Remove a hit before storing the previous image so eviction cannot discard the target.
+        let cached = self
+            .cached
+            .iter()
+            .position(|image| image.id == id)
+            .and_then(|index| self.cached.remove(index));
         self.cancel();
         self.selected = Some(id.to_owned());
-        if self.cached.as_ref().is_some_and(|image| image.id == id) {
-            self.image = self.cached.take();
+        if let Some(image) = cached {
+            self.image = Some(image);
             return;
         }
         match ThumbnailJob::start(id) {
@@ -83,10 +91,18 @@ impl Thumbnail {
         if let Some(image) = self.image.take()
             && image.complete
         {
-            self.cached = Some(image);
+            if self.cached.len() == CACHE_SIZE {
+                self.cached.pop_front();
+            }
+            self.cached.push_back(image);
         }
     }
 
+    pub fn clear_cache(&mut self) {
+        self.cancel();
+        self.cached.clear();
+    }
+
     pub fn is_loading(&self) -> bool {
         self.selected.is_some()
             && self.error.is_none()
@@ -180,7 +196,6 @@ impl Thumbnail {
                     // area again before rendering, so rapid resizes cannot draw an old size.
                     if image.protocol.update_resized_protocol(response) {
                         image.complete = true;
-                        self.cached = None;
                     }
                 }
                 Ok(Err(error)) => {
@@ -281,6 +296,62 @@ mod tests {
         assert!(thumbnail.error.is_none());
     }
 
+    #[test]
+    fn thumbnail_hits_survive_a_full_buffer_and_promote_recent_images() {
+        let mut thumbnail = Thumbnail::new();
+        let completed = |id: String| {
+            let (requests, _receiver) = mpsc::channel();
+            let (_sender, replies) = mpsc::channel();
+            ThumbnailImage {
+                id,
+                protocol: ThreadProtocol::new(
+                    requests,
+                    Some(
+                        Picker::halfblocks()
+                            .new_resize_protocol(image::DynamicImage::new_rgb8(1, 1)),
+                    ),
+                ),
+                replies,
+                complete: true,
+            }
+        };
+        for index in 0..CACHE_SIZE {
+            thumbnail.image = Some(completed(format!("video{index:06}")));
+            thumbnail.cancel();
+        }
+        thumbnail.image = Some(completed("video000008".into()));
+        thumbnail.selected = Some("video000008".into());
+        // This is the oldest cached image: storing the outgoing image first would evict it.
+        thumbnail.select("video000000");
+        assert_eq!(thumbnail.image.as_ref().unwrap().id, "video000000");
+        assert!(thumbnail.fetch.is_none());
+        thumbnail.select("video000001");
+        assert_eq!(thumbnail.image.as_ref().unwrap().id, "video000001");
+        assert!(thumbnail.fetch.is_none());
+        thumbnail.cancel();
+        assert!(
+            !thumbnail
+                .cached
+                .iter()
+                .any(|image| image.id == "video000002")
+        );
+        assert!(
+            thumbnail
+                .cached
+                .iter()
+                .any(|image| image.id == "video000000")
+        );
+        assert!(
+            thumbnail
+                .cached
+                .iter()
+                .any(|image| image.id == "video000008")
+        );
+        thumbnail.clear_cache();
+        assert!(thumbnail.cached.is_empty());
+        assert!(thumbnail.image.is_none());
+    }
+
     #[test]
     fn small_thumbnail_fills_popup_proportionally_and_resizes() {
         use ratatui::{Terminal, backend::TestBackend, style::Color};