Use intrusive list to track LRU order

src/lib.rs

@@ -112,7 +112,7 @@ fn try_allocate(atlas: &mut InnerAtlas, width: usize, height: usize) -> Option<A
         }
 
         // Try to free least recently used allocation
-        let (key, value) = atlas.glyph_cache.entries_least_recently_used().next()?;
+        let (key, value) = atlas.glyph_cache.pop()?;
         atlas
             .packer
             .deallocate(value.atlas_id.expect("cache corrupt"));

src/recently_used.rs

@@ -1,60 +1,175 @@
 use std::{
     borrow::Borrow,
     collections::{hash_map::Entry, HashMap},
+    fmt,
     hash::Hash,
 };
 
 struct RecentlyUsedItem<V> {
-    entry_idx: usize,
+    node_idx: usize,
     value: V,
 }
 
-pub struct RecentlyUsedMap<K: Clone + Copy + Eq + Hash, V> {
-    keys: Vec<K>,
-    map: HashMap<K, RecentlyUsedItem<V>>,
+#[derive(Debug)]
+enum Node<V> {
+    Value {
+        value: V,
+        prev_idx: Option<usize>,
+        next_idx: Option<usize>,
+    },
+    Free {
+        next_idx: Option<usize>,
+    },
 }
 
-impl<K: Clone + Copy + Eq + Hash, V> RecentlyUsedMap<K, V> {
+pub struct RecentlyUsedMap<K: Clone + Copy + Eq + Hash, V> {
+    nodes: Vec<Node<K>>,
+    least_recent_idx: Option<usize>,
+    most_recent_idx: Option<usize>,
+    map: HashMap<K, RecentlyUsedItem<V>>,
+    free: Option<usize>,
+}
+
+impl<K: Clone + Copy + Eq + Hash + fmt::Debug, V> RecentlyUsedMap<K, V> {
     pub fn new() -> Self {
         Self::with_capacity(0)
     }
 
     pub fn with_capacity(capacity: usize) -> Self {
+        assert!(capacity < usize::MAX - 1, "capacity too large");
         Self {
-            keys: Vec::with_capacity(capacity),
+            nodes: Vec::with_capacity(capacity),
+            least_recent_idx: None,
+            most_recent_idx: None,
+            free: None,
             map: HashMap::with_capacity(capacity),
         }
     }
 
-    pub fn insert(&mut self, key: K, value: V) {
-        let new_idx = self.keys.len();
-        match self.map.entry(key) {
-            Entry::Occupied(mut occupied) => {
-                let old = occupied.insert(RecentlyUsedItem {
-                    entry_idx: new_idx,
-                    value,
-                });
-                let removed = self.keys.remove(old.entry_idx);
-                self.keys.push(removed);
+    fn allocate_node(&mut self, node: Node<K>) -> usize {
+        match self.free {
+            Some(idx) => {
+                // Reuse a node from storage
+                match self.nodes[idx] {
+                    Node::Free { next_idx } => {
+                        self.free = next_idx;
                     }
-            Entry::Vacant(vacant) => {
-                vacant.insert(RecentlyUsedItem {
-                    entry_idx: new_idx,
-                    value,
-                });
-                self.keys.push(key);
+                    Node::Value { .. } => unreachable!(),
+                }
+
+                self.nodes[idx] = node;
+
+                idx
+            }
+            None => {
+                // Create a new storage node
+                self.nodes.push(node);
+                self.nodes.len() - 1
             }
         }
     }
 
-    pub fn remove<Q: ?Sized>(&mut self, key: &Q)
+    pub fn insert(&mut self, key: K, value: V) {
+        let new_idx = self.allocate_node(Node::Value {
+            value: key,
+            prev_idx: self.most_recent_idx,
+            next_idx: None,
+        });
+
+        let prior_most_recent = self.most_recent_idx.map(|idx| &mut self.nodes[idx]);
+        match prior_most_recent {
+            Some(p) => {
+                // Update prior most recent if one exists
+                match p {
+                    Node::Value { next_idx, .. } => {
+                        *next_idx = Some(new_idx);
+                    }
+                    Node::Free { .. } => unreachable!(),
+                }
+            }
+            None => {
+                // Update least recent if this is the first node
+                self.least_recent_idx = Some(new_idx);
+            }
+        }
+
+        self.most_recent_idx = Some(new_idx);
+
+        match self.map.entry(key) {
+            Entry::Occupied(mut occupied) => {
+                let old = occupied.insert(RecentlyUsedItem {
+                    node_idx: new_idx,
+                    value,
+                });
+
+                // Remove the old occurrence of this key
+                self.remove_node(old.node_idx);
+            }
+            Entry::Vacant(vacant) => {
+                vacant.insert(RecentlyUsedItem {
+                    node_idx: new_idx,
+                    value,
+                });
+            }
+        }
+    }
+
+    fn remove_node(&mut self, idx: usize) {
+        match self.nodes[idx] {
+            Node::Value {
+                prev_idx, next_idx, ..
+            } => {
+                match prev_idx {
+                    Some(prev) => match &mut self.nodes[prev] {
+                        Node::Value {
+                            next_idx: old_next_idx,
+                            ..
+                        } => {
+                            *old_next_idx = next_idx;
+                        }
+                        Node::Free { .. } => unreachable!(),
+                    },
+                    None => {
+                        // This node was the least recent
+                        self.least_recent_idx = next_idx;
+                    }
+                }
+
+                match next_idx {
+                    Some(next) => match &mut self.nodes[next] {
+                        Node::Value {
+                            prev_idx: old_prev_idx,
+                            ..
+                        } => {
+                            *old_prev_idx = prev_idx;
+                        }
+                        Node::Free { .. } => unreachable!(),
+                    },
+                    None => {
+                        // This node was the most recent
+                        self.most_recent_idx = prev_idx;
+                    }
+                }
+            }
+            Node::Free { .. } => unreachable!(),
+        }
+
+        // Add to free list
+        self.nodes[idx] = Node::Free {
+            next_idx: self.free,
+        };
+        self.free = Some(idx);
+    }
+
+    pub fn remove<Q: ?Sized>(&mut self, key: &Q) -> Option<V>
     where
         K: Borrow<Q>,
         Q: Hash + Eq,
     {
-        if let Some(entry) = self.map.remove(key.borrow()) {
-            self.keys.remove(entry.entry_idx);
-        }
+        self.map.remove(key.borrow()).map(|entry| {
+            self.remove_node(entry.node_idx);
+            entry.value
+        })
     }
 
     pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
@@ -73,8 +188,17 @@ impl<K: Clone + Copy + Eq + Hash, V> RecentlyUsedMap<K, V> {
         self.map.contains_key(k)
     }
 
-    pub fn entries_least_recently_used(&self) -> impl Iterator<Item = (K, &V)> + '_ {
-        self.keys.iter().map(|k| (*k, &self.map[k].value))
+    pub fn pop(&mut self) -> Option<(K, V)> {
+        let least_recent_idx = self.least_recent_idx?;
+        let least_recent_node = &self.nodes[least_recent_idx];
+
+        match *least_recent_node {
+            Node::Value { value: key, .. } => {
+                let value = self.remove(&key);
+                value.map(|v| (key, v))
+            }
+            Node::Free { .. } => unreachable!(),
+        }
     }
 }
 
@@ -87,12 +211,11 @@ mod tests {
         let mut rus = RecentlyUsedMap::new();
         rus.insert("a", ());
         rus.insert("b", ());
-        assert_eq!(
-            rus.entries_least_recently_used()
-                .map(|(k, _)| k)
-                .collect::<String>(),
-            "ab"
-        );
+        rus.insert("c", ());
+        assert_eq!(rus.pop(), Some(("a", ())));
+        assert_eq!(rus.pop(), Some(("b", ())));
+        assert_eq!(rus.pop(), Some(("c", ())));
+        assert_eq!(rus.pop(), None);
     }
 
     #[test]
@@ -102,12 +225,10 @@ mod tests {
         rus.insert("b", ());
         rus.insert("c", ());
         rus.insert("a", ());
-        assert_eq!(
-            rus.entries_least_recently_used()
-                .map(|(k, _)| k)
-                .collect::<String>(),
-            "bca"
-        );
+        assert_eq!(rus.pop(), Some(("b", ())));
+        assert_eq!(rus.pop(), Some(("c", ())));
+        assert_eq!(rus.pop(), Some(("a", ())));
+        assert_eq!(rus.pop(), None);
     }
 
     #[test]
@@ -117,11 +238,22 @@ mod tests {
         rus.insert("b", ());
         rus.remove("a");
         rus.remove("c");
-        assert_eq!(
-            rus.entries_least_recently_used()
-                .map(|(k, _)| k)
-                .collect::<String>(),
-            "b"
-        );
+        assert_eq!(rus.pop(), Some(("b", ())));
+        assert_eq!(rus.pop(), None);
+    }
+
+    #[test]
+    fn reuses_free_nodes() {
+        let mut rus = RecentlyUsedMap::new();
+        rus.insert("a", ());
+        rus.insert("b", ());
+        rus.insert("c", ());
+        rus.remove("b");
+        rus.remove("c");
+        rus.remove("a");
+        rus.insert("d", ());
+        rus.insert("e", ());
+        rus.insert("f", ());
+        assert_eq!(rus.nodes.len(), 3);
     }
 }