vtcode_commons/
lr_map.rs

1//! Lock-free concurrent map built on [`left_right`].
2//!
3//! [`LrMap`] keeps two copies of a `HashMap` — readers see one copy while the
4//! writer mutates the other. On publish the copies swap, giving readers a
5//! consistent, wait-free snapshot.
6//!
7//! Best for read-heavy workloads with infrequent writes (caches, registries).
8
9use hashbrown::HashMap;
10use left_right::{Absorb, ReadHandleFactory, WriteHandle};
11use std::hash::Hash;
12use std::sync::Mutex;
13
14enum MapOp<K, V> {
15    Insert(K, V),
16    Clear,
17}
18
19impl<K: Eq + Hash + Clone, V: Clone> Absorb<MapOp<K, V>> for HashMap<K, V> {
20    fn absorb_first(&mut self, operation: &mut MapOp<K, V>, _other: &Self) {
21        match operation {
22            MapOp::Insert(k, v) => {
23                self.insert(k.clone(), v.clone());
24            }
25            MapOp::Clear => self.clear(),
26        }
27    }
28
29    fn sync_with(&mut self, first: &Self) {
30        self.clone_from(first);
31    }
32}
33
34/// A concurrent map optimized for read-heavy workloads.
35///
36/// Readers never block — not even while a write is in progress. Writers are
37/// serialized through an internal [`Mutex`].
38///
39/// Trade-off: doubled memory (two copies of the map).
40pub struct LrMap<K: Eq + Hash + Clone, V: Clone> {
41    reader_factory: ReadHandleFactory<HashMap<K, V>>,
42    writer: Mutex<WriteHandle<HashMap<K, V>, MapOp<K, V>>>,
43}
44
45impl<K, V> LrMap<K, V>
46where
47    K: Eq + Hash + Clone + Send + Sync,
48    V: Clone + Send + Sync,
49{
50    pub fn new() -> Self {
51        let (writer, reader) = left_right::new_from_empty(HashMap::new());
52        let factory = reader.factory();
53        Self {
54            reader_factory: factory,
55            writer: Mutex::new(writer),
56        }
57    }
58
59    /// Lock-free lookup returning a clone of the value.
60    pub fn get<Q>(&self, key: &Q) -> Option<V>
61    where
62        K: std::borrow::Borrow<Q>,
63        Q: Hash + Eq + ?Sized,
64    {
65        let reader = self.reader_factory.handle();
66        reader.enter().and_then(|map| map.get(key).cloned())
67    }
68
69    pub fn insert(&self, key: K, value: V) {
70        match self.writer.lock() {
71            Ok(mut w) => {
72                w.append(MapOp::Insert(key, value));
73                w.publish();
74            }
75            Err(e) => {
76                // Log the lock poisoning error instead of silently dropping the write
77                eprintln!(
78                    "WARNING: LrMap::insert failed due to poisoned mutex: {e}. Write dropped."
79                );
80            }
81        }
82    }
83
84    pub fn clear(&self) {
85        match self.writer.lock() {
86            Ok(mut w) => {
87                w.append(MapOp::Clear);
88                w.publish();
89            }
90            Err(e) => {
91                // Log the lock poisoning error instead of silently dropping the write
92                eprintln!(
93                    "WARNING: LrMap::clear failed due to poisoned mutex: {e}. Operation dropped."
94                );
95            }
96        }
97    }
98
99    pub fn len(&self) -> usize {
100        let reader = self.reader_factory.handle();
101        reader.enter().map(|m| m.len()).unwrap_or(0)
102    }
103
104    pub fn is_empty(&self) -> bool {
105        self.len() == 0
106    }
107}
108
109impl<K, V> Default for LrMap<K, V>
110where
111    K: Eq + Hash + Clone + Send + Sync,
112    V: Clone + Send + Sync,
113{
114    fn default() -> Self {
115        Self::new()
116    }
117}
118
119#[cfg(test)]
120mod tests {
121    use super::*;
122    use std::sync::Arc;
123
124    #[test]
125    fn insert_and_get() {
126        let map: LrMap<String, i32> = LrMap::new();
127        map.insert("a".into(), 1);
128        assert_eq!(map.get("a"), Some(1));
129        assert_eq!(map.get("b"), None);
130    }
131
132    #[test]
133    fn overwrite_key() {
134        let map: LrMap<String, i32> = LrMap::new();
135        map.insert("a".into(), 1);
136        map.insert("a".into(), 2);
137        assert_eq!(map.get("a"), Some(2));
138    }
139
140    #[test]
141    fn clear_removes_all() {
142        let map: LrMap<String, i32> = LrMap::new();
143        map.insert("a".into(), 1);
144        map.insert("b".into(), 2);
145        map.clear();
146        assert!(map.is_empty());
147    }
148
149    #[test]
150    fn concurrent_reads() {
151        let map: Arc<LrMap<String, i32>> = Arc::new(LrMap::new());
152        map.insert("key".into(), 42);
153
154        let handles: Vec<_> = (0..4)
155            .map(|_| {
156                let m = Arc::clone(&map);
157                std::thread::spawn(move || {
158                    for _ in 0..100 {
159                        assert_eq!(m.get("key"), Some(42));
160                    }
161                })
162            })
163            .collect();
164
165        for h in handles {
166            h.join().expect("reader thread panicked");
167        }
168    }
169}
vtcode_commons/lr_map.rs

vtcode_commons/
lr_map.rs
