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                tracing::warn!("LrMap::insert failed due to poisoned mutex: {e}. Write dropped.");
77            }
78        }
79    }
80
81    pub fn clear(&self) {
82        match self.writer.lock() {
83            Ok(mut w) => {
84                w.append(MapOp::Clear);
85                w.publish();
86            }
87            Err(e) => {
88                tracing::warn!(
89                    "LrMap::clear failed due to poisoned mutex: {e}. Operation dropped."
90                );
91            }
92        }
93    }
94
95    pub fn len(&self) -> usize {
96        let reader = self.reader_factory.handle();
97        reader.enter().map(|m| m.len()).unwrap_or(0)
98    }
99
100    pub fn is_empty(&self) -> bool {
101        self.len() == 0
102    }
103}
104
105impl<K, V> Default for LrMap<K, V>
106where
107    K: Eq + Hash + Clone + Send + Sync,
108    V: Clone + Send + Sync,
109{
110    fn default() -> Self {
111        Self::new()
112    }
113}
114
115#[cfg(test)]
116mod tests {
117    use super::*;
118    use std::sync::Arc;
119
120    #[test]
121    fn insert_and_get() {
122        let map: LrMap<String, i32> = LrMap::new();
123        map.insert("a".into(), 1);
124        assert_eq!(map.get("a"), Some(1));
125        assert_eq!(map.get("b"), None);
126    }
127
128    #[test]
129    fn overwrite_key() {
130        let map: LrMap<String, i32> = LrMap::new();
131        map.insert("a".into(), 1);
132        map.insert("a".into(), 2);
133        assert_eq!(map.get("a"), Some(2));
134    }
135
136    #[test]
137    fn clear_removes_all() {
138        let map: LrMap<String, i32> = LrMap::new();
139        map.insert("a".into(), 1);
140        map.insert("b".into(), 2);
141        map.clear();
142        assert!(map.is_empty());
143    }
144
145    #[test]
146    fn concurrent_reads() {
147        let map: Arc<LrMap<String, i32>> = Arc::new(LrMap::new());
148        map.insert("key".into(), 42);
149
150        let handles: Vec<_> = (0..4)
151            .map(|_| {
152                let m = Arc::clone(&map);
153                std::thread::spawn(move || {
154                    for _ in 0..100 {
155                        assert_eq!(m.get("key"), Some(42));
156                    }
157                })
158            })
159            .collect();
160
161        for h in handles {
162            h.join().expect("reader thread panicked");
163        }
164    }
165}
vtcode_commons/lr_map.rs

vtcode_commons/
lr_map.rs
