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 left_right::{Absorb, ReadHandleFactory, WriteHandle};
10use std::collections::HashMap;
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        if let Ok(mut w) = self.writer.lock() {
71            w.append(MapOp::Insert(key, value));
72            w.publish();
73        }
74    }
75
76    pub fn clear(&self) {
77        if let Ok(mut w) = self.writer.lock() {
78            w.append(MapOp::Clear);
79            w.publish();
80        }
81    }
82
83    pub fn len(&self) -> usize {
84        let reader = self.reader_factory.handle();
85        reader.enter().map(|m| m.len()).unwrap_or(0)
86    }
87
88    pub fn is_empty(&self) -> bool {
89        self.len() == 0
90    }
91}
92
93impl<K, V> Default for LrMap<K, V>
94where
95    K: Eq + Hash + Clone + Send + Sync,
96    V: Clone + Send + Sync,
97{
98    fn default() -> Self {
99        Self::new()
100    }
101}
102
103#[cfg(test)]
104mod tests {
105    use super::*;
106    use std::sync::Arc;
107
108    #[test]
109    fn insert_and_get() {
110        let map: LrMap<String, i32> = LrMap::new();
111        map.insert("a".into(), 1);
112        assert_eq!(map.get("a"), Some(1));
113        assert_eq!(map.get("b"), None);
114    }
115
116    #[test]
117    fn overwrite_key() {
118        let map: LrMap<String, i32> = LrMap::new();
119        map.insert("a".into(), 1);
120        map.insert("a".into(), 2);
121        assert_eq!(map.get("a"), Some(2));
122    }
123
124    #[test]
125    fn clear_removes_all() {
126        let map: LrMap<String, i32> = LrMap::new();
127        map.insert("a".into(), 1);
128        map.insert("b".into(), 2);
129        map.clear();
130        assert!(map.is_empty());
131    }
132
133    #[test]
134    fn concurrent_reads() {
135        let map: Arc<LrMap<String, i32>> = Arc::new(LrMap::new());
136        map.insert("key".into(), 42);
137
138        let handles: Vec<_> = (0..4)
139            .map(|_| {
140                let m = Arc::clone(&map);
141                std::thread::spawn(move || {
142                    for _ in 0..100 {
143                        assert_eq!(m.get("key"), Some(42));
144                    }
145                })
146            })
147            .collect();
148
149        for h in handles {
150            h.join().expect("reader thread panicked");
151        }
152    }
153}
vtcode_commons/lr_map.rs

vtcode_commons/
lr_map.rs
