search the Vec for small LazyIndexMaps

benches/main.rs

@@ -4,7 +4,7 @@ use std::hint::black_box;
 use std::fs::File;
 use std::io::Read;
 
-use jiter::{Jiter, JsonValue, Peek};
+use jiter::{Jiter, JsonValue, LazyIndexMap, Peek};
 use serde_json::Value;
 
 fn read_file(path: &str) -> String {
@@ -215,6 +215,33 @@ test_cases!(floats_array);
 // src/github.com/json-iterator/go-benchmark/benchmark.go#L30C17-L30C29
 test_cases!(medium_response);
 
+fn lazy_map_lookup(length: i64, bench: &mut Bencher) {
+    bench.iter(|| {
+        let mut map: LazyIndexMap<String, JsonValue> = LazyIndexMap::new();
+        for i in 0..length {
+            let key = i.to_string();
+            map.insert(key, JsonValue::Int(i));
+        }
+
+        // best case we get the next value each time
+        for i in 0..length {
+            black_box(map.get(&i.to_string()).unwrap());
+        }
+    })
+}
+
+fn lazy_map_lookup_1_10(bench: &mut Bencher) {
+    lazy_map_lookup(10, bench);
+}
+
+fn lazy_map_lookup_2_20(bench: &mut Bencher) {
+    lazy_map_lookup(20, bench);
+}
+
+fn lazy_map_lookup_3_50(bench: &mut Bencher) {
+    lazy_map_lookup(50, bench);
+}
+
 benchmark_group!(
     benches,
     big_jiter_iter,
@@ -246,6 +273,9 @@ benchmark_group!(
     true_array_serde_value,
     true_object_jiter_iter,
     true_object_jiter_value,
-    true_object_serde_value
+    true_object_serde_value,
+    lazy_map_lookup_1_10,
+    lazy_map_lookup_2_20,
+    lazy_map_lookup_3_50,
 );
 benchmark_main!(benches);

src/lazy_index_map.rs

@@ -3,7 +3,7 @@
 use std::fmt;
 use std::hash::Hash;
 use std::slice::Iter as SliceIter;
-use std::sync::OnceLock;
+use std::sync::{Arc, Mutex, OnceLock};
 
 use ahash::AHashMap;
 use smallvec::SmallVec;
@@ -13,6 +13,7 @@
 pub struct LazyIndexMap<K, V> {
     vec: SmallVec<[(K, V); 8]>,
     map: OnceLock<AHashMap<K, usize>>,
+    last_find: Arc<Mutex<usize>>,
 }
 
 impl<K, V> fmt::Debug for LazyIndexMap<K, V>
@@ -25,6 +26,9 @@
     }
 }
 
+// picked to be a good tradeoff after experimenting with `lazy_map_lookup` benchmark, should cover most models
+const HASHMAP_THRESHOLD: usize = 16;
+
 /// Like [IndexMap](https://docs.rs/indexmap/latest/indexmap/) but only builds the lookup map when it's needed.
 impl<K, V> LazyIndexMap<K, V>
 where
@@ -35,6 +39,7 @@
         Self {
             vec: SmallVec::new(),
             map: OnceLock::new(),
+            last_find: Arc::new(Mutex::new(0)),
         }
     }
 
@@ -58,14 +63,29 @@
         K: Borrow<Q> + PartialEq<Q>,
         Q: Hash + Eq,
     {
-        let map = self.map.get_or_init(|| {
-            self.vec
-                .iter()
-                .enumerate()
-                .map(|(index, (key, _))| (key.clone(), index))
-                .collect()
-        });
-        map.get(key).map(|&i| &self.vec[i].1)
+        let vec_len = self.vec.len();
+        // if the vec is longer than the threshold, we use the hashmap for lookups
+        if vec_len > HASHMAP_THRESHOLD {
+            self.get_map().get(key).map(|&i| &self.vec[i].1)
+        } else {
+            // otherwise we find the value in the vec
+            // we assume the most likely position for the match is at `last_find + 1`
+            match self.last_find.lock() {
+                Ok(mut last_find) => {
+                    let first_try = *last_find + 1;
+                    for i in first_try..first_try + vec_len {
+                        let index = i % vec_len;
+                        let (k, v) = &self.vec[index];
+                        if k == key {
+                            *last_find = index;
+                            return Some(v);
+                        }
+                    }
+                    None
+                }
+                Err(_) => None,
+            }
+        }
     }
 
     pub fn keys(&self) -> impl Iterator<Item = &K> {

tests/main.rs

@@ -873,13 +873,39 @@ fn test_4302_int_err() {
 }
 
 #[test]
-fn lazy_index_map_prety() {
+fn lazy_index_map_pretty() {
     let mut map = LazyIndexMap::new();
     map.insert("foo".to_string(), JsonValue::Str("bar".to_string()));
     map.insert("spam".to_string(), JsonValue::Null);
     assert_eq!(format!("{map:?}"), r#"{"foo": Str("bar"), "spam": Null}"#);
 }
 
+#[test]
+fn lazy_index_map_small_get() {
+    let mut map = LazyIndexMap::new();
+    map.insert("foo".to_string(), JsonValue::Str("bar".to_string()));
+    map.insert("spam".to_string(), JsonValue::Null);
+
+    assert_eq!(map.get("foo"), Some(&JsonValue::Str("bar".to_string())));
+    assert_eq!(map.get("spam"), Some(&JsonValue::Null));
+    assert_eq!(map.get("spam"), Some(&JsonValue::Null));
+    assert_eq!(map.get("foo"), Some(&JsonValue::Str("bar".to_string())));
+}
+
+#[test]
+fn lazy_index_map_big_get() {
+    let mut map = LazyIndexMap::new();
+
+    for i in 0..25 {
+        let key = i.to_string();
+        map.insert(key, JsonValue::Int(i));
+    }
+
+    assert_eq!(map.get("0"), Some(&JsonValue::Int(0)));
+    assert_eq!(map.get("10"), Some(&JsonValue::Int(10)));
+    assert_eq!(map.get("22"), Some(&JsonValue::Int(22)));
+}
+
 #[test]
 fn readme_jiter() {
     let json_data = r#"