1use crate::attribute::OwnedAttribute;
2use crate::name::OwnedName;
3
4use std::collections::hash_map::RandomState;
5use std::collections::HashSet;
6use std::hash::{BuildHasher, Hash, Hasher};
7
8/// An ordered set
9pub struct AttributesSet {
10 vec: Vec<OwnedAttribute>,
11 /// Uses a no-op hasher, because these u64s are hashes already
12 may_contain: HashSet<u64, U64HasherBuilder>,
13 /// This is real hasher for the `OwnedName`
14 hasher: RandomState,
15}
16
17/// Use linear search and don't allocate `HashSet` if there are few attributes,
18/// because allocation costs more than a few comparisons.
19const HASH_THRESHOLD: usize = 8;
20
21impl AttributesSet {
22 pub fn new() -> Self {
23 Self {
24 vec: Vec::new(),
25 hasher: RandomState::new(),
26 may_contain: HashSet::default(),
27 }
28 }
29
30 fn hash(&self, val: &OwnedName) -> u64 {
31 let mut h = self.hasher.build_hasher();
32 val.hash(&mut h);
33 h.finish()
34 }
35
36 pub fn len(&self) -> usize {
37 self.vec.len()
38 }
39
40 pub fn contains(&self, name: &OwnedName) -> bool {
41 // fall back to linear search only on duplicate or hash collision
42 (self.vec.len() < HASH_THRESHOLD || self.may_contain.contains(&self.hash(name))) &&
43 self.vec.iter().any(move |a| &a.name == name)
44 }
45
46 pub fn push(&mut self, attr: OwnedAttribute) {
47 if self.vec.len() >= HASH_THRESHOLD {
48 if self.vec.len() == HASH_THRESHOLD {
49 self.may_contain.reserve(HASH_THRESHOLD * 2);
50 for attr in &self.vec {
51 self.may_contain.insert(self.hash(&attr.name));
52 }
53 }
54 self.may_contain.insert(self.hash(&attr.name));
55 }
56 self.vec.push(attr);
57 }
58
59 pub fn into_vec(self) -> Vec<OwnedAttribute> {
60 self.vec
61 }
62}
63
64#[test]
65fn indexset() {
66 let mut s = AttributesSet::new();
67 let not_here = OwnedName {
68 local_name: "attr1000".into(),
69 namespace: Some("test".into()),
70 prefix: None,
71 };
72
73 // this test will take a lot of time if the `contains()` is linear, and the loop is quadratic
74 for i in 0..50000 {
75 let name = OwnedName {
76 local_name: format!("attr{i}"), namespace: None, prefix: None,
77 };
78 assert!(!s.contains(&name));
79
80 s.push(OwnedAttribute { name, value: String::new() });
81 assert!(!s.contains(&not_here));
82 }
83
84 assert!(s.contains(&OwnedName {
85 local_name: "attr1234".into(), namespace: None, prefix: None,
86 }));
87 assert!(s.contains(&OwnedName {
88 local_name: "attr0".into(), namespace: None, prefix: None,
89 }));
90 assert!(s.contains(&OwnedName {
91 local_name: "attr49999".into(), namespace: None, prefix: None,
92 }));
93}
94
95/// Hashser that does nothing except passing u64 through
96struct U64Hasher(u64);
97
98impl Hasher for U64Hasher {
99 fn finish(&self) -> u64 { self.0 }
100 fn write(&mut self, slice: &[u8]) {
101 for &v: u8 in slice { self.0 ^= u64::from(v) } // unused in practice
102 }
103 fn write_u64(&mut self, i: u64) {
104 self.0 ^= i;
105 }
106}
107
108#[derive(Default)]
109struct U64HasherBuilder;
110
111impl BuildHasher for U64HasherBuilder {
112 type Hasher = U64Hasher;
113 fn build_hasher(&self) -> U64Hasher { U64Hasher(0) }
114}
115