iddqd/bi_hash_map/
ref_mut.rs

1use crate::{BiHashItem, DefaultHashBuilder, support::map_hash::MapHash};
2use core::{
3    fmt,
4    hash::BuildHasher,
5    ops::{Deref, DerefMut},
6};
7
8/// A mutable reference to a [`BiHashMap`] item.
9///
10/// This is a wrapper around a `&mut T` that panics when dropped, if the
11/// borrowed value's keys have changed since the wrapper was created.
12///
13/// # Change detection
14///
15/// It is illegal to change the keys of a borrowed `&mut T`. `RefMut` attempts
16/// to enforce this invariant.
17///
18/// `RefMut` stores the `Hash` output of keys at creation time, and recomputes
19/// these hashes when it is dropped or when [`Self::into_ref`] is called. If a
20/// key changes, there's a small but non-negligible chance that its hash value
21/// stays the same[^collision-chance]. In that case, as long as the new key is
22/// not the same as another existing one, internal invariants are not violated
23/// and the [`BiHashMap`] will continue to work correctly. (But don't do this!)
24///
25/// It is also possible to deliberately write pathological `Hash`
26/// implementations that collide more often. (Don't do this either.)
27///
28/// Also, `RefMut`'s hash detection will not function if [`mem::forget`] is
29/// called on it. If a key is changed and `mem::forget` is then called on the
30/// `RefMut`, the `BiHashMap` will stop functioning correctly. This will not
31/// introduce memory safety issues, however.
32///
33/// The issues here are similar to using interior mutability (e.g. `RefCell` or
34/// `Mutex`) to mutate keys in a regular `HashMap`.
35///
36/// [`mem::forget`]: std::mem::forget
37///
38/// [^collision-chance]: The output of `Hash` is a [`u64`], so the probability
39/// of an individual hash colliding by chance is 1/2⁶⁴. Due to the [birthday
40/// problem], the probability of a collision by chance reaches 10⁻⁶ within
41/// around 6 × 10⁶ elements.
42///
43/// [`BiHashMap`]: crate::BiHashMap
44/// [birthday problem]: https://en.wikipedia.org/wiki/Birthday_problem#Probability_table
45pub struct RefMut<
46    'a,
47    T: BiHashItem,
48    S: Clone + BuildHasher = DefaultHashBuilder,
49> {
50    inner: Option<RefMutInner<'a, T, S>>,
51}
52
53impl<'a, T: BiHashItem, S: Clone + BuildHasher> RefMut<'a, T, S> {
54    pub(super) fn new(hashes: [MapHash<S>; 2], borrowed: &'a mut T) -> Self {
55        Self { inner: Some(RefMutInner { hashes, borrowed }) }
56    }
57
58    /// Borrows self into a shorter-lived `RefMut`.
59    ///
60    /// This `RefMut` will also check hash equality on drop.
61    pub fn reborrow(&mut self) -> RefMut<'_, T, S> {
62        let inner = self.inner.as_mut().unwrap();
63        let borrowed = &mut *inner.borrowed;
64        RefMut::new(inner.hashes.clone(), borrowed)
65    }
66
67    /// Converts this `RefMut` into a `&'a T`.
68    pub fn into_ref(mut self) -> &'a T {
69        let inner = self.inner.take().unwrap();
70        inner.into_ref()
71    }
72}
73
74impl<T: BiHashItem, S: Clone + BuildHasher> Drop for RefMut<'_, T, S> {
75    fn drop(&mut self) {
76        if let Some(inner) = self.inner.take() {
77            inner.into_ref();
78        }
79    }
80}
81
82impl<T: BiHashItem, S: Clone + BuildHasher> Deref for RefMut<'_, T, S> {
83    type Target = T;
84
85    fn deref(&self) -> &Self::Target {
86        self.inner.as_ref().unwrap().borrowed
87    }
88}
89
90impl<T: BiHashItem, S: Clone + BuildHasher> DerefMut for RefMut<'_, T, S> {
91    fn deref_mut(&mut self) -> &mut Self::Target {
92        self.inner.as_mut().unwrap().borrowed
93    }
94}
95
96impl<T: BiHashItem + fmt::Debug, S: Clone + BuildHasher> fmt::Debug
97    for RefMut<'_, T, S>
98{
99    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
100        match self.inner {
101            Some(ref inner) => inner.fmt(f),
102            None => {
103                f.debug_struct("RefMut").field("borrowed", &"missing").finish()
104            }
105        }
106    }
107}
108
109struct RefMutInner<'a, T: BiHashItem, S> {
110    hashes: [MapHash<S>; 2],
111    borrowed: &'a mut T,
112}
113
114impl<'a, T: BiHashItem, S: BuildHasher> RefMutInner<'a, T, S> {
115    fn into_ref(self) -> &'a T {
116        if !self.hashes[0].is_same_hash(self.borrowed.key1()) {
117            panic!("key1 changed during RefMut borrow");
118        }
119        if !self.hashes[1].is_same_hash(self.borrowed.key2()) {
120            panic!("key2 changed during RefMut borrow");
121        }
122
123        self.borrowed
124    }
125}
126
127impl<T: BiHashItem + fmt::Debug, S> fmt::Debug for RefMutInner<'_, T, S> {
128    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
129        self.borrowed.fmt(f)
130    }
131}
iddqd/bi_hash_map/ref_mut.rs

iddqd/bi_hash_map/
ref_mut.rs
