core/hash/mod.rs
1//! Generic hashing support.
2//!
3//! This module provides a generic way to compute the [hash] of a value.
4//! Hashes are most commonly used with [`HashMap`] and [`HashSet`].
5//!
6//! [hash]: https://en.wikipedia.org/wiki/Hash_function
7//! [`HashMap`]: ../../std/collections/struct.HashMap.html
8//! [`HashSet`]: ../../std/collections/struct.HashSet.html
9//!
10//! The simplest way to make a type hashable is to use `#[derive(Hash)]`:
11//!
12//! # Examples
13//!
14//! ```rust
15//! use std::hash::{DefaultHasher, Hash, Hasher};
16//!
17//! #[derive(Hash)]
18//! struct Person {
19//! id: u32,
20//! name: String,
21//! phone: u64,
22//! }
23//!
24//! let person1 = Person {
25//! id: 5,
26//! name: "Janet".to_string(),
27//! phone: 555_666_7777,
28//! };
29//! let person2 = Person {
30//! id: 5,
31//! name: "Bob".to_string(),
32//! phone: 555_666_7777,
33//! };
34//!
35//! assert!(calculate_hash(&person1) != calculate_hash(&person2));
36//!
37//! fn calculate_hash<T: Hash>(t: &T) -> u64 {
38//! let mut s = DefaultHasher::new();
39//! t.hash(&mut s);
40//! s.finish()
41//! }
42//! ```
43//!
44//! If you need more control over how a value is hashed, you need to implement
45//! the [`Hash`] trait:
46//!
47//! ```rust
48//! use std::hash::{DefaultHasher, Hash, Hasher};
49//!
50//! struct Person {
51//! id: u32,
52//! # #[allow(dead_code)]
53//! name: String,
54//! phone: u64,
55//! }
56//!
57//! impl Hash for Person {
58//! fn hash<H: Hasher>(&self, state: &mut H) {
59//! self.id.hash(state);
60//! self.phone.hash(state);
61//! }
62//! }
63//!
64//! let person1 = Person {
65//! id: 5,
66//! name: "Janet".to_string(),
67//! phone: 555_666_7777,
68//! };
69//! let person2 = Person {
70//! id: 5,
71//! name: "Bob".to_string(),
72//! phone: 555_666_7777,
73//! };
74//!
75//! assert_eq!(calculate_hash(&person1), calculate_hash(&person2));
76//!
77//! fn calculate_hash<T: Hash>(t: &T) -> u64 {
78//! let mut s = DefaultHasher::new();
79//! t.hash(&mut s);
80//! s.finish()
81//! }
82//! ```
83
84#![stable(feature = "rust1", since = "1.0.0")]
85
86#[stable(feature = "rust1", since = "1.0.0")]
87#[allow(deprecated)]
88pub use self::sip::SipHasher;
89#[unstable(feature = "hashmap_internals", issue = "none")]
90#[allow(deprecated)]
91#[doc(hidden)]
92pub use self::sip::SipHasher13;
93use crate::{fmt, marker};
94
95mod sip;
96
97/// A hashable type.
98///
99/// Types implementing `Hash` are able to be [`hash`]ed with an instance of
100/// [`Hasher`].
101///
102/// ## Implementing `Hash`
103///
104/// You can derive `Hash` with `#[derive(Hash)]` if all fields implement `Hash`.
105/// The resulting hash will be the combination of the values from calling
106/// [`hash`] on each field.
107///
108/// ```
109/// #[derive(Hash)]
110/// struct Rustacean {
111/// name: String,
112/// country: String,
113/// }
114/// ```
115///
116/// If you need more control over how a value is hashed, you can of course
117/// implement the `Hash` trait yourself:
118///
119/// ```
120/// use std::hash::{Hash, Hasher};
121///
122/// struct Person {
123/// id: u32,
124/// name: String,
125/// phone: u64,
126/// }
127///
128/// impl Hash for Person {
129/// fn hash<H: Hasher>(&self, state: &mut H) {
130/// self.id.hash(state);
131/// self.phone.hash(state);
132/// }
133/// }
134/// ```
135///
136/// ## `Hash` and `Eq`
137///
138/// When implementing both `Hash` and [`Eq`], it is important that the following
139/// property holds:
140///
141/// ```text
142/// k1 == k2 -> hash(k1) == hash(k2)
143/// ```
144///
145/// In other words, if two keys are equal, their hashes must also be equal.
146/// [`HashMap`] and [`HashSet`] both rely on this behavior.
147///
148/// Thankfully, you won't need to worry about upholding this property when
149/// deriving both [`Eq`] and `Hash` with `#[derive(PartialEq, Eq, Hash)]`.
150///
151/// Violating this property is a logic error. The behavior resulting from a logic error is not
152/// specified, but users of the trait must ensure that such logic errors do *not* result in
153/// undefined behavior. This means that `unsafe` code **must not** rely on the correctness of these
154/// methods.
155///
156/// ## Prefix collisions
157///
158/// Implementations of `hash` should ensure that the data they
159/// pass to the `Hasher` are prefix-free. That is,
160/// values which are not equal should cause two different sequences of values to be written,
161/// and neither of the two sequences should be a prefix of the other.
162///
163/// For example, the standard implementation of [`Hash` for `&str`][impl] passes an extra
164/// `0xFF` byte to the `Hasher` so that the values `("ab", "c")` and `("a",
165/// "bc")` hash differently.
166///
167/// ## Portability
168///
169/// Due to differences in endianness and type sizes, data fed by `Hash` to a `Hasher`
170/// should not be considered portable across platforms. Additionally the data passed by most
171/// standard library types should not be considered stable between compiler versions.
172///
173/// This means tests shouldn't probe hard-coded hash values or data fed to a `Hasher` and
174/// instead should check consistency with `Eq`.
175///
176/// Serialization formats intended to be portable between platforms or compiler versions should
177/// either avoid encoding hashes or only rely on `Hash` and `Hasher` implementations that
178/// provide additional guarantees.
179///
180/// [`HashMap`]: ../../std/collections/struct.HashMap.html
181/// [`HashSet`]: ../../std/collections/struct.HashSet.html
182/// [`hash`]: Hash::hash
183/// [impl]: ../../std/primitive.str.html#impl-Hash-for-str
184#[stable(feature = "rust1", since = "1.0.0")]
185#[rustc_diagnostic_item = "Hash"]
186pub trait Hash: marker::PointeeSized {
187 /// Feeds this value into the given [`Hasher`].
188 ///
189 /// # Examples
190 ///
191 /// ```
192 /// use std::hash::{DefaultHasher, Hash, Hasher};
193 ///
194 /// let mut hasher = DefaultHasher::new();
195 /// 7920.hash(&mut hasher);
196 /// println!("Hash is {:x}!", hasher.finish());
197 /// ```
198 #[stable(feature = "rust1", since = "1.0.0")]
199 fn hash<H: Hasher>(&self, state: &mut H);
200
201 /// Feeds a slice of this type into the given [`Hasher`].
202 ///
203 /// This method is meant as a convenience, but its implementation is
204 /// also explicitly left unspecified. It isn't guaranteed to be
205 /// equivalent to repeated calls of [`hash`] and implementations of
206 /// [`Hash`] should keep that in mind and call [`hash`] themselves
207 /// if the slice isn't treated as a whole unit in the [`PartialEq`]
208 /// implementation.
209 ///
210 /// For example, a [`VecDeque`] implementation might naïvely call
211 /// [`as_slices`] and then [`hash_slice`] on each slice, but this
212 /// is wrong since the two slices can change with a call to
213 /// [`make_contiguous`] without affecting the [`PartialEq`]
214 /// result. Since these slices aren't treated as singular
215 /// units, and instead part of a larger deque, this method cannot
216 /// be used.
217 ///
218 /// # Examples
219 ///
220 /// ```
221 /// use std::hash::{DefaultHasher, Hash, Hasher};
222 ///
223 /// let mut hasher = DefaultHasher::new();
224 /// let numbers = [6, 28, 496, 8128];
225 /// Hash::hash_slice(&numbers, &mut hasher);
226 /// println!("Hash is {:x}!", hasher.finish());
227 /// ```
228 ///
229 /// [`VecDeque`]: ../../std/collections/struct.VecDeque.html
230 /// [`as_slices`]: ../../std/collections/struct.VecDeque.html#method.as_slices
231 /// [`make_contiguous`]: ../../std/collections/struct.VecDeque.html#method.make_contiguous
232 /// [`hash`]: Hash::hash
233 /// [`hash_slice`]: Hash::hash_slice
234 #[stable(feature = "hash_slice", since = "1.3.0")]
235 fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)
236 where
237 Self: Sized,
238 {
239 for piece in data {
240 piece.hash(state)
241 }
242 }
243}
244
245// Separate module to reexport the macro `Hash` from prelude without the trait `Hash`.
246pub(crate) mod macros {
247 /// Derive macro generating an impl of the trait `Hash`.
248 #[rustc_builtin_macro]
249 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
250 #[allow_internal_unstable(core_intrinsics)]
251 pub macro Hash($item:item) {
252 /* compiler built-in */
253 }
254}
255#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
256#[doc(inline)]
257pub use macros::Hash;
258
259/// A trait for hashing an arbitrary stream of bytes.
260///
261/// Instances of `Hasher` usually represent state that is changed while hashing
262/// data.
263///
264/// `Hasher` provides a fairly basic interface for retrieving the generated hash
265/// (with [`finish`]), and writing integers as well as slices of bytes into an
266/// instance (with [`write`] and [`write_u8`] etc.). Most of the time, `Hasher`
267/// instances are used in conjunction with the [`Hash`] trait.
268///
269/// This trait provides no guarantees about how the various `write_*` methods are
270/// defined and implementations of [`Hash`] should not assume that they work one
271/// way or another. You cannot assume, for example, that a [`write_u32`] call is
272/// equivalent to four calls of [`write_u8`]. Nor can you assume that adjacent
273/// `write` calls are merged, so it's possible, for example, that
274/// ```
275/// # fn foo(hasher: &mut impl std::hash::Hasher) {
276/// hasher.write(&[1, 2]);
277/// hasher.write(&[3, 4, 5, 6]);
278/// # }
279/// ```
280/// and
281/// ```
282/// # fn foo(hasher: &mut impl std::hash::Hasher) {
283/// hasher.write(&[1, 2, 3, 4]);
284/// hasher.write(&[5, 6]);
285/// # }
286/// ```
287/// end up producing different hashes.
288///
289/// Thus to produce the same hash value, [`Hash`] implementations must ensure
290/// for equivalent items that exactly the same sequence of calls is made -- the
291/// same methods with the same parameters in the same order.
292///
293/// # Examples
294///
295/// ```
296/// use std::hash::{DefaultHasher, Hasher};
297///
298/// let mut hasher = DefaultHasher::new();
299///
300/// hasher.write_u32(1989);
301/// hasher.write_u8(11);
302/// hasher.write_u8(9);
303/// hasher.write(b"Huh?");
304///
305/// println!("Hash is {:x}!", hasher.finish());
306/// ```
307///
308/// [`finish`]: Hasher::finish
309/// [`write`]: Hasher::write
310/// [`write_u8`]: Hasher::write_u8
311/// [`write_u32`]: Hasher::write_u32
312#[stable(feature = "rust1", since = "1.0.0")]
313pub trait Hasher {
314 /// Returns the hash value for the values written so far.
315 ///
316 /// Despite its name, the method does not reset the hasher’s internal
317 /// state. Additional [`write`]s will continue from the current value.
318 /// If you need to start a fresh hash value, you will have to create
319 /// a new hasher.
320 ///
321 /// # Examples
322 ///
323 /// ```
324 /// use std::hash::{DefaultHasher, Hasher};
325 ///
326 /// let mut hasher = DefaultHasher::new();
327 /// hasher.write(b"Cool!");
328 ///
329 /// println!("Hash is {:x}!", hasher.finish());
330 /// ```
331 ///
332 /// [`write`]: Hasher::write
333 #[stable(feature = "rust1", since = "1.0.0")]
334 #[must_use]
335 fn finish(&self) -> u64;
336
337 /// Writes some data into this `Hasher`.
338 ///
339 /// # Examples
340 ///
341 /// ```
342 /// use std::hash::{DefaultHasher, Hasher};
343 ///
344 /// let mut hasher = DefaultHasher::new();
345 /// let data = [0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef];
346 ///
347 /// hasher.write(&data);
348 ///
349 /// println!("Hash is {:x}!", hasher.finish());
350 /// ```
351 ///
352 /// # Note to Implementers
353 ///
354 /// You generally should not do length-prefixing as part of implementing
355 /// this method. It's up to the [`Hash`] implementation to call
356 /// [`Hasher::write_length_prefix`] before sequences that need it.
357 #[stable(feature = "rust1", since = "1.0.0")]
358 fn write(&mut self, bytes: &[u8]);
359
360 /// Writes a single `u8` into this hasher.
361 #[inline]
362 #[stable(feature = "hasher_write", since = "1.3.0")]
363 fn write_u8(&mut self, i: u8) {
364 self.write(&[i])
365 }
366 /// Writes a single `u16` into this hasher.
367 #[inline]
368 #[stable(feature = "hasher_write", since = "1.3.0")]
369 fn write_u16(&mut self, i: u16) {
370 self.write(&i.to_ne_bytes())
371 }
372 /// Writes a single `u32` into this hasher.
373 #[inline]
374 #[stable(feature = "hasher_write", since = "1.3.0")]
375 fn write_u32(&mut self, i: u32) {
376 self.write(&i.to_ne_bytes())
377 }
378 /// Writes a single `u64` into this hasher.
379 #[inline]
380 #[stable(feature = "hasher_write", since = "1.3.0")]
381 fn write_u64(&mut self, i: u64) {
382 self.write(&i.to_ne_bytes())
383 }
384 /// Writes a single `u128` into this hasher.
385 #[inline]
386 #[stable(feature = "i128", since = "1.26.0")]
387 fn write_u128(&mut self, i: u128) {
388 self.write(&i.to_ne_bytes())
389 }
390 /// Writes a single `usize` into this hasher.
391 #[inline]
392 #[stable(feature = "hasher_write", since = "1.3.0")]
393 fn write_usize(&mut self, i: usize) {
394 self.write(&i.to_ne_bytes())
395 }
396
397 /// Writes a single `i8` into this hasher.
398 #[inline]
399 #[stable(feature = "hasher_write", since = "1.3.0")]
400 fn write_i8(&mut self, i: i8) {
401 self.write_u8(i as u8)
402 }
403 /// Writes a single `i16` into this hasher.
404 #[inline]
405 #[stable(feature = "hasher_write", since = "1.3.0")]
406 fn write_i16(&mut self, i: i16) {
407 self.write_u16(i as u16)
408 }
409 /// Writes a single `i32` into this hasher.
410 #[inline]
411 #[stable(feature = "hasher_write", since = "1.3.0")]
412 fn write_i32(&mut self, i: i32) {
413 self.write_u32(i as u32)
414 }
415 /// Writes a single `i64` into this hasher.
416 #[inline]
417 #[stable(feature = "hasher_write", since = "1.3.0")]
418 fn write_i64(&mut self, i: i64) {
419 self.write_u64(i as u64)
420 }
421 /// Writes a single `i128` into this hasher.
422 #[inline]
423 #[stable(feature = "i128", since = "1.26.0")]
424 fn write_i128(&mut self, i: i128) {
425 self.write_u128(i as u128)
426 }
427 /// Writes a single `isize` into this hasher.
428 #[inline]
429 #[stable(feature = "hasher_write", since = "1.3.0")]
430 fn write_isize(&mut self, i: isize) {
431 self.write_usize(i as usize)
432 }
433
434 /// Writes a length prefix into this hasher, as part of being prefix-free.
435 ///
436 /// If you're implementing [`Hash`] for a custom collection, call this before
437 /// writing its contents to this `Hasher`. That way
438 /// `(collection![1, 2, 3], collection![4, 5])` and
439 /// `(collection![1, 2], collection![3, 4, 5])` will provide different
440 /// sequences of values to the `Hasher`
441 ///
442 /// The `impl<T> Hash for [T]` includes a call to this method, so if you're
443 /// hashing a slice (or array or vector) via its `Hash::hash` method,
444 /// you should **not** call this yourself.
445 ///
446 /// This method is only for providing domain separation. If you want to
447 /// hash a `usize` that represents part of the *data*, then it's important
448 /// that you pass it to [`Hasher::write_usize`] instead of to this method.
449 ///
450 /// # Examples
451 ///
452 /// ```
453 /// #![feature(hasher_prefixfree_extras)]
454 /// # // Stubs to make the `impl` below pass the compiler
455 /// # #![allow(non_local_definitions)]
456 /// # struct MyCollection<T>(Option<T>);
457 /// # impl<T> MyCollection<T> {
458 /// # fn len(&self) -> usize { todo!() }
459 /// # }
460 /// # impl<'a, T> IntoIterator for &'a MyCollection<T> {
461 /// # type Item = T;
462 /// # type IntoIter = std::iter::Empty<T>;
463 /// # fn into_iter(self) -> Self::IntoIter { todo!() }
464 /// # }
465 ///
466 /// use std::hash::{Hash, Hasher};
467 /// impl<T: Hash> Hash for MyCollection<T> {
468 /// fn hash<H: Hasher>(&self, state: &mut H) {
469 /// state.write_length_prefix(self.len());
470 /// for elt in self {
471 /// elt.hash(state);
472 /// }
473 /// }
474 /// }
475 /// ```
476 ///
477 /// # Note to Implementers
478 ///
479 /// If you've decided that your `Hasher` is willing to be susceptible to
480 /// Hash-DoS attacks, then you might consider skipping hashing some or all
481 /// of the `len` provided in the name of increased performance.
482 #[inline]
483 #[unstable(feature = "hasher_prefixfree_extras", issue = "96762")]
484 fn write_length_prefix(&mut self, len: usize) {
485 self.write_usize(len);
486 }
487
488 /// Writes a single `str` into this hasher.
489 ///
490 /// If you're implementing [`Hash`], you generally do not need to call this,
491 /// as the `impl Hash for str` does, so you should prefer that instead.
492 ///
493 /// This includes the domain separator for prefix-freedom, so you should
494 /// **not** call `Self::write_length_prefix` before calling this.
495 ///
496 /// # Note to Implementers
497 ///
498 /// There are at least two reasonable default ways to implement this.
499 /// Which one will be the default is not yet decided, so for now
500 /// you probably want to override it specifically.
501 ///
502 /// ## The general answer
503 ///
504 /// It's always correct to implement this with a length prefix:
505 ///
506 /// ```
507 /// # #![feature(hasher_prefixfree_extras)]
508 /// # struct Foo;
509 /// # impl std::hash::Hasher for Foo {
510 /// # fn finish(&self) -> u64 { unimplemented!() }
511 /// # fn write(&mut self, _bytes: &[u8]) { unimplemented!() }
512 /// fn write_str(&mut self, s: &str) {
513 /// self.write_length_prefix(s.len());
514 /// self.write(s.as_bytes());
515 /// }
516 /// # }
517 /// ```
518 ///
519 /// And, if your `Hasher` works in `usize` chunks, this is likely a very
520 /// efficient way to do it, as anything more complicated may well end up
521 /// slower than just running the round with the length.
522 ///
523 /// ## If your `Hasher` works byte-wise
524 ///
525 /// One nice thing about `str` being UTF-8 is that the `b'\xFF'` byte
526 /// never happens. That means that you can append that to the byte stream
527 /// being hashed and maintain prefix-freedom:
528 ///
529 /// ```
530 /// # #![feature(hasher_prefixfree_extras)]
531 /// # struct Foo;
532 /// # impl std::hash::Hasher for Foo {
533 /// # fn finish(&self) -> u64 { unimplemented!() }
534 /// # fn write(&mut self, _bytes: &[u8]) { unimplemented!() }
535 /// fn write_str(&mut self, s: &str) {
536 /// self.write(s.as_bytes());
537 /// self.write_u8(0xff);
538 /// }
539 /// # }
540 /// ```
541 ///
542 /// This does require that your implementation not add extra padding, and
543 /// thus generally requires that you maintain a buffer, running a round
544 /// only once that buffer is full (or `finish` is called).
545 ///
546 /// That's because if `write` pads data out to a fixed chunk size, it's
547 /// likely that it does it in such a way that `"a"` and `"a\x00"` would
548 /// end up hashing the same sequence of things, introducing conflicts.
549 #[inline]
550 #[unstable(feature = "hasher_prefixfree_extras", issue = "96762")]
551 fn write_str(&mut self, s: &str) {
552 self.write(s.as_bytes());
553 self.write_u8(0xff);
554 }
555}
556
557#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
558impl<H: Hasher + ?Sized> Hasher for &mut H {
559 fn finish(&self) -> u64 {
560 (**self).finish()
561 }
562 fn write(&mut self, bytes: &[u8]) {
563 (**self).write(bytes)
564 }
565 fn write_u8(&mut self, i: u8) {
566 (**self).write_u8(i)
567 }
568 fn write_u16(&mut self, i: u16) {
569 (**self).write_u16(i)
570 }
571 fn write_u32(&mut self, i: u32) {
572 (**self).write_u32(i)
573 }
574 fn write_u64(&mut self, i: u64) {
575 (**self).write_u64(i)
576 }
577 fn write_u128(&mut self, i: u128) {
578 (**self).write_u128(i)
579 }
580 fn write_usize(&mut self, i: usize) {
581 (**self).write_usize(i)
582 }
583 fn write_i8(&mut self, i: i8) {
584 (**self).write_i8(i)
585 }
586 fn write_i16(&mut self, i: i16) {
587 (**self).write_i16(i)
588 }
589 fn write_i32(&mut self, i: i32) {
590 (**self).write_i32(i)
591 }
592 fn write_i64(&mut self, i: i64) {
593 (**self).write_i64(i)
594 }
595 fn write_i128(&mut self, i: i128) {
596 (**self).write_i128(i)
597 }
598 fn write_isize(&mut self, i: isize) {
599 (**self).write_isize(i)
600 }
601 fn write_length_prefix(&mut self, len: usize) {
602 (**self).write_length_prefix(len)
603 }
604 fn write_str(&mut self, s: &str) {
605 (**self).write_str(s)
606 }
607}
608
609/// A trait for creating instances of [`Hasher`].
610///
611/// A `BuildHasher` is typically used (e.g., by [`HashMap`]) to create
612/// [`Hasher`]s for each key such that they are hashed independently of one
613/// another, since [`Hasher`]s contain state.
614///
615/// For each instance of `BuildHasher`, the [`Hasher`]s created by
616/// [`build_hasher`] should be identical. That is, if the same stream of bytes
617/// is fed into each hasher, the same output will also be generated.
618///
619/// # Examples
620///
621/// ```
622/// use std::hash::{BuildHasher, Hasher, RandomState};
623///
624/// let s = RandomState::new();
625/// let mut hasher_1 = s.build_hasher();
626/// let mut hasher_2 = s.build_hasher();
627///
628/// hasher_1.write_u32(8128);
629/// hasher_2.write_u32(8128);
630///
631/// assert_eq!(hasher_1.finish(), hasher_2.finish());
632/// ```
633///
634/// [`build_hasher`]: BuildHasher::build_hasher
635/// [`HashMap`]: ../../std/collections/struct.HashMap.html
636#[stable(since = "1.7.0", feature = "build_hasher")]
637pub trait BuildHasher {
638 /// Type of the hasher that will be created.
639 #[stable(since = "1.7.0", feature = "build_hasher")]
640 type Hasher: Hasher;
641
642 /// Creates a new hasher.
643 ///
644 /// Each call to `build_hasher` on the same instance should produce identical
645 /// [`Hasher`]s.
646 ///
647 /// # Examples
648 ///
649 /// ```
650 /// use std::hash::{BuildHasher, RandomState};
651 ///
652 /// let s = RandomState::new();
653 /// let new_s = s.build_hasher();
654 /// ```
655 #[stable(since = "1.7.0", feature = "build_hasher")]
656 fn build_hasher(&self) -> Self::Hasher;
657
658 /// Calculates the hash of a single value.
659 ///
660 /// This is intended as a convenience for code which *consumes* hashes, such
661 /// as the implementation of a hash table or in unit tests that check
662 /// whether a custom [`Hash`] implementation behaves as expected.
663 ///
664 /// This must not be used in any code which *creates* hashes, such as in an
665 /// implementation of [`Hash`]. The way to create a combined hash of
666 /// multiple values is to call [`Hash::hash`] multiple times using the same
667 /// [`Hasher`], not to call this method repeatedly and combine the results.
668 ///
669 /// # Example
670 ///
671 /// ```
672 /// use std::cmp::{max, min};
673 /// use std::hash::{BuildHasher, Hash, Hasher};
674 /// struct OrderAmbivalentPair<T: Ord>(T, T);
675 /// impl<T: Ord + Hash> Hash for OrderAmbivalentPair<T> {
676 /// fn hash<H: Hasher>(&self, hasher: &mut H) {
677 /// min(&self.0, &self.1).hash(hasher);
678 /// max(&self.0, &self.1).hash(hasher);
679 /// }
680 /// }
681 ///
682 /// // Then later, in a `#[test]` for the type...
683 /// let bh = std::hash::RandomState::new();
684 /// assert_eq!(
685 /// bh.hash_one(OrderAmbivalentPair(1, 2)),
686 /// bh.hash_one(OrderAmbivalentPair(2, 1))
687 /// );
688 /// assert_eq!(
689 /// bh.hash_one(OrderAmbivalentPair(10, 2)),
690 /// bh.hash_one(&OrderAmbivalentPair(2, 10))
691 /// );
692 /// ```
693 #[stable(feature = "build_hasher_simple_hash_one", since = "1.71.0")]
694 fn hash_one<T: Hash>(&self, x: T) -> u64
695 where
696 Self: Sized,
697 Self::Hasher: Hasher,
698 {
699 let mut hasher = self.build_hasher();
700 x.hash(&mut hasher);
701 hasher.finish()
702 }
703}
704
705/// Used to create a default [`BuildHasher`] instance for types that implement
706/// [`Hasher`] and [`Default`].
707///
708/// `BuildHasherDefault<H>` can be used when a type `H` implements [`Hasher`] and
709/// [`Default`], and you need a corresponding [`BuildHasher`] instance, but none is
710/// defined.
711///
712/// Any `BuildHasherDefault` is [zero-sized]. It can be created with
713/// [`default`][method.default]. When using `BuildHasherDefault` with [`HashMap`] or
714/// [`HashSet`], this doesn't need to be done, since they implement appropriate
715/// [`Default`] instances themselves.
716///
717/// # Examples
718///
719/// Using `BuildHasherDefault` to specify a custom [`BuildHasher`] for
720/// [`HashMap`]:
721///
722/// ```
723/// use std::collections::HashMap;
724/// use std::hash::{BuildHasherDefault, Hasher};
725///
726/// #[derive(Default)]
727/// struct MyHasher;
728///
729/// impl Hasher for MyHasher {
730/// fn write(&mut self, bytes: &[u8]) {
731/// // Your hashing algorithm goes here!
732/// unimplemented!()
733/// }
734///
735/// fn finish(&self) -> u64 {
736/// // Your hashing algorithm goes here!
737/// unimplemented!()
738/// }
739/// }
740///
741/// type MyBuildHasher = BuildHasherDefault<MyHasher>;
742///
743/// let hash_map = HashMap::<u32, u32, MyBuildHasher>::default();
744/// ```
745///
746/// [method.default]: BuildHasherDefault::default
747/// [`HashMap`]: ../../std/collections/struct.HashMap.html
748/// [`HashSet`]: ../../std/collections/struct.HashSet.html
749/// [zero-sized]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts
750#[stable(since = "1.7.0", feature = "build_hasher")]
751pub struct BuildHasherDefault<H>(marker::PhantomData<fn() -> H>);
752
753impl<H> BuildHasherDefault<H> {
754 /// Creates a new BuildHasherDefault for Hasher `H`.
755 #[stable(feature = "build_hasher_default_const_new", since = "1.85.0")]
756 #[rustc_const_stable(feature = "build_hasher_default_const_new", since = "1.85.0")]
757 pub const fn new() -> Self {
758 BuildHasherDefault(marker::PhantomData)
759 }
760}
761
762#[stable(since = "1.9.0", feature = "core_impl_debug")]
763impl<H> fmt::Debug for BuildHasherDefault<H> {
764 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
765 f.debug_struct("BuildHasherDefault").finish()
766 }
767}
768
769#[stable(since = "1.7.0", feature = "build_hasher")]
770impl<H: Default + Hasher> BuildHasher for BuildHasherDefault<H> {
771 type Hasher = H;
772
773 fn build_hasher(&self) -> H {
774 H::default()
775 }
776}
777
778#[stable(since = "1.7.0", feature = "build_hasher")]
779impl<H> Clone for BuildHasherDefault<H> {
780 fn clone(&self) -> BuildHasherDefault<H> {
781 BuildHasherDefault(marker::PhantomData)
782 }
783}
784
785#[stable(since = "1.7.0", feature = "build_hasher")]
786impl<H> Default for BuildHasherDefault<H> {
787 fn default() -> BuildHasherDefault<H> {
788 Self::new()
789 }
790}
791
792#[stable(since = "1.29.0", feature = "build_hasher_eq")]
793impl<H> PartialEq for BuildHasherDefault<H> {
794 fn eq(&self, _other: &BuildHasherDefault<H>) -> bool {
795 true
796 }
797}
798
799#[stable(since = "1.29.0", feature = "build_hasher_eq")]
800impl<H> Eq for BuildHasherDefault<H> {}
801
802mod impls {
803 use super::*;
804 use crate::slice;
805
806 macro_rules! impl_write {
807 ($(($ty:ident, $meth:ident),)*) => {$(
808 #[stable(feature = "rust1", since = "1.0.0")]
809 impl Hash for $ty {
810 #[inline]
811 fn hash<H: Hasher>(&self, state: &mut H) {
812 state.$meth(*self)
813 }
814
815 #[inline]
816 fn hash_slice<H: Hasher>(data: &[$ty], state: &mut H) {
817 let newlen = size_of_val(data);
818 let ptr = data.as_ptr() as *const u8;
819 // SAFETY: `ptr` is valid and aligned, as this macro is only used
820 // for numeric primitives which have no padding. The new slice only
821 // spans across `data` and is never mutated, and its total size is the
822 // same as the original `data` so it can't be over `isize::MAX`.
823 state.write(unsafe { slice::from_raw_parts(ptr, newlen) })
824 }
825 }
826 )*}
827 }
828
829 impl_write! {
830 (u8, write_u8),
831 (u16, write_u16),
832 (u32, write_u32),
833 (u64, write_u64),
834 (usize, write_usize),
835 (i8, write_i8),
836 (i16, write_i16),
837 (i32, write_i32),
838 (i64, write_i64),
839 (isize, write_isize),
840 (u128, write_u128),
841 (i128, write_i128),
842 }
843
844 #[stable(feature = "rust1", since = "1.0.0")]
845 impl Hash for bool {
846 #[inline]
847 fn hash<H: Hasher>(&self, state: &mut H) {
848 state.write_u8(*self as u8)
849 }
850 }
851
852 #[stable(feature = "rust1", since = "1.0.0")]
853 impl Hash for char {
854 #[inline]
855 fn hash<H: Hasher>(&self, state: &mut H) {
856 state.write_u32(*self as u32)
857 }
858 }
859
860 #[stable(feature = "rust1", since = "1.0.0")]
861 impl Hash for str {
862 #[inline]
863 fn hash<H: Hasher>(&self, state: &mut H) {
864 state.write_str(self);
865 }
866 }
867
868 #[stable(feature = "never_hash", since = "1.29.0")]
869 impl Hash for ! {
870 #[inline]
871 fn hash<H: Hasher>(&self, _: &mut H) {
872 *self
873 }
874 }
875
876 macro_rules! impl_hash_tuple {
877 () => (
878 #[stable(feature = "rust1", since = "1.0.0")]
879 impl Hash for () {
880 #[inline]
881 fn hash<H: Hasher>(&self, _state: &mut H) {}
882 }
883 );
884
885 ( $($name:ident)+) => (
886 maybe_tuple_doc! {
887 $($name)+ @
888 #[stable(feature = "rust1", since = "1.0.0")]
889 impl<$($name: Hash),+> Hash for ($($name,)+) {
890 #[allow(non_snake_case)]
891 #[inline]
892 fn hash<S: Hasher>(&self, state: &mut S) {
893 let ($(ref $name,)+) = *self;
894 $($name.hash(state);)+
895 }
896 }
897 }
898 );
899 }
900
901 macro_rules! maybe_tuple_doc {
902 ($a:ident @ #[$meta:meta] $item:item) => {
903 #[doc(fake_variadic)]
904 #[doc = "This trait is implemented for tuples up to twelve items long."]
905 #[$meta]
906 $item
907 };
908 ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
909 #[doc(hidden)]
910 #[$meta]
911 $item
912 };
913 }
914
915 impl_hash_tuple! {}
916 impl_hash_tuple! { T }
917 impl_hash_tuple! { T B }
918 impl_hash_tuple! { T B C }
919 impl_hash_tuple! { T B C D }
920 impl_hash_tuple! { T B C D E }
921 impl_hash_tuple! { T B C D E F }
922 impl_hash_tuple! { T B C D E F G }
923 impl_hash_tuple! { T B C D E F G H }
924 impl_hash_tuple! { T B C D E F G H I }
925 impl_hash_tuple! { T B C D E F G H I J }
926 impl_hash_tuple! { T B C D E F G H I J K }
927 impl_hash_tuple! { T B C D E F G H I J K L }
928
929 #[stable(feature = "rust1", since = "1.0.0")]
930 impl<T: Hash> Hash for [T] {
931 #[inline]
932 fn hash<H: Hasher>(&self, state: &mut H) {
933 state.write_length_prefix(self.len());
934 Hash::hash_slice(self, state)
935 }
936 }
937
938 #[stable(feature = "rust1", since = "1.0.0")]
939 impl<T: ?Sized + marker::PointeeSized + Hash> Hash for &T {
940 #[inline]
941 fn hash<H: Hasher>(&self, state: &mut H) {
942 (**self).hash(state);
943 }
944 }
945
946 #[stable(feature = "rust1", since = "1.0.0")]
947 impl<T: ?Sized + marker::PointeeSized + Hash> Hash for &mut T {
948 #[inline]
949 fn hash<H: Hasher>(&self, state: &mut H) {
950 (**self).hash(state);
951 }
952 }
953
954 #[stable(feature = "rust1", since = "1.0.0")]
955 impl<T: ?Sized + marker::PointeeSized> Hash for *const T {
956 #[inline]
957 fn hash<H: Hasher>(&self, state: &mut H) {
958 let (address, metadata) = self.to_raw_parts();
959 state.write_usize(address.addr());
960 metadata.hash(state);
961 }
962 }
963
964 #[stable(feature = "rust1", since = "1.0.0")]
965 impl<T: ?Sized + marker::PointeeSized> Hash for *mut T {
966 #[inline]
967 fn hash<H: Hasher>(&self, state: &mut H) {
968 let (address, metadata) = self.to_raw_parts();
969 state.write_usize(address.addr());
970 metadata.hash(state);
971 }
972 }
973}