hstr/
lib.rs

1#![cfg_attr(feature = "atom_size_128", feature(integer_atomics))]
2//! See [Atom] for more information.
3
4use core::str;
5use std::{
6    borrow::Borrow,
7    fmt::{Debug, Display},
8    hash::Hash,
9    mem::{forget, transmute, ManuallyDrop},
10    num::NonZeroU8,
11    ops::Deref,
12    str::from_utf8_unchecked,
13};
14
15use debug_unreachable::debug_unreachable;
16use once_cell::sync::Lazy;
17
18pub use crate::dynamic::{global_atom_store_gc, AtomStore};
19use crate::{
20    macros::{get_hash, impl_from_alias},
21    tagged_value::TaggedValue,
22};
23
24mod dynamic;
25mod global_store;
26mod macros;
27mod tagged_value;
28#[cfg(test)]
29mod tests;
30pub mod wtf8;
31mod wtf8_atom;
32
33pub use wtf8_atom::Wtf8Atom;
34
35/// An immutable string which is cheap to clone, compare, hash, and has small
36/// size.
37///
38/// # Usecase
39///
40/// This type is designed for the compilers or build tools like a bundler
41/// written in Rust. String interning is much costly than simply allocating a
42/// string, but in compilers, hashing and comparison of strings are very
43/// frequent operations for some of strings, and the other strings are mostly
44/// just passed around. According to DoD, we should optimize types
45/// for operations that occur frequently, and this type is the result.
46///
47/// # Features
48///
49/// ## No mutex on creation and destruction.
50///
51/// This type also considers concurrent processing of AST nodes. Parsers
52/// generates lots of [Atom]s, so the creation of [Atom] should never involve a
53/// global mutex, because parsers are embarrassingly parallel. Also, the AST
54/// nodes are typically dropped in parallel. So [Drop] implementation of [Atom]
55/// should not involve a global mutex, too.
56///
57///
58/// ## Small size (One `u64`)
59///
60/// The most of strings are simply passed around, so the size of [Atom] should
61/// be small as possible.
62///
63/// ```rust
64/// # use std::mem::size_of;
65/// # if !cfg!(feature = "atom_size_128") {
66/// use hstr::Atom;
67/// assert!(size_of::<Atom>() == size_of::<u64>());
68/// assert!(size_of::<Option<Atom>>() == size_of::<u64>());
69/// # }
70/// ````
71///
72///
73/// ## Fast equality check (in most cases)
74///
75/// Equality comparison is O(1) in most cases. If two atoms are from the same
76/// [AtomStore], or they are from different stores but they are
77/// [`AtomStore::merge`]d, they are compared by numeric equality.
78///
79/// If two strings are created from different [AtomStore]s, they are compared
80/// using `strcmp` by default. But `hstr` allows you to make `==` faster -
81/// [`AtomStore::merge`].
82///
83///
84/// ## Fast [Hash] implementation
85///
86/// [Atom] precompute the hash value of long strings when they are created, so
87/// it is `O(1)` to compute hash.
88///
89///
90/// ## Small strings as inline data
91///
92/// Small strings are stored in the [Atom] itself without any allocation.
93///
94///
95/// ## Creating atoms
96///
97/// If you are working on a module which creates lots of [Atom]s, you are
98/// recommended to use [AtomStore] API because it's faster. But if you are not,
99/// you can use global APIs for convenience.
100///
101/// ## Dealloc
102///
103/// - Atoms stored in the local `AtomStore` have the same lifetime as the store
104///   itself, and will be deallocated when the store is dropped.
105/// - Atoms created via the `atom!` macro or `String::into` are stored in the
106///   global atom store. By default, these atoms are never deallocated. To clean
107///   up unused atoms, call [global_atom_store_gc].
108#[repr(transparent)]
109pub struct Atom {
110    // If this Atom is a dynamic one, this is *const Entry
111    unsafe_data: TaggedValue,
112}
113
114#[doc(hidden)]
115pub type CachedAtom = Lazy<Atom>;
116
117#[doc(hidden)]
118pub const fn inline_atom(s: &str) -> Option<Atom> {
119    dynamic::inline_atom(s)
120}
121
122/// Create an atom from a string literal. This atom is never dropped.
123#[macro_export]
124macro_rules! atom {
125    ($s:expr) => {{
126        const INLINE: ::core::option::Option<$crate::Atom> = $crate::inline_atom($s);
127        // This condition can be evaluated at compile time to enable inlining as a
128        // simple constant
129        if INLINE.is_some() {
130            INLINE.unwrap()
131        } else {
132            // Otherwise we use a
133            #[inline(never)]
134            fn get_atom() -> $crate::Atom {
135                static CACHE: $crate::CachedAtom =
136                    $crate::CachedAtom::new(|| $crate::Atom::from($s));
137
138                (*CACHE).clone()
139            }
140
141            get_atom()
142        }
143    }};
144}
145
146impl Default for Atom {
147    #[inline(never)]
148    fn default() -> Self {
149        atom!("")
150    }
151}
152
153/// Immutable, so it's safe to be shared between threads
154unsafe impl Send for Atom {}
155
156/// Immutable, so it's safe to be shared between threads
157unsafe impl Sync for Atom {}
158
159impl Display for Atom {
160    #[inline]
161    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
162        Display::fmt(self.as_str(), f)
163    }
164}
165
166impl Debug for Atom {
167    #[inline]
168    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
169        Debug::fmt(self.as_str(), f)
170    }
171}
172
173#[cfg(feature = "serde")]
174impl serde::ser::Serialize for Atom {
175    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
176    where
177        S: serde::ser::Serializer,
178    {
179        serializer.serialize_str(self)
180    }
181}
182
183#[cfg(feature = "serde")]
184impl<'de> serde::de::Deserialize<'de> for Atom {
185    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
186    where
187        D: serde::Deserializer<'de>,
188    {
189        String::deserialize(deserializer).map(Self::new)
190    }
191}
192const DYNAMIC_TAG: u8 = 0b_00;
193const INLINE_TAG: u8 = 0b_01; // len in upper nybble
194const INLINE_TAG_INIT: NonZeroU8 = unsafe { NonZeroU8::new_unchecked(INLINE_TAG) };
195const TAG_MASK: u8 = 0b_11;
196const LEN_OFFSET: usize = 4;
197const LEN_MASK: u8 = 0xf0;
198
199// const STATIC_SHIFT_BITS: usize = 32;
200
201impl Atom {
202    #[inline(always)]
203    pub fn new<S>(s: S) -> Self
204    where
205        Self: From<S>,
206    {
207        Self::from(s)
208    }
209
210    #[inline(always)]
211    fn tag(&self) -> u8 {
212        self.unsafe_data.tag() & TAG_MASK
213    }
214
215    /// Return true if this is a dynamic Atom.
216    #[inline(always)]
217    fn is_dynamic(&self) -> bool {
218        self.tag() == DYNAMIC_TAG
219    }
220}
221
222impl Atom {
223    fn from_mutated_str<F: FnOnce(&mut str)>(s: &str, f: F) -> Self {
224        let mut buffer = [0u8; 64];
225
226        if let Some(buffer_prefix) = buffer.get_mut(..s.len()) {
227            buffer_prefix.copy_from_slice(s.as_bytes());
228            // SAFETY: `buffer_prefix` was just initialized from `s`, which is
229            // valid UTF-8. `f` receives a `str`, so it must preserve UTF-8.
230            let as_str = unsafe { ::std::str::from_utf8_unchecked_mut(buffer_prefix) };
231            f(as_str);
232            Atom::from(&*as_str)
233        } else {
234            let mut string = s.to_owned();
235            f(&mut string);
236            Atom::from(string)
237        }
238    }
239
240    /// Like [`to_ascii_uppercase`].
241    ///
242    /// [`to_ascii_uppercase`]: https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.to_ascii_uppercase
243    pub fn to_ascii_uppercase(&self) -> Self {
244        for (i, b) in self.bytes().enumerate() {
245            if let b'a'..=b'z' = b {
246                return Atom::from_mutated_str(self, |s| s[i..].make_ascii_uppercase());
247            }
248        }
249        self.clone()
250    }
251
252    /// Like [`to_ascii_lowercase`].
253    ///
254    /// [`to_ascii_lowercase`]: https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.to_ascii_lowercase
255    pub fn to_ascii_lowercase(&self) -> Self {
256        for (i, b) in self.bytes().enumerate() {
257            if let b'A'..=b'Z' = b {
258                return Atom::from_mutated_str(self, |s| s[i..].make_ascii_lowercase());
259            }
260        }
261        self.clone()
262    }
263}
264
265impl Atom {
266    fn get_hash(&self) -> u64 {
267        get_hash!(self)
268    }
269
270    #[inline]
271    fn as_str(&self) -> &str {
272        match self.tag() {
273            DYNAMIC_TAG => unsafe {
274                let item = crate::dynamic::deref_from(self.unsafe_data);
275                from_utf8_unchecked(transmute::<&[u8], &'static [u8]>(&item.slice))
276            },
277            INLINE_TAG => {
278                let len = (self.unsafe_data.tag() & LEN_MASK) >> LEN_OFFSET;
279                let src = self.unsafe_data.data();
280                unsafe { std::str::from_utf8_unchecked(&src[..(len as usize)]) }
281            }
282            _ => unsafe { debug_unreachable!() },
283        }
284    }
285}
286
287#[cfg(test)]
288impl Atom {
289    pub(crate) fn ref_count(&self) -> usize {
290        match self.tag() {
291            DYNAMIC_TAG => {
292                let ptr = unsafe { crate::dynamic::deref_from(self.unsafe_data) };
293
294                triomphe::ThinArc::strong_count(&ptr.0)
295            }
296            _ => 1,
297        }
298    }
299}
300
301impl PartialEq for Atom {
302    #[inline]
303    fn eq(&self, other: &Self) -> bool {
304        let unsafe_data = self.unsafe_data;
305        let other_unsafe_data = other.unsafe_data;
306
307        if unsafe_data == other_unsafe_data {
308            return true;
309        }
310
311        let tag = unsafe_data.tag() & TAG_MASK;
312
313        if tag != (other_unsafe_data.tag() & TAG_MASK) {
314            return false;
315        }
316
317        match tag {
318            // Inline atoms encode both their length and bytes in `unsafe_data`, so
319            // different raw values mean different strings.
320            INLINE_TAG => false,
321            DYNAMIC_TAG => {
322                let this = unsafe { crate::dynamic::deref_from(unsafe_data) };
323                let other = unsafe { crate::dynamic::deref_from(other_unsafe_data) };
324
325                if this.header.header.hash != other.header.header.hash {
326                    return false;
327                }
328
329                this.slice == other.slice
330            }
331            _ => unsafe { debug_unreachable!() },
332        }
333    }
334}
335
336impl Eq for Atom {}
337
338impl Hash for Atom {
339    #[inline(always)]
340    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
341        state.write_u64(self.get_hash());
342    }
343}
344
345impl Drop for Atom {
346    #[inline(always)]
347    fn drop(&mut self) {
348        if self.is_dynamic() {
349            unsafe { drop(crate::dynamic::restore_arc(self.unsafe_data)) }
350        }
351    }
352}
353
354impl Clone for Atom {
355    #[inline(always)]
356    fn clone(&self) -> Self {
357        Self::from_alias(self.unsafe_data)
358    }
359}
360
361impl_from_alias!(Atom);
362
363impl Deref for Atom {
364    type Target = str;
365
366    #[inline(always)]
367    fn deref(&self) -> &Self::Target {
368        self.as_str()
369    }
370}
371
372impl AsRef<str> for Atom {
373    #[inline(always)]
374    fn as_ref(&self) -> &str {
375        self.as_str()
376    }
377}
378
379impl PartialEq<str> for Atom {
380    #[inline]
381    fn eq(&self, other: &str) -> bool {
382        self.as_str() == other
383    }
384}
385
386impl PartialEq<&'_ str> for Atom {
387    #[inline]
388    fn eq(&self, other: &&str) -> bool {
389        self.as_str() == *other
390    }
391}
392
393impl PartialEq<Atom> for str {
394    #[inline]
395    fn eq(&self, other: &Atom) -> bool {
396        self == other.as_str()
397    }
398}
399
400impl Borrow<Wtf8Atom> for Atom {
401    #[inline(always)]
402    fn borrow(&self) -> &Wtf8Atom {
403        // SAFETY:
404        // 1. Wtf8Atom is #[repr(transparent)] over TaggedValue
405        // 2. Atom is #[repr(transparent)] over TaggedValue
406        // 3. hstr::Atom and hstr::Wtf8Atom share the same TaggedValue
407        const _: () = assert!(std::mem::size_of::<Atom>() == std::mem::size_of::<Wtf8Atom>());
408        const _: () = assert!(std::mem::align_of::<Atom>() == std::mem::align_of::<Wtf8Atom>());
409        unsafe { transmute::<&Atom, &Wtf8Atom>(self) }
410    }
411}
412
413impl Atom {
414    /// Converts a WTF-8 encoded [Wtf8Atom] to a regular UTF-8 [Atom] without
415    /// validation.
416    ///
417    /// # Safety
418    ///
419    /// The caller must ensure that the WTF-8 atom contains only valid UTF-8
420    /// data (no unpaired surrogates). This function performs no validation
421    /// and will create an invalid `Atom` if the input contains unpaired
422    /// surrogates.
423    ///
424    /// This is a zero-cost conversion that preserves all internal optimizations
425    /// (inline storage, precomputed hashes, etc.) since both types have
426    /// identical internal representation.
427    pub unsafe fn from_wtf8_unchecked(s: Wtf8Atom) -> Self {
428        let s = ManuallyDrop::new(s);
429        Atom {
430            unsafe_data: s.unsafe_data,
431        }
432    }
433}
434
435#[cfg(test)]
436mod macro_tests {
437
438    use super::*;
439    #[test]
440    fn test_atom() {
441        // Test enough to exceed the small string optimization
442        assert_eq!(atom!(""), Atom::default());
443        assert_eq!(atom!(""), Atom::from(""));
444        assert_eq!(atom!("a"), Atom::from("a"));
445        assert_eq!(atom!("ab"), Atom::from("ab"));
446        assert_eq!(atom!("abc"), Atom::from("abc"));
447        assert_eq!(atom!("abcd"), Atom::from("abcd"));
448        assert_eq!(atom!("abcde"), Atom::from("abcde"));
449        assert_eq!(atom!("abcdef"), Atom::from("abcdef"));
450        assert_eq!(atom!("abcdefg"), Atom::from("abcdefg"));
451        assert_eq!(atom!("abcdefgh"), Atom::from("abcdefgh"));
452        assert_eq!(atom!("abcdefghi"), Atom::from("abcdefghi"));
453    }
454
455    #[test]
456    fn test_inline_atom() {
457        // This is a silly test, just asserts that rustc can evaluate the pattern from
458        // our macro in a constant context.
459        const STR: Atom = {
460            let inline = inline_atom("hello");
461            if inline.is_some() {
462                inline.unwrap()
463            } else {
464                unreachable!();
465            }
466        };
467        assert_eq!(STR, Atom::from("hello"));
468    }
469}
hstr/lib.rs

hstr/
lib.rs
