string_cache/
atom.rs

1// Copyright 2014 The Servo Project Developers. See the COPYRIGHT
2// file at the top-level directory of this distribution.
3//
4// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
5// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
7// option. This file may not be copied, modified, or distributed
8// except according to those terms.
9
10use crate::dynamic_set::{dynamic_set, Entry};
11use crate::static_sets::StaticAtomSet;
12use debug_unreachable::debug_unreachable;
13
14use std::borrow::Cow;
15use std::cmp::Ordering::{self, Equal};
16use std::fmt;
17use std::hash::{Hash, Hasher};
18use std::marker::PhantomData;
19use std::mem;
20use std::num::NonZeroU64;
21use std::ops;
22use std::slice;
23use std::str;
24use std::sync::atomic::Ordering::SeqCst;
25
26const DYNAMIC_TAG: u8 = 0b_00;
27const INLINE_TAG: u8 = 0b_01; // len in upper nybble
28const STATIC_TAG: u8 = 0b_10;
29const TAG_MASK: u64 = 0b_11;
30const LEN_OFFSET: u64 = 4;
31const LEN_MASK: u64 = 0xF0;
32
33const MAX_INLINE_LEN: usize = 7;
34const STATIC_SHIFT_BITS: usize = 32;
35
36/// Represents a string that has been interned.
37///
38/// While the type definition for `Atom` indicates that it generic on a particular
39/// implementation of an atom set, you don't need to worry about this.  Atoms can be static
40/// and come from a `StaticAtomSet` generated by the `string_cache_codegen` crate, or they
41/// can be dynamic and created by you on an `EmptyStaticAtomSet`.
42///
43/// `Atom` implements `Clone` but not `Copy`, since internally atoms are reference-counted;
44/// this means that you may need to `.clone()` an atom to keep copies to it in different
45/// places, or when passing it to a function that takes an `Atom` rather than an `&Atom`.
46///
47/// ## Creating an atom at runtime
48///
49/// If you use `string_cache_codegen` to generate a precomputed list of atoms, your code
50/// may then do something like read data from somewhere and extract tokens that need to be
51/// compared to the atoms.  In this case, you can use `Atom::from(&str)` or
52/// `Atom::from(String)`.  These create a reference-counted atom which will be
53/// automatically freed when all references to it are dropped.
54///
55/// This means that your application can safely have a loop which tokenizes data, creates
56/// atoms from the tokens, and compares the atoms to a predefined set of keywords, without
57/// running the risk of arbitrary memory consumption from creating large numbers of atoms —
58/// as long as your application does not store clones of the atoms it creates along the
59/// way.
60///
61/// For example, the following is safe and will not consume arbitrary amounts of memory:
62///
63/// ```ignore
64/// let untrusted_data = "large amounts of text ...";
65///
66/// for token in untrusted_data.split_whitespace() {
67///     let atom = Atom::from(token); // interns the string
68///
69///     if atom == Atom::from("keyword") {
70///         // handle that keyword
71///     } else if atom == Atom::from("another_keyword") {
72///         // handle that keyword
73///     } else {
74///         println!("unknown keyword");
75///     }
76/// } // atom is dropped here, so it is not kept around in memory
77/// ```
78#[derive(PartialEq, Eq)]
79// NOTE: Deriving PartialEq requires that a given string must always be interned the same way.
80pub struct Atom<Static> {
81    unsafe_data: NonZeroU64,
82    phantom: PhantomData<Static>,
83}
84
85// FIXME: bound removed from the struct definition before of this error for pack_static:
86// "error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable"
87// https://github.com/rust-lang/rust/issues/57563
88impl<Static> Atom<Static> {
89    /// For the atom!() macros
90    #[inline(always)]
91    #[doc(hidden)]
92    pub const fn pack_static(n: u32) -> Self {
93        Self {
94            unsafe_data: unsafe {
95                // STATIC_TAG ensures this is non-zero
96                NonZeroU64::new_unchecked((STATIC_TAG as u64) | ((n as u64) << STATIC_SHIFT_BITS))
97            },
98            phantom: PhantomData,
99        }
100    }
101
102    /// For the atom!() macros
103    #[inline(always)]
104    #[doc(hidden)]
105    pub const fn pack_inline(mut n: u64, len: u8) -> Self {
106        if cfg!(target_endian = "big") {
107            // Reverse order of top 7 bytes.
108            // Bottom 8 bits of `n` are zero, and we need that to remain so.
109            // String data is stored in top 7 bytes, tag and length in bottom byte.
110            n = n.to_le() << 8;
111        }
112
113        let data: u64 = (INLINE_TAG as u64) | ((len as u64) << LEN_OFFSET) | n;
114        Self {
115            // INLINE_TAG ensures this is never zero
116            unsafe_data: unsafe { NonZeroU64::new_unchecked(data) },
117            phantom: PhantomData,
118        }
119    }
120
121    fn tag(&self) -> u8 {
122        (self.unsafe_data.get() & TAG_MASK) as u8
123    }
124}
125
126impl<Static: StaticAtomSet> Atom<Static> {
127    /// Return the internal representation. For testing.
128    #[doc(hidden)]
129    pub fn unsafe_data(&self) -> u64 {
130        self.unsafe_data.get()
131    }
132
133    /// Return true if this is a static Atom. For testing.
134    #[doc(hidden)]
135    pub fn is_static(&self) -> bool {
136        self.tag() == STATIC_TAG
137    }
138
139    /// Return true if this is a dynamic Atom. For testing.
140    #[doc(hidden)]
141    pub fn is_dynamic(&self) -> bool {
142        self.tag() == DYNAMIC_TAG
143    }
144
145    /// Return true if this is an inline Atom. For testing.
146    #[doc(hidden)]
147    pub fn is_inline(&self) -> bool {
148        self.tag() == INLINE_TAG
149    }
150
151    fn static_index(&self) -> u64 {
152        self.unsafe_data.get() >> STATIC_SHIFT_BITS
153    }
154
155    /// Get the hash of the string as it is stored in the set.
156    pub fn get_hash(&self) -> u32 {
157        match self.tag() {
158            DYNAMIC_TAG => {
159                let entry = self.unsafe_data.get() as *const Entry;
160                unsafe { (*entry).hash }
161            }
162            STATIC_TAG => Static::get().hashes[self.static_index() as usize],
163            INLINE_TAG => {
164                let data = self.unsafe_data.get();
165                // This may or may not be great...
166                ((data >> 32) ^ data) as u32
167            }
168            _ => unsafe { debug_unreachable!() },
169        }
170    }
171
172    pub fn try_static(string_to_add: &str) -> Option<Self> {
173        Self::try_static_internal(string_to_add).ok()
174    }
175
176    fn try_static_internal(string_to_add: &str) -> Result<Self, phf_shared::Hashes> {
177        let static_set = Static::get();
178        let hash = phf_shared::hash(&*string_to_add, &static_set.key);
179        let index = phf_shared::get_index(&hash, static_set.disps, static_set.atoms.len());
180
181        if static_set.atoms[index as usize] == string_to_add {
182            Ok(Self::pack_static(index))
183        } else {
184            Err(hash)
185        }
186    }
187}
188
189impl<Static: StaticAtomSet> Default for Atom<Static> {
190    #[inline]
191    fn default() -> Self {
192        Atom::pack_static(Static::empty_string_index())
193    }
194}
195
196impl<Static: StaticAtomSet> Hash for Atom<Static> {
197    #[inline]
198    fn hash<H>(&self, state: &mut H)
199    where
200        H: Hasher,
201    {
202        state.write_u32(self.get_hash())
203    }
204}
205
206impl<'a, Static: StaticAtomSet> From<Cow<'a, str>> for Atom<Static> {
207    fn from(string_to_add: Cow<'a, str>) -> Self {
208        let len = string_to_add.len();
209        if len == 0 {
210            Self::pack_static(Static::empty_string_index())
211        } else if len <= MAX_INLINE_LEN {
212            let mut data: u64 = (INLINE_TAG as u64) | ((len as u64) << LEN_OFFSET);
213            {
214                let dest = inline_atom_slice_mut(&mut data);
215                dest[..len].copy_from_slice(string_to_add.as_bytes());
216            }
217            Atom {
218                // INLINE_TAG ensures this is never zero
219                unsafe_data: unsafe { NonZeroU64::new_unchecked(data) },
220                phantom: PhantomData,
221            }
222        } else {
223            Self::try_static_internal(&*string_to_add).unwrap_or_else(|hash| {
224                let ptr: std::ptr::NonNull<Entry> = dynamic_set().insert(string_to_add, hash.g);
225                let data = ptr.as_ptr() as u64;
226                debug_assert!(0 == data & TAG_MASK);
227                Atom {
228                    // The address of a ptr::NonNull is non-zero
229                    unsafe_data: unsafe { NonZeroU64::new_unchecked(data) },
230                    phantom: PhantomData,
231                }
232            })
233        }
234    }
235}
236
237impl<Static: StaticAtomSet> Clone for Atom<Static> {
238    #[inline(always)]
239    fn clone(&self) -> Self {
240        if self.tag() == DYNAMIC_TAG {
241            let entry = self.unsafe_data.get() as *const Entry;
242            unsafe { &*entry }.ref_count.fetch_add(1, SeqCst);
243        }
244        Atom { ..*self }
245    }
246}
247
248impl<Static> Drop for Atom<Static> {
249    #[inline]
250    fn drop(&mut self) {
251        if self.tag() == DYNAMIC_TAG {
252            let entry = self.unsafe_data.get() as *const Entry;
253            if unsafe { &*entry }.ref_count.fetch_sub(1, SeqCst) == 1 {
254                drop_slow(self)
255            }
256        }
257
258        // Out of line to guide inlining.
259        fn drop_slow<Static>(this: &mut Atom<Static>) {
260            dynamic_set().remove(this.unsafe_data.get() as *mut Entry);
261        }
262    }
263}
264
265impl<Static: StaticAtomSet> ops::Deref for Atom<Static> {
266    type Target = str;
267
268    #[inline]
269    fn deref(&self) -> &str {
270        unsafe {
271            match self.tag() {
272                DYNAMIC_TAG => {
273                    let entry = self.unsafe_data.get() as *const Entry;
274                    &(*entry).string
275                }
276                INLINE_TAG => {
277                    let len = (self.unsafe_data() & LEN_MASK) >> LEN_OFFSET;
278                    debug_assert!(len as usize <= MAX_INLINE_LEN);
279                    let src = inline_atom_slice(&self.unsafe_data);
280                    str::from_utf8_unchecked(src.get_unchecked(..(len as usize)))
281                }
282                STATIC_TAG => Static::get().atoms[self.static_index() as usize],
283                _ => debug_unreachable!(),
284            }
285        }
286    }
287}
288
289impl<Static: StaticAtomSet> fmt::Debug for Atom<Static> {
290    #[inline]
291    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
292        let ty_str = unsafe {
293            match self.tag() {
294                DYNAMIC_TAG => "dynamic",
295                INLINE_TAG => "inline",
296                STATIC_TAG => "static",
297                _ => debug_unreachable!(),
298            }
299        };
300
301        write!(f, "Atom('{}' type={})", &*self, ty_str)
302    }
303}
304
305impl<Static: StaticAtomSet> PartialOrd for Atom<Static> {
306    #[inline]
307    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
308        if self.unsafe_data == other.unsafe_data {
309            return Some(Equal);
310        }
311        self.as_ref().partial_cmp(other.as_ref())
312    }
313}
314
315impl<Static: StaticAtomSet> Ord for Atom<Static> {
316    #[inline]
317    fn cmp(&self, other: &Self) -> Ordering {
318        if self.unsafe_data == other.unsafe_data {
319            return Equal;
320        }
321        self.as_ref().cmp(other.as_ref())
322    }
323}
324
325// AsciiExt requires mutating methods, so we just implement the non-mutating ones.
326// We don't need to implement is_ascii because there's no performance improvement
327// over the one from &str.
328impl<Static: StaticAtomSet> Atom<Static> {
329    fn from_mutated_str<F: FnOnce(&mut str)>(s: &str, f: F) -> Self {
330        let mut buffer = mem::MaybeUninit::<[u8; 64]>::uninit();
331        let buffer = unsafe { &mut *buffer.as_mut_ptr() };
332
333        if let Some(buffer_prefix) = buffer.get_mut(..s.len()) {
334            buffer_prefix.copy_from_slice(s.as_bytes());
335            let as_str = unsafe { ::std::str::from_utf8_unchecked_mut(buffer_prefix) };
336            f(as_str);
337            Atom::from(&*as_str)
338        } else {
339            let mut string = s.to_owned();
340            f(&mut string);
341            Atom::from(string)
342        }
343    }
344
345    /// Like [`to_ascii_uppercase`].
346    ///
347    /// [`to_ascii_uppercase`]: https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.to_ascii_uppercase
348    pub fn to_ascii_uppercase(&self) -> Self {
349        for (i, b) in self.bytes().enumerate() {
350            if let b'a'..=b'z' = b {
351                return Atom::from_mutated_str(self, |s| s[i..].make_ascii_uppercase());
352            }
353        }
354        self.clone()
355    }
356
357    /// Like [`to_ascii_lowercase`].
358    ///
359    /// [`to_ascii_lowercase`]: https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.to_ascii_lowercase
360    pub fn to_ascii_lowercase(&self) -> Self {
361        for (i, b) in self.bytes().enumerate() {
362            if let b'A'..=b'Z' = b {
363                return Atom::from_mutated_str(self, |s| s[i..].make_ascii_lowercase());
364            }
365        }
366        self.clone()
367    }
368
369    /// Like [`eq_ignore_ascii_case`].
370    ///
371    /// [`eq_ignore_ascii_case`]: https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.eq_ignore_ascii_case
372    pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
373        (self == other) || self.eq_str_ignore_ascii_case(&**other)
374    }
375
376    /// Like [`eq_ignore_ascii_case`], but takes an unhashed string as `other`.
377    ///
378    /// [`eq_ignore_ascii_case`]: https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.eq_ignore_ascii_case
379    pub fn eq_str_ignore_ascii_case(&self, other: &str) -> bool {
380        (&**self).eq_ignore_ascii_case(other)
381    }
382}
383
384#[inline(always)]
385fn inline_atom_slice(x: &NonZeroU64) -> &[u8] {
386        let x: *const NonZeroU64 = x;
387        let mut data = x as *const u8;
388        // All except the lowest byte, which is first in little-endian, last in big-endian.
389        if cfg!(target_endian = "little") {
390            data = unsafe { data.offset(1) };
391        }
392        let len = 7;
393        unsafe { slice::from_raw_parts(data, len) }   
394}
395
396#[inline(always)]
397fn inline_atom_slice_mut(x: &mut u64) -> &mut [u8] {   
398        let x: *mut u64 = x;
399        let mut data = x as *mut u8;
400        // All except the lowest byte, which is first in little-endian, last in big-endian.
401        if cfg!(target_endian = "little") {
402            data = unsafe { data.offset(1) };
403        }
404        let len = 7;
405        unsafe { slice::from_raw_parts_mut(data, len) }
406}
string_cache/atom.rs

string_cache/
atom.rs
