ef7a0da8cb
This is based on this matklad post: https://matklad.github.io/2020/03/22/fast-simple-rust-interner.html It's modified slightly to provide a safer interface and slightly more readable implementation: * interned string IDs are wrapped in a newtype that is not publicly constructible * unsafe block is reduced to only the small scope in which it is needed * lookup lifetime is pinned explicitly to make the intent clearer when reading this code Change-Id: Ia3dae988f33f8e5e7d8dc0c1a9216914a945b036 Reviewed-on: https://cl.tvl.fyi/c/depot/+/2578 Tested-by: BuildkiteCI Reviewed-by: tazjin <mail@tazj.in>
87 lines
2.4 KiB
Rust
87 lines
2.4 KiB
Rust
//! String-interning implementation for values that are likely to
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//! benefit from fast comparisons and deduplication (e.g. instances of
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//! variable names).
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//!
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//! This uses a trick from the typed-arena crate for guaranteeing
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//! stable addresses by never resizing the existing String buffer, and
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//! collecting full buffers in a vector.
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use std::collections::HashMap;
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#[cfg(test)]
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mod tests;
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#[derive(Clone, Copy, Debug, PartialEq, Hash)]
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pub struct InternedStr {
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id: usize,
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}
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#[derive(Default)]
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pub struct Interner {
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map: HashMap<&'static str, InternedStr>,
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vec: Vec<&'static str>,
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buf: String,
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full: Vec<String>,
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}
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impl Interner {
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pub fn with_capacity(cap: usize) -> Self {
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Interner {
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buf: String::with_capacity(cap),
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..Default::default()
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}
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}
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pub fn intern<S: AsRef<str>>(&mut self, name: S) -> InternedStr {
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let name = name.as_ref();
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if let Some(&id) = self.map.get(name) {
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return id;
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}
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let name = self.alloc(name);
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let id = InternedStr {
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id: self.vec.len() as usize,
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};
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self.map.insert(name, id);
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self.vec.push(name);
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debug_assert!(self.lookup(id) == name);
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debug_assert!(self.intern(name) == id);
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id
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}
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pub fn lookup<'a>(&'a self, id: InternedStr) -> &'a str {
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self.vec[id.id]
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}
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fn alloc<'a>(&'a mut self, name: &str) -> &'static str {
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let cap = self.buf.capacity();
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if cap < self.buf.len() + name.len() {
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let new_cap = (cap.max(name.len()) + 1).next_power_of_two();
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let new_buf = String::with_capacity(new_cap);
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let old_buf = std::mem::replace(&mut self.buf, new_buf);
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self.full.push(old_buf);
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}
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let interned: &'a str = {
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let start = self.buf.len();
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self.buf.push_str(name);
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&self.buf[start..]
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};
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unsafe {
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// This is sound for two reasons:
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//
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// 1. This function (Interner::alloc) is private, which
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// prevents users from allocating a supposedly static
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// reference.
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//
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// 2. Interner::lookup explicitly shortens the lifetime of
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// references that are handed out to that of the
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// reference to self.
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return &*(interned as *const str);
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}
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}
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}
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