111 lines
2.6 KiB
Markdown
111 lines
2.6 KiB
Markdown
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# weak-table: weak hash maps and sets for Rust
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[](https://github.com/tov/weak-table-rs/actions)
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[](https://crates.io/crates/weak-table)
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[](LICENSE-MIT)
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This crate defines several kinds of weak hash maps and sets. See
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the [full API documentation](http://docs.rs/weak-table/) for details.
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### Rust version support
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This crate supports Rust version 1.46 and later.
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### Crate features
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`weak-table` is built with the `std` feature, which enables
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functionality dependent on the `std` library, enabled by default.
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Optionally, the following dependency may be enabled:
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- `ahash`: use `ahash`’s hasher rather than the `std` hasher
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If the `std` feature is disabled (for no_std) then the `ahash` dependency **must** be enabled.
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### Examples
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Here we create a weak hash map and demonstrate that it forgets mappings
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whose keys expire:
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```rust
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use weak_table::WeakKeyHashMap;
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use std::sync::{Arc, Weak};
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let mut table = <WeakKeyHashMap<Weak<str>, u32>>::new();
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let one = Arc::<str>::from("one");
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let two = Arc::<str>::from("two");
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table.insert(one.clone(), 1);
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assert_eq!( table.get("one"), Some(&1) );
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assert_eq!( table.get("two"), None );
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table.insert(two.clone(), 2);
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*table.get_mut(&one).unwrap() += 10;
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assert_eq!( table.get("one"), Some(&11) );
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assert_eq!( table.get("two"), Some(&2) );
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drop(one);
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assert_eq!( table.get("one"), None );
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assert_eq!( table.get("two"), Some(&2) );
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```
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Here we use a weak hash set to implement a simple string interning facility:
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```rust
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use weak_table::WeakHashSet;
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use std::ops::Deref;
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use std::rc::{Rc, Weak};
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#[derive(Clone, Debug)]
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pub struct Symbol(Rc<str>);
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impl PartialEq for Symbol {
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fn eq(&self, other: &Symbol) -> bool {
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Rc::ptr_eq(&self.0, &other.0)
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}
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}
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impl Eq for Symbol {}
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impl Deref for Symbol {
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type Target = str;
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fn deref(&self) -> &str {
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&self.0
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}
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}
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#[derive(Debug, Default)]
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pub struct SymbolTable(WeakHashSet<Weak<str>>);
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impl SymbolTable {
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pub fn new() -> Self {
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Self::default()
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}
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pub fn intern(&mut self, name: &str) -> Symbol {
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if let Some(rc) = self.0.get(name) {
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Symbol(rc)
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} else {
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let rc = Rc::<str>::from(name);
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self.0.insert(Rc::clone(&rc));
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Symbol(rc)
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}
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}
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}
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#[test]
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fn interning() {
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let mut tab = SymbolTable::new();
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let a0 = tab.intern("a");
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let a1 = tab.intern("a");
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let b = tab.intern("b");
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assert_eq!(a0, a1);
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assert_ne!(a0, b);
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}
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```
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