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Avoid holding two copies of each term in the term pool
Now, instead of storing terms with a `HashMap<Term, Rc<Term>>`, the pool uses a hash set.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,66 @@ | ||
| //* The behaviour of the term pool could be modeled by a hash map from `Term` to `Rc<Term>`, but | ||
| //* that would require allocating two copies of each term, one in the key of the hash map, and one | ||
| //* inside the `Rc`. Instead, we store a hash set of `Rc<Term>`s, combining the key and the value | ||
| //* into a single object. We access this hash set using a `&Term`, and if the entry is present, we | ||
| //* clone it; otherwise, we allocate a new `Rc`. | ||
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| use crate::ast::*; | ||
| use std::borrow::Borrow; | ||
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| /// Since `ast::Rc` intentionally implements hashing and equality by reference (instead of by | ||
| /// value), we cannot safely implement `Borrow<Term>` for `Rc<Term>`, so we cannot access a | ||
| /// `HashSet<Rc<Term>>` using a `&Term` as a key. To go around that, we use this struct that wraps | ||
| /// an `Rc<Term>` and that re-implements hashing and equality by value, meaning we can implement | ||
| /// `Borrow<Term>` for it, and use it as the contents of the hash set instead. | ||
| #[derive(Debug, Clone, Eq)] | ||
| struct ByValue(Rc<Term>); | ||
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| impl PartialEq for ByValue { | ||
| fn eq(&self, other: &Self) -> bool { | ||
| self.0.as_ref() == other.0.as_ref() | ||
| } | ||
| } | ||
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| impl Hash for ByValue { | ||
| fn hash<H: std::hash::Hasher>(&self, state: &mut H) { | ||
| self.0.as_ref().hash(state); | ||
| } | ||
| } | ||
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| impl Borrow<Term> for ByValue { | ||
| fn borrow(&self) -> &Term { | ||
| self.0.as_ref() | ||
| } | ||
| } | ||
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| #[derive(Debug, Clone, Default)] | ||
| pub struct Storage(AHashSet<ByValue>); | ||
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| impl Storage { | ||
| pub fn new() -> Self { | ||
| Self::default() | ||
| } | ||
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| pub fn add(&mut self, term: Term) -> Rc<Term> { | ||
| // If the `hash_set_entry` feature was stable, this would be much simpler to do using | ||
| // `get_or_insert_with` (and would avoid rehashing the term) | ||
| match self.0.get(&term) { | ||
| Some(t) => t.0.clone(), | ||
| None => { | ||
| let result = Rc::new(term); | ||
| self.0.insert(ByValue(result.clone())); | ||
| result | ||
| } | ||
| } | ||
| } | ||
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| pub fn get(&self, term: &Term) -> Option<&Rc<Term>> { | ||
| self.0.get(term).map(|t| &t.0) | ||
| } | ||
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| // This method is only necessary for the hash consing tests | ||
| #[cfg(test)] | ||
| pub fn into_vec(self) -> Vec<Rc<Term>> { | ||
| self.0.into_iter().map(|ByValue(t)| t).collect() | ||
| } | ||
| } |
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