perf: small memory performance optimizations (#35)

reduce DecisionAndLevel from 8 to 4 bytes and use `ahash`.

Cargo.toml

@@ -13,6 +13,7 @@ readme = "README.md"
 resolver = "2"
 
 [dependencies]
+ahash = "0.8.11"
 itertools = "0.13"
 petgraph = "0.6.5"
 tracing = "0.1.40"

src/internal/arena.rs

@@ -80,15 +80,15 @@ impl<TId: ArenaId, TValue> Arena<TId, TValue> {
     }
 
     /// Returns an iterator over the elements of the arena.
-    pub fn iter(&self) -> ArenaIter<TId, TValue> {
+    pub fn iter(&self) -> ArenaIter<'_, TId, TValue> {
         ArenaIter {
             arena: self,
             index: 0,
         }
     }
 
     /// Returns an mutable iterator over the elements of the arena.
-    pub fn iter_mut(&mut self) -> ArenaIterMut<TId, TValue> {
+    pub fn iter_mut(&mut self) -> ArenaIterMut<'_, TId, TValue> {
         ArenaIterMut {
             arena: self,
             index: 0,

src/internal/frozen_copy_map.rs

@@ -1,12 +1,11 @@
 use std::borrow::Borrow;
 use std::cell::UnsafeCell;
-use std::collections::hash_map::RandomState;
 use std::collections::HashMap;
 use std::hash::{BuildHasher, Hash};
 
 /// An insert only map where items can only be returned by cloning the values. This ensures that the
 /// map can safely be used in an immutable context.
-pub struct FrozenCopyMap<K, V, S = RandomState> {
+pub struct FrozenCopyMap<K, V, S = ahash::RandomState> {
     map: UnsafeCell<HashMap<K, V, S>>,
 }
 

src/internal/id.rs

@@ -74,7 +74,7 @@ impl SolvableId {
     pub fn display<VS: VersionSet, N: PackageName + Display>(
         self,
         pool: &Pool<VS, N>,
-    ) -> DisplaySolvable<VS, N> {
+    ) -> DisplaySolvable<'_, VS, N> {
         pool.resolve_internal_solvable(self).display(pool)
     }
 }

src/internal/mapping.rs

@@ -135,7 +135,7 @@ impl<TId: ArenaId, TValue> Mapping<TId, TValue> {
     }
 
     /// Returns an iterator over all the existing key value pairs.
-    pub fn iter(&self) -> MappingIter<TId, TValue> {
+    pub fn iter(&self) -> MappingIter<'_, TId, TValue> {
         MappingIter {
             mapping: self,
             offset: 0,

src/pool.rs

@@ -25,19 +25,19 @@ pub struct Pool<VS: VersionSet, N: PackageName = String> {
     package_names: Arena<NameId, N>,
 
     /// Map from package names to the id of their interned counterpart
-    pub(crate) names_to_ids: FrozenCopyMap<N, NameId>,
+    pub(crate) names_to_ids: FrozenCopyMap<N, NameId, ahash::RandomState>,
 
     /// Interned strings
     strings: Arena<StringId, String>,
 
     /// Map from package names to the id of their interned counterpart
-    pub(crate) string_to_ids: FrozenCopyMap<String, StringId>,
+    pub(crate) string_to_ids: FrozenCopyMap<String, StringId, ahash::RandomState>,
 
     /// Interned match specs
     pub(crate) version_sets: Arena<VersionSetId, (NameId, VS)>,
 
     /// Map from version set to the id of their interned counterpart
-    version_set_to_id: FrozenCopyMap<(NameId, VS), VersionSetId>,
+    version_set_to_id: FrozenCopyMap<(NameId, VS), VersionSetId, ahash::RandomState>,
 }
 
 impl<VS: VersionSet, N: PackageName> Default for Pool<VS, N> {

src/problem.rs

@@ -1,6 +1,7 @@
 //! Types to examine why a problem was unsatisfiable, and to report the causes to the user.
 
-use std::collections::{HashMap, HashSet};
+use ahash::HashMap;
+use std::collections::HashSet;
 use std::fmt;
 use std::fmt::{Display, Formatter};
 use std::hash::Hash;
@@ -277,7 +278,7 @@ impl ProblemGraph {
         let merged_nodes = if simplify {
             self.simplify(pool)
         } else {
-            HashMap::new()
+            HashMap::default()
         };
 
         write!(f, "digraph {{")?;
@@ -357,7 +358,7 @@ impl ProblemGraph {
         let graph = &self.graph;
 
         // Gather information about nodes that can be merged
-        let mut maybe_merge = HashMap::new();
+        let mut maybe_merge = HashMap::default();
         for node_id in graph.node_indices() {
             let candidate = match graph[node_id] {
                 ProblemNode::UnresolvedDependency | ProblemNode::Excluded(_) => continue,
@@ -653,7 +654,7 @@ impl<'pool, VS: VersionSet, N: PackageName + Display, M: SolvableDisplay<VS, N>>
     fn fmt_graph(
         &self,
         f: &mut Formatter<'_>,
-        top_level_edges: &[EdgeReference<ProblemEdge>],
+        top_level_edges: &[EdgeReference<'_, ProblemEdge>],
         top_level_indent: bool,
     ) -> fmt::Result
     where

src/solver/cache.rs

@@ -8,10 +8,11 @@ use crate::{
     Candidates, Dependencies, DependencyProvider, NameId, PackageName, Pool, SolvableId,
     VersionSet, VersionSetId,
 };
+use ahash::HashMap;
 use bitvec::vec::BitVec;
 use elsa::FrozenMap;
 use event_listener::Event;
-use std::{any::Any, cell::RefCell, collections::HashMap, marker::PhantomData, rc::Rc};
+use std::{any::Any, cell::RefCell, marker::PhantomData, rc::Rc};
 
 /// Keeps a cache of previously computed and/or requested information about solvables and version
 /// sets.
@@ -24,14 +25,15 @@ pub struct SolverCache<VS: VersionSet, N: PackageName, D: DependencyProvider<VS,
     package_name_to_candidates_in_flight: RefCell<HashMap<NameId, Rc<Event>>>,
 
     /// A mapping of `VersionSetId` to the candidates that match that set.
-    version_set_candidates: FrozenMap<VersionSetId, Vec<SolvableId>>,
+    version_set_candidates: FrozenMap<VersionSetId, Vec<SolvableId>, ahash::RandomState>,
 
     /// A mapping of `VersionSetId` to the candidates that do not match that set (only candidates
     /// of the package indicated by the version set are included).
-    version_set_inverse_candidates: FrozenMap<VersionSetId, Vec<SolvableId>>,
+    version_set_inverse_candidates: FrozenMap<VersionSetId, Vec<SolvableId>, ahash::RandomState>,
 
     /// A mapping of `VersionSetId` to a sorted list of candidates that match that set.
-    pub(crate) version_set_to_sorted_candidates: FrozenMap<VersionSetId, Vec<SolvableId>>,
+    pub(crate) version_set_to_sorted_candidates:
+        FrozenMap<VersionSetId, Vec<SolvableId>, ahash::RandomState>,
 
     /// A mapping from a solvable to a list of dependencies
     solvable_dependencies: Arena<DependenciesId, Dependencies>,

src/solver/clause.rs

@@ -207,7 +207,11 @@ impl Clause {
     pub fn visit_literals(
         &self,
         learnt_clauses: &Arena<LearntClauseId, Vec<Literal>>,
-        version_set_to_sorted_candidates: &FrozenMap<VersionSetId, Vec<SolvableId>>,
+        version_set_to_sorted_candidates: &FrozenMap<
+            VersionSetId,
+            Vec<SolvableId>,
+            ahash::RandomState,
+        >,
         mut visit: impl FnMut(Literal),
     ) {
         match *self {
@@ -487,7 +491,11 @@ impl ClauseState {
     pub fn next_unwatched_variable(
         &self,
         learnt_clauses: &Arena<LearntClauseId, Vec<Literal>>,
-        version_set_to_sorted_candidates: &FrozenMap<VersionSetId, Vec<SolvableId>>,
+        version_set_to_sorted_candidates: &FrozenMap<
+            VersionSetId,
+            Vec<SolvableId>,
+            ahash::RandomState,
+        >,
         decision_map: &DecisionMap,
     ) -> Option<SolvableId> {
         // The next unwatched variable (if available), is a variable that is:

src/solver/decision_map.rs

@@ -10,7 +10,7 @@ use std::fmt::{Display, Formatter};
 /// < 0: level of decision when the solvable is set to false
 #[repr(transparent)]
 #[derive(Copy, Clone)]
-struct DecisionAndLevel(i64);
+struct DecisionAndLevel(i32);
 
 impl DecisionAndLevel {
     fn undecided() -> DecisionAndLevel {
@@ -26,11 +26,12 @@ impl DecisionAndLevel {
     }
 
     fn level(self) -> u32 {
-        self.0.unsigned_abs() as u32
+        self.0.unsigned_abs()
     }
 
     fn with_value_and_level(value: bool, level: u32) -> Self {
-        Self(if value { level as i64 } else { -(level as i64) })
+        debug_assert!(level <= (i32::MAX as u32), "level is too large");
+        Self(if value { level as i32 } else { -(level as i32) })
     }
 }
 

src/solver/mod.rs

@@ -1019,9 +1019,10 @@ impl<VS: VersionSet, N: PackageName + Display, D: DependencyProvider<VS, N>, RT:
             let mut predecessor_clause_id: Option<ClauseId> = None;
             let mut clause_id = self.watches.first_clause_watching_solvable(pkg);
             while !clause_id.is_null() {
-                if predecessor_clause_id == Some(clause_id) {
-                    panic!("Linked list is circular!");
-                }
+                debug_assert!(
+                    predecessor_clause_id != Some(clause_id),
+                    "Linked list is circular!"
+                );
 
                 // Get mutable access to both clauses.
                 let mut clauses = self.clauses.borrow_mut();

tests/solver.rs

@@ -1,7 +1,7 @@
 use std::{
     any::Any,
     cell::{Cell, RefCell},
-    collections::{HashMap, HashSet},
+    collections::HashSet,
     fmt::{Debug, Display, Formatter},
     io::{stderr, Write},
     num::ParseIntError,
@@ -14,6 +14,7 @@ use std::{
     time::Duration,
 };
 
+use ahash::HashMap;
 use indexmap::IndexMap;
 use itertools::Itertools;
 use resolvo::{