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perf: encode shared constraints with an auxiliary variable per version set #236

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124 changes: 123 additions & 1 deletion src/conflict.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ use petgraph::{

use crate::{
DenseIndex, DependencyProvider, Interner, Requirement, SolvableId, SolverId, StringId,
VersionSetId,
VariableId, VersionSetId,
internal::{id::ClauseId, solver_id::SolvableIdOrRoot},
runtime::AsyncRuntime,
solver::{Solver, clause::Clause, variable_map::VariableOrigin},
Expand Down Expand Up @@ -56,6 +56,72 @@ impl Conflict {
let unresolved_node = graph.add_node(ConflictNode::UnresolvedDependency);
let mut last_node_by_name: HashMap<D::NameId, NodeIndex> = HashMap::default();

// The shared constrains encoding splits each (parent, excluded
// candidate) pair over two clauses linked by an auxiliary variable.
// Collect both sides per auxiliary variable so the original
// `parent -> candidate` edges can be reconstructed in the loop below.
let mut constrains_aux_parents: HashMap<VariableId, Vec<SolvableIdOrRoot<D::SolvableId>>> =
HashMap::default();
let mut constrains_aux_candidates: HashMap<
VariableId,
Vec<SolvableIdOrRoot<D::SolvableId>>,
> = HashMap::default();
for clause_id in &self.clauses {
match state.clauses.kinds[clause_id.to_index()] {
Clause::ConstrainsParent(parent, aux, _) => {
let parent = parent
.as_solvable_or_root(&state.variable_map)
.expect("constrains parents are solvables or root");
constrains_aux_parents.entry(aux).or_default().push(parent);
}
Clause::ConstrainsExcluded(candidate, aux, _) => {
let candidate = candidate
.as_solvable_or_root(&state.variable_map)
.expect("excluded candidates are solvables");
constrains_aux_candidates
.entry(aux)
.or_default()
.push(candidate);
}
_ => {}
}
}

// If only one side of a chain is part of the conflict, recover the
// other side from the assignment reason of the auxiliary variable.
let reason_clause = |aux: VariableId| {
state
.decision_tracker
.find_clause_for_assignment(aux)
.map(|clause_id| state.clauses.kinds[clause_id.to_index()])
};
for &aux in constrains_aux_parents.keys() {
if constrains_aux_candidates.contains_key(&aux) {
continue;
}
if let Some(Clause::ConstrainsExcluded(candidate, _, _)) = reason_clause(aux) {
let candidate = candidate
.as_solvable_or_root(&state.variable_map)
.expect("excluded candidates are solvables");
constrains_aux_candidates.insert(aux, vec![candidate]);
}
}
for &aux in constrains_aux_candidates.keys() {
if constrains_aux_parents.contains_key(&aux) {
continue;
}
if let Some(Clause::ConstrainsParent(parent, _, _)) = reason_clause(aux) {
let parent = parent
.as_solvable_or_root(&state.variable_map)
.expect("constrains parents are solvables or root");
constrains_aux_parents.insert(aux, vec![parent]);
}
}

// Avoids adding the same edge once for each side of a chain.
type ConstrainsEdge<S> = (SolvableIdOrRoot<S>, SolvableIdOrRoot<S>, VersionSetId);
let mut constrains_edges: HashSet<ConstrainsEdge<D::SolvableId>> = HashSet::new();

for clause_id in &self.clauses {
let clause = &state.clauses.kinds[clause_id.to_index()];
match clause {
Expand Down Expand Up @@ -160,6 +226,62 @@ impl Conflict {
ConflictEdge::Conflict(ConflictCause::Constrains(version_set_id)),
);
}
&Clause::ConstrainsParent(package_id, aux, version_set_id) => {
let package_solvable = package_id
.as_solvable_or_root(&state.variable_map)
.expect("constrains parents are solvables or root");

let Some(candidates) = constrains_aux_candidates.get(&aux) else {
continue;
};

for &dependency_solvable in candidates {
if !constrains_edges.insert((
package_solvable,
dependency_solvable,
version_set_id,
)) {
continue;
}

let package_node = Self::add_node(&mut graph, &mut nodes, package_solvable);
let dep_node = Self::add_node(&mut graph, &mut nodes, dependency_solvable);

graph.add_edge(
package_node,
dep_node,
ConflictEdge::Conflict(ConflictCause::Constrains(version_set_id)),
);
}
}
&Clause::ConstrainsExcluded(dep_id, aux, version_set_id) => {
let dependency_solvable = dep_id
.as_solvable_or_root(&state.variable_map)
.expect("excluded candidates are solvables");

let Some(parents) = constrains_aux_parents.get(&aux) else {
continue;
};

for &package_solvable in parents {
if !constrains_edges.insert((
package_solvable,
dependency_solvable,
version_set_id,
)) {
continue;
}

let package_node = Self::add_node(&mut graph, &mut nodes, package_solvable);
let dep_node = Self::add_node(&mut graph, &mut nodes, dependency_solvable);

graph.add_edge(
package_node,
dep_node,
ConflictEdge::Conflict(ConflictCause::Constrains(version_set_id)),
);
}
}
Clause::AnyOf(selected, _variable) => {
// Assumption: since `AnyOf` of clause can never be false, we dont add an edge
// for it.
Expand Down
124 changes: 124 additions & 0 deletions src/solver/clause.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -81,6 +81,30 @@ pub(crate) enum Clause<N> {
///
/// In SAT terms: (¬A ∨ ¬B)
Constrains(VariableId, VariableId, VersionSetId),
/// Makes the constraint's auxiliary variable turn true when a candidate
/// that is excluded by the constraint's version set is installed.
///
/// Usage: constraints whose version set excludes many candidates share a
/// single auxiliary variable per version set. The clauses that link the
/// excluded candidates to the auxiliary variable are emitted once per
/// version set, and each (parent, version set) pair only adds a single
/// [`Clause::ConstrainsParent`] clause. Together these encode the same
/// restriction as [`Clause::Constrains`] with O(candidates + parents)
/// instead of O(candidates * parents) clauses.
///
/// Only this implication direction is needed (the Plaisted-Greenbaum
/// one-sided form): the auxiliary variable is never decided directly, it
/// only receives a value through propagation.
///
/// In SAT terms: (¬candidate ∨ aux)
ConstrainsExcluded(VariableId, VariableId, VersionSetId),
/// Forbids a parent solvable from being installed together with the
/// constraint's auxiliary variable, i.e. together with any candidate that
/// is excluded by the constraint's version set. See
/// [`Clause::ConstrainsExcluded`] for an overview of the encoding.
///
/// In SAT terms: (¬parent ∨ ¬aux)
ConstrainsParent(VariableId, VariableId, VersionSetId),
/// Forbids the package on the right-hand side
///
/// Note that the package on the left-hand side is not part of the clause,
Expand Down Expand Up @@ -117,6 +141,8 @@ impl<N> Clause<N> {
matches!(
self,
Clause::Constrains(..)
| Clause::ConstrainsExcluded(..)
| Clause::ConstrainsParent(..)
| Clause::ForbidMultipleInstances(..)
| Clause::Lock(..)
| Clause::AnyOf(..)
Expand Down Expand Up @@ -218,6 +244,44 @@ impl<N: SolverId> Clause<N> {
)
}

/// Returns the building blocks needed for a new [WatchedLiterals] of the
/// [Clause::ConstrainsExcluded] kind.
fn constrains_excluded(
candidate: VariableId,
aux: VariableId,
via: VersionSetId,
) -> (Self, Option<[Literal; 2]>) {
(
Clause::ConstrainsExcluded(candidate, aux, via),
Some([candidate.negative(), aux.positive()]),
)
}

/// Returns the building blocks needed for a new [WatchedLiterals] of the
/// [Clause::ConstrainsParent] kind. See [`Clause::constrains`] for the
/// meaning of the returned values.
fn constrains_parent(
parent: VariableId,
aux: VariableId,
via: VersionSetId,
decision_tracker: &DecisionTracker,
) -> (Self, Option<[Literal; 2]>, bool) {
// It only makes sense to introduce a constrains clause when the parent
// solvable is undecided or going to be installed
assert_ne!(decision_tracker.assigned_value(parent), Some(false));

// If the auxiliary variable is already true, an excluded candidate is
// installed, which implies the parent solvable would be false (and we
// just asserted that it is not).
let conflict = decision_tracker.assigned_value(aux) == Some(true);

(
Clause::ConstrainsParent(parent, aux, via),
Some([parent.negative(), aux.negative()]),
conflict,
)
}

/// Returns the ids of the solvables that will be watched as well as the
/// clause itself.
fn forbid_multiple(
Expand Down Expand Up @@ -311,6 +375,12 @@ impl<N: SolverId> Clause<N> {
Clause::Constrains(s1, s2, _) => [s1.negative(), s2.negative()]
.into_iter()
.try_fold(init, visit),
Clause::ConstrainsExcluded(candidate, aux, _) => [candidate.negative(), aux.positive()]
.into_iter()
.try_fold(init, visit),
Clause::ConstrainsParent(parent, aux, _) => [parent.negative(), aux.negative()]
.into_iter()
.try_fold(init, visit),
Clause::ForbidMultipleInstances(s1, s2, _) => {
[s1.negative(), s2].into_iter().try_fold(init, visit)
}
Expand Down Expand Up @@ -437,6 +507,38 @@ impl WatchedLiterals {
)
}

/// Shorthand method to construct a [Clause::ConstrainsExcluded] without
/// requiring complicated arguments.
pub fn constrains_excluded<N: SolverId>(
candidate: VariableId,
aux: VariableId,
requirement: VersionSetId,
) -> (Option<Self>, Clause<N>) {
let (kind, watched_literals) = Clause::constrains_excluded(candidate, aux, requirement);
(Self::from_kind_and_initial_watches(watched_literals), kind)
}

/// Shorthand method to construct a [Clause::ConstrainsParent] without
/// requiring complicated arguments.
///
/// The returned boolean value is true when adding the clause resulted in a
/// conflict.
pub fn constrains_parent<N: SolverId>(
parent: VariableId,
aux: VariableId,
requirement: VersionSetId,
decision_tracker: &DecisionTracker,
) -> (Option<Self>, bool, Clause<N>) {
let (kind, watched_literals, conflict) =
Clause::constrains_parent(parent, aux, requirement, decision_tracker);

(
Self::from_kind_and_initial_watches(watched_literals),
conflict,
kind,
)
}

pub fn lock<N: SolverId>(
locked_candidate: VariableId,
other_candidate: VariableId,
Expand Down Expand Up @@ -652,6 +754,28 @@ impl<I: Interner> Display for ClauseDisplay<'_, I> {
self.interner.display_version_set(version_set_id)
)
}
Clause::ConstrainsExcluded(candidate, aux, version_set_id) => {
write!(
f,
"ConstrainsExcluded({}({:?}), {}({:?}), {})",
candidate.display(self.variable_map, self.interner),
candidate,
aux.display(self.variable_map, self.interner),
aux,
self.interner.display_version_set(version_set_id)
)
}
Clause::ConstrainsParent(parent, aux, version_set_id) => {
write!(
f,
"ConstrainsParent({}({:?}), {}({:?}), {})",
parent.display(self.variable_map, self.interner),
parent,
aux.display(self.variable_map, self.interner),
aux,
self.interner.display_version_set(version_set_id)
)
}
Clause::ForbidMultipleInstances(v1, v2, name) => {
write!(
f,
Expand Down
6 changes: 5 additions & 1 deletion src/solver/diagnostics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,9 @@ impl<D: DependencyProvider, RT: AsyncRuntime> Solver<D, RT> {
Clause::Learnt(..) => {
learned_clauses += 1;
}
Clause::Constrains(..) => {
Clause::Constrains(..)
| Clause::ConstrainsExcluded(..)
| Clause::ConstrainsParent(..) => {
constrains_clauses += 1;
}
Clause::AnyOf(..) => {
Expand All @@ -60,6 +62,8 @@ impl<D: DependencyProvider, RT: AsyncRuntime> Solver<D, RT> {
None => assertion_clauses += 1,
Some(_) => match clause {
Clause::Constrains(..)
| Clause::ConstrainsExcluded(..)
| Clause::ConstrainsParent(..)
| Clause::ForbidMultipleInstances(..)
| Clause::Lock(..)
| Clause::AnyOf(..) => binary_watched += 1,
Expand Down
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