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Remove fixed assignments #16

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merged 1 commit into from
Jan 25, 2024
Merged

Remove fixed assignments #16

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53 changes: 4 additions & 49 deletions src/solver/decision_tracker.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,11 +10,6 @@ pub(crate) struct DecisionTracker {
map: DecisionMap,
stack: Vec<Decision>,
propagate_index: usize,

// Fixed assignments are decisions that are true regardless of previous decisions. These
// assignments are not cleared after backtracked.
fixed_assignments: Vec<Decision>,
fixed_assignment_index: usize,
}

impl DecisionTracker {
Expand All @@ -23,24 +18,13 @@ impl DecisionTracker {
map: DecisionMap::new(),
stack: Vec::new(),
propagate_index: 0,
fixed_assignment_index: 0,
fixed_assignments: Vec::new(),
}
}

pub(crate) fn clear(&mut self) {
self.map = DecisionMap::new();
self.stack = Vec::new();
self.propagate_index = 0;

// The fixed assignment decisions are kept but the propagation index is. This assures that
// during the next propagation all fixed assignment decisions are repropagated.
self.fixed_assignment_index = 0;

// Re-apply all the fixed decisions
for decision in self.fixed_assignments.iter() {
self.map.set(decision.solvable_id, decision.value, 1);
}
}

pub(crate) fn is_empty(&self) -> bool {
Expand All @@ -56,10 +40,7 @@ impl DecisionTracker {
}

pub(crate) fn stack(&self) -> impl Iterator<Item = Decision> + DoubleEndedIterator + '_ {
self.fixed_assignments
.iter()
.copied()
.chain(self.stack.iter().copied())
self.stack.iter().copied()
}

pub(crate) fn level(&self, solvable_id: SolvableId) -> u32 {
Expand Down Expand Up @@ -92,26 +73,6 @@ impl DecisionTracker {
}
}

/// Attempts to add a fixed assignment decision. A fixed assignment is different from a regular
/// decision in that its value is persistent and cannot be reverted by backtracking. This is
/// useful for assertion clauses.
///
/// Returns true if the solvable was undecided, false if it was already decided to the same
/// value.
///
/// Returns an error if the solvable was decided to a different value (which means there is a conflict)
pub(crate) fn try_add_fixed_assignment(&mut self, decision: Decision) -> Result<bool, ()> {
match self.map.value(decision.solvable_id) {
None => {
self.map.set(decision.solvable_id, decision.value, 1);
self.fixed_assignments.push(decision);
Ok(true)
}
Some(value) if value == decision.value => Ok(false),
_ => Err(()),
}
}

pub(crate) fn undo_until(&mut self, level: u32) {
while let Some(decision) = self.stack.last() {
if self.level(decision.solvable_id) <= level {
Expand All @@ -136,14 +97,8 @@ impl DecisionTracker {
///
/// Side-effect: the decision will be marked as propagated
pub(crate) fn next_unpropagated(&mut self) -> Option<Decision> {
if self.fixed_assignment_index < self.fixed_assignments.len() {
let &decision = &self.fixed_assignments[self.fixed_assignment_index];
self.fixed_assignment_index += 1;
Some(decision)
} else {
let &decision = self.stack[self.propagate_index..].iter().next()?;
self.propagate_index += 1;
Some(decision)
}
let &decision = self.stack[self.propagate_index..].iter().next()?;
self.propagate_index += 1;
Some(decision)
}
}
23 changes: 9 additions & 14 deletions src/solver/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -293,9 +293,8 @@ impl<VS: VersionSet, N: PackageName + Display, D: DependencyProvider<VS, N>> Sol
.alloc(ClauseState::forbid_multiple(candidate, other_candidate));

let clause = &mut self.clauses[clause_id];
if clause.has_watches() {
self.watches.start_watching(clause, clause_id);
}
debug_assert!(clause.has_watches());
self.watches.start_watching(clause, clause_id);
}
}

Expand All @@ -308,9 +307,9 @@ impl<VS: VersionSet, N: PackageName + Display, D: DependencyProvider<VS, N>> Sol
.alloc(ClauseState::lock(locked_solvable_id, other_candidate));

let clause = &mut self.clauses[clause_id];
if clause.has_watches() {
self.watches.start_watching(clause, clause_id);
}

debug_assert!(clause.has_watches());
self.watches.start_watching(clause, clause_id);
}
}
}
Expand All @@ -319,15 +318,11 @@ impl<VS: VersionSet, N: PackageName + Display, D: DependencyProvider<VS, N>> Sol
for (solvable, reason) in package_candidates.excluded.iter().copied() {
let clause_id = self.clauses.alloc(ClauseState::exclude(solvable, reason));

// Immediately add the decision to unselect this solvable. This should be fine because
// no other decision should have been made about this solvable in the first place.
let exclude_descision = Decision::new(solvable, false, clause_id);
self.decision_tracker
.try_add_fixed_assignment(exclude_descision)
.expect("already decided about a solvable that wasn't properly added yet.");
// Exclusions are negative assertions, tracked outside of the watcher system
self.negative_assertions.push((solvable, clause_id));

// No need to add watches for this clause because the clause is an assertion on which
// we already decided.
// Conflicts should be impossible here
debug_assert!(self.decision_tracker.assigned_value(solvable) != Some(true));
}

self.clauses_added_for_package.insert(package_name);
Expand Down