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logic.rs
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logic.rs
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use crate::context::{
Context, ContextOps, Floundered, InferenceTable, ResolventOps, TruncateOps, UnificationOps,
};
use crate::fallible::NoSolution;
use crate::forest::Forest;
use crate::hh::HhGoal;
use crate::strand::{CanonicalStrand, SelectedSubgoal, Strand};
use crate::table::AnswerIndex;
use crate::{
Answer, CompleteAnswer, ExClause, FlounderedSubgoal, Literal, Minimums, TableIndex, TimeStamp,
};
type RootSearchResult<T> = Result<T, RootSearchFail>;
/// The different ways that a *root* search (which potentially pursues
/// many strands) can fail. A root search is one that begins with an
/// empty stack.
#[derive(Debug)]
pub(super) enum RootSearchFail {
/// The table we were trying to solve cannot succeed.
NoMoreSolutions,
/// The table cannot be solved without more type information.
Floundered,
/// We did not find a solution, but we still have things to try.
/// Repeat the request, and we'll give one of those a spin.
///
/// (In a purely depth-first-based solver, like Prolog, this
/// doesn't appear.)
QuantumExceeded,
/// A negative cycle was found. This is fail-fast, so even if there was
/// possibly a solution (ambiguous or not), it may not have been found.
NegativeCycle,
/// The current answer index is not useful. Currently, this is returned
/// because the current answer needs refining.
InvalidAnswer,
}
/// This is returned when we try to select a subgoal for a strand.
enum SubGoalSelection {
/// A subgoal was successfully selected. It has already been checked
/// to not be floundering. However, it may have an answer already, be
/// coinductive, or create a cycle.
Selected,
/// This strand has no remaining subgoals.
NoRemainingSubgoals,
/// This strand has floundered. Either all the positive subgoals
/// have floundered or a single negative subgoal has floundered.
Floundered,
}
/// This is returned `on_no_remaining_subgoals`
enum NoRemainingSubgoalsResult {
/// There is an answer available for the root table
RootAnswerAvailable,
/// There was a `RootSearchFail`
RootSearchFail(RootSearchFail),
// This was a success
Success,
}
impl<C: Context> Forest<C> {
/// Returns an answer with a given index for the given table. This
/// may require activating a strand and following it. It returns
/// `Ok(answer)` if they answer is available and otherwise a
/// `RootSearchFail` result.
pub(super) fn root_answer(
&mut self,
context: &impl ContextOps<C>,
table: TableIndex,
answer_index: AnswerIndex,
) -> RootSearchResult<CompleteAnswer<C>> {
assert!(self.stack.is_empty());
match self.ensure_root_answer(context, table, answer_index) {
Ok(()) => {
let answer = self.answer(table, answer_index);
let has_delayed_subgoals = C::has_delayed_subgoals(&answer.subst);
if has_delayed_subgoals {
return Err(RootSearchFail::InvalidAnswer);
}
Ok(CompleteAnswer {
subst: C::canonical_constrained_subst_from_canonical_constrained_answer(
&answer.subst,
),
ambiguous: answer.ambiguous,
})
}
Err(err) => Err(err),
}
}
/// Ensures that answer with the given index is available from the
/// given table. Returns `Ok` if there is an answer.
///
/// This function first attempts to fetch answer that is cached in
/// the table. If none is found, then it will recursively search
/// to find an answer.
fn ensure_root_answer(
&mut self,
context: &impl ContextOps<C>,
initial_table: TableIndex,
initial_answer: AnswerIndex,
) -> RootSearchResult<()> {
info_heading!(
"ensure_answer(table={:?}, answer={:?})",
initial_table,
initial_answer
);
info!("table goal = {:#?}", self.tables[initial_table].table_goal);
// Check if this table has floundered.
if self.tables[initial_table].is_floundered() {
return Err(RootSearchFail::Floundered);
}
// Check for a tabled answer.
if let Some(answer) = self.tables[initial_table].answer(initial_answer) {
info!("answer cached = {:?}", answer);
return Ok(());
}
// If no tabled answer is present, we ought to be requesting
// the next available index.
assert_eq!(
self.tables[initial_table].next_answer_index(),
initial_answer
);
self.stack.push(initial_table, Minimums::MAX);
loop {
// FIXME: use depth for debug/info printing
let clock = self.stack.top().clock;
// If we had an active strand, continue to pursue it
let table = self.stack.top().table;
// We track when we last pursued each strand. If all the strands have been
// pursued at this depth, then that means they all encountered a cycle.
// We also know that if the first strand has been pursued at this depth,
// then all have. Otherwise, an answer to any strand would have provided an
// answer for the table.
let next_strand = self.stack.top().active_strand.take().or_else(|| {
self.tables[table]
.dequeue_next_strand_if(|strand| strand.last_pursued_time < clock)
.map(|canonical_strand| {
let num_universes = C::num_universes(&self.tables[table].table_goal);
let CanonicalStrand {
canonical_ex_clause,
selected_subgoal,
last_pursued_time,
} = canonical_strand;
let (infer, ex_clause) =
context.instantiate_ex_clause(num_universes, &canonical_ex_clause);
let strand = Strand {
infer,
ex_clause,
selected_subgoal: selected_subgoal.clone(),
last_pursued_time,
};
strand
})
});
match next_strand {
Some(mut strand) => {
debug!("next strand: {:#?}", strand);
strand.last_pursued_time = clock;
match self.select_subgoal(context, &mut strand) {
SubGoalSelection::Selected => {
// A subgoal has been selected. We now check this subgoal
// table for an existing answer or if it's in a cycle.
// If neither of those are the case, a strand is selected
// and the next loop iteration happens.
self.on_subgoal_selected(strand)?;
continue;
}
SubGoalSelection::NoRemainingSubgoals => {
match self.on_no_remaining_subgoals(context, strand) {
NoRemainingSubgoalsResult::RootAnswerAvailable => return Ok(()),
NoRemainingSubgoalsResult::RootSearchFail(e) => return Err(e),
NoRemainingSubgoalsResult::Success => {}
};
continue;
}
SubGoalSelection::Floundered => {
// The strand floundered when trying to select a subgoal.
// This will always return a `RootSearchFail`, either because the
// root table floundered or we yield with `QuantumExceeded`.
return Err(self.on_subgoal_selection_flounder(strand));
}
}
}
None => {
self.on_no_strands_left()?;
continue;
}
}
}
}
pub(super) fn any_future_answer(
&self,
table: TableIndex,
answer: AnswerIndex,
mut test: impl FnMut(&C::InferenceNormalizedSubst) -> bool,
) -> bool {
if let Some(answer) = self.tables[table].answer(answer) {
info!("answer cached = {:?}", answer);
return test(C::inference_normalized_subst_from_subst(&answer.subst));
}
self.tables[table].strands().any(|strand| {
test(C::inference_normalized_subst_from_ex_clause(
&strand.canonical_ex_clause,
))
})
}
/// This is called when an answer is available for the selected subgoal
/// of the strand. First, if the selected subgoal is a `Positive` subgoal,
/// it first clones the strand pursuing the next answer. Then, it merges the
/// answer into the provided `Strand`.
/// On success, `Ok` is returned and the `Strand` can be continued to process
/// On failure, `Err` is returned and the `Strand` should be discarded
fn merge_answer_into_strand(&mut self, strand: &mut Strand<C>) -> RootSearchResult<()> {
// At this point, we know we have an answer for
// the selected subgoal of the strand.
// Now, we have to unify that answer onto the strand.
// If this subgoal was a `Positive` one, whichever way this
// particular answer turns out, there may yet be *more* answers.
// Enqueue that alternative for later.
// NOTE: this is separate from the match below because we `take` the selected_subgoal
// below, but here we keep it for the new `Strand`.
let selected_subgoal = strand.selected_subgoal.as_ref().unwrap();
if let Literal::Positive(_) = strand.ex_clause.subgoals[selected_subgoal.subgoal_index] {
let mut next_subgoal = selected_subgoal.clone();
next_subgoal.answer_index.increment();
let next_strand = Strand {
infer: strand.infer.clone(),
ex_clause: strand.ex_clause.clone(),
selected_subgoal: Some(next_subgoal),
last_pursued_time: strand.last_pursued_time.clone(),
};
let table = self.stack.top().table;
let canonical_next_strand = Self::canonicalize_strand(next_strand);
self.tables[table].enqueue_strand(canonical_next_strand);
}
// Deselect and remove the selected subgoal, now that we have an answer for it.
let selected_subgoal = strand.selected_subgoal.take().unwrap();
let subgoal = strand
.ex_clause
.subgoals
.remove(selected_subgoal.subgoal_index);
match subgoal {
Literal::Positive(subgoal) => {
let SelectedSubgoal {
subgoal_index: _,
subgoal_table,
answer_index,
ref universe_map,
} = selected_subgoal;
let table_goal = &C::map_goal_from_canonical(
&universe_map,
&C::canonical(&self.tables[subgoal_table].table_goal),
);
let answer_subst = &C::map_subst_from_canonical(
&universe_map,
&self.answer(subgoal_table, answer_index).subst,
);
match strand.infer.apply_answer_subst(
&mut strand.ex_clause,
&subgoal,
table_goal,
answer_subst,
) {
Ok(()) => {
let Strand {
infer: _,
ex_clause,
selected_subgoal: _,
last_pursued_time: _,
} = strand;
// If the answer had was ambiguous, we have to
// ensure that `ex_clause` is also ambiguous. This is
// the SLG FACTOR operation, though NFTD just makes it
// part of computing the SLG resolvent.
if self.answer(subgoal_table, answer_index).ambiguous {
ex_clause.ambiguous = true;
}
// Increment the answer time for the `ex_clause`. Floundered
// subgoals may be eligble to be pursued again.
ex_clause.answer_time.increment();
// Ok, we've applied the answer to this Strand.
return Ok(());
}
// This answer led nowhere. Give up for now, but of course
// there may still be other strands to pursue, so return
// `QuantumExceeded`.
Err(NoSolution) => {
info!("answer not unifiable -> NoSolution");
// This strand as no solution. It is no longer active,
// so it dropped at the end of this scope.
// Now we want to propogate back to the up with `QuantumExceeded`
self.unwind_stack();
return Err(RootSearchFail::QuantumExceeded);
}
}
}
Literal::Negative(_) => {
let SelectedSubgoal {
subgoal_index: _,
subgoal_table,
answer_index,
universe_map: _,
} = selected_subgoal;
// We got back an answer. This is bad, because we want
// to disprove the subgoal, but it may be
// "conditional" (maybe true, maybe not).
let answer = self.answer(subgoal_table, answer_index);
// By construction, we do not expect negative subgoals
// to have delayed subgoals. This is because we do not
// need to permit `not { L }` where `L` is a
// coinductive goal. We could improve this if needed,
// but it keeps things simple.
if C::has_delayed_subgoals(&answer.subst) {
panic!("Negative subgoal had delayed_subgoals");
}
if !answer.ambiguous {
// We want to disproval the subgoal, but we
// have an unconditional answer for the subgoal,
// therefore we have failed to disprove it.
info!("found unconditional answer to neg literal -> NoSolution");
// This strand as no solution. By returning an Err,
// the caller should discard this `Strand`.
// Now we want to propogate back to the up with `QuantumExceeded`
self.unwind_stack();
return Err(RootSearchFail::QuantumExceeded);
}
// Otherwise, the answer is ambiguous. We can keep going,
// but we have to mark our strand, too, as ambiguous.
//
// We want to disproval the subgoal, but we
// have an unconditional answer for the subgoal,
// therefore we have failed to disprove it.
strand.ex_clause.ambiguous = true;
// Strand is ambigious.
return Ok(());
}
};
}
/// This is called when the selected subgoal for a strand has floundered.
/// We have to decide what this means for the strand.
/// - If the strand was positively dependent on the subgoal, we flounder,
/// the subgoal, then return `false`. This strand may be able to be
/// retried later.
/// - If the strand was negatively dependent on the subgoal, then strand
/// has led nowhere of interest and we return `true`. This strand should
/// be discarded.
///
/// In other words, we return whether this strand flounders.
fn propagate_floundered_subgoal(&mut self, strand: &mut Strand<C>) -> bool {
// This subgoal selection for the strand is finished, so take it
let selected_subgoal = strand.selected_subgoal.take().unwrap();
match strand.ex_clause.subgoals[selected_subgoal.subgoal_index] {
Literal::Positive(_) => {
// If this strand depends on this positively, then we can
// come back to it later. So, we mark that subgoal as
// floundered and yield `QuantumExceeded` up the stack
// If this subgoal floundered, push it onto the
// floundered list, along with the time that it
// floundered. We'll try to solve some other subgoals
// and maybe come back to it.
self.flounder_subgoal(&mut strand.ex_clause, selected_subgoal.subgoal_index);
return false;
}
Literal::Negative(_) => {
// Floundering on a negative literal isn't like a
// positive search: we only pursue negative literals
// when we already know precisely the type we are
// looking for. So there's no point waiting for other
// subgoals, we'll never recover more information.
//
// In fact, floundering on negative searches shouldn't
// normally happen, since there are no uninferred
// variables in the goal, but it can with forall
// goals:
//
// forall<T> { not { T: Debug } }
//
// Here, the table we will be searching for answers is
// `?T: Debug`, so it could well flounder.
// This strand has no solution. It is no longer active,
// so it dropped at the end of this scope.
return true;
}
}
}
/// This is called if the selected subgoal for a `Strand` is
/// a coinductive cycle.
fn on_coinductive_subgoal(&mut self, mut strand: Strand<C>) -> Result<(), RootSearchFail> {
// This is a co-inductive cycle. That is, this table
// appears somewhere higher on the stack, and has now
// recursively requested an answer for itself. This
// means that we have to delay this subgoal until we
// reach a trivial self-cycle.
// This subgoal selection for the strand is finished, so take it
let selected_subgoal = strand.selected_subgoal.take().unwrap();
match strand
.ex_clause
.subgoals
.remove(selected_subgoal.subgoal_index)
{
Literal::Positive(subgoal) => {
// We delay this subgoal
let table = self.stack.top().table;
assert!(
self.tables[table].coinductive_goal
&& self.tables[selected_subgoal.subgoal_table].coinductive_goal
);
strand.ex_clause.delayed_subgoals.push(subgoal);
self.stack.top().active_strand = Some(strand);
return Ok(());
}
Literal::Negative(_) => {
// We don't allow coinduction for negative literals
info!("found coinductive answer to negative literal");
panic!("Coinductive cycle with negative literal");
}
}
}
/// This is called if the selected subgoal for `strand` is
/// a positive, non-coinductive cycle.
///
/// # Parameters
///
/// * `strand` the strand from the top of the stack we are pursuing
/// * `minimums` is the collected minimum clock times
fn on_positive_cycle(
&mut self,
strand: Strand<C>,
minimums: Minimums,
) -> Result<(), RootSearchFail> {
// We can't take this because we might need it later to clear the cycle
let selected_subgoal = strand.selected_subgoal.as_ref().unwrap();
match strand.ex_clause.subgoals[selected_subgoal.subgoal_index] {
Literal::Positive(_) => {
self.stack.top().cyclic_minimums.take_minimums(&minimums);
}
Literal::Negative(_) => {
// We depend on `not(subgoal)`. For us to continue,
// `subgoal` must be completely evaluated. Therefore,
// we depend (negatively) on the minimum link of
// `subgoal` as a whole -- it doesn't matter whether
// it's pos or neg.
let mins = Minimums {
positive: self.stack.top().clock,
negative: minimums.minimum_of_pos_and_neg(),
};
self.stack.top().cyclic_minimums.take_minimums(&mins);
}
}
// Ok, we've taken the minimums from this cycle above. Now,
// we just return the strand to the table. The table only
// pulls strands if they have not been checked at this
// depth.
//
// We also can't mark these and return early from this
// because the stack above us might change.
let table = self.stack.top().table;
let canonical_strand = Self::canonicalize_strand(strand);
self.tables[table].enqueue_strand(canonical_strand);
// The strand isn't active, but the table is, so just continue
Ok(())
}
/// Invoked after we've selected a (new) subgoal for the top-most
/// strand. Attempts to pursue this selected subgoal.
///
/// Returns:
///
/// * `Ok` if we should keep searching.
/// * `Err` if the subgoal failed in some way such that the strand can be abandoned.
fn on_subgoal_selected(&mut self, mut strand: Strand<C>) -> Result<(), RootSearchFail> {
// This may be a newly selected subgoal or an existing selected subgoal.
let SelectedSubgoal {
subgoal_index: _,
subgoal_table,
answer_index,
universe_map: _,
} = *strand.selected_subgoal.as_ref().unwrap();
debug!(
"table selection {:?} with goal: {:#?}",
subgoal_table, self.tables[subgoal_table].table_goal
);
// This is checked inside select_subgoal
assert!(!self.tables[subgoal_table].is_floundered());
// Check for a tabled answer.
if let Some(answer) = self.tables[subgoal_table].answer(answer_index) {
info!("answer cached = {:?}", answer);
// There was a previous answer available for this table
// We need to check if we can merge it into the current `Strand`.
match self.merge_answer_into_strand(&mut strand) {
Err(e) => {
debug!("could not merge into current strand");
drop(strand);
return Err(e);
}
Ok(_) => {
debug!("merged answer into current strand");
self.stack.top().active_strand = Some(strand);
return Ok(());
}
}
}
// If no tabled answer is present, we ought to be requesting
// the next available index.
assert_eq!(self.tables[subgoal_table].next_answer_index(), answer_index);
// Next, check if the table is already active. If so, then we
// have a recursive attempt.
if let Some(cyclic_depth) = self.stack.is_active(subgoal_table) {
info!("cycle detected at depth {:?}", cyclic_depth);
let minimums = Minimums {
positive: self.stack[cyclic_depth].clock,
negative: TimeStamp::MAX,
};
if self.top_of_stack_is_coinductive_from(cyclic_depth) {
debug!("table is coinductive");
return self.on_coinductive_subgoal(strand);
}
debug!("table encountered a positive cycle");
return self.on_positive_cycle(strand, minimums);
}
// We don't know anything about the selected subgoal table.
// Set this strand as active and push it onto the stack.
self.stack.top().active_strand = Some(strand);
let cyclic_minimums = Minimums::MAX;
self.stack.push(subgoal_table, cyclic_minimums);
Ok(())
}
fn on_no_remaining_subgoals(
&mut self,
context: &impl ContextOps<C>,
strand: Strand<C>,
) -> NoRemainingSubgoalsResult {
debug!("no remaining subgoals for the table");
match self.pursue_answer(strand) {
Some(answer_index) => {
debug!("answer is available");
// We found an answer for this strand, and therefore an
// answer for this table. Now, this table was either a
// subgoal for another strand, or was the root table.
let table = self.stack.top().table;
let mut caller_strand = match self.stack.pop_and_take_caller_strand() {
Some(s) => s,
None => {
// That was the root table, so we are done --
// *well*, unless there were delayed
// subgoals. In that case, we want to evaluate
// those delayed subgoals to completion, so we
// have to create a fresh strand that will
// take them as goals. Note that we *still
// need the original answer in place*, because
// we might have to build on it (see the
// Delayed Trivial Self Cycle, Variant 3
// example).
let (_, _, _, table_goal) =
context.instantiate_ucanonical_goal(&self.tables[table].table_goal);
let answer = self.answer(table, answer_index);
if let Some(strand) =
self.create_refinement_strand(context, table, answer, table_goal)
{
self.tables[table].enqueue_strand(strand);
}
return NoRemainingSubgoalsResult::RootAnswerAvailable;
}
};
match self.merge_answer_into_strand(&mut caller_strand) {
Err(e) => {
drop(caller_strand);
return NoRemainingSubgoalsResult::RootSearchFail(e);
}
Ok(_) => {
self.stack.top().active_strand = Some(caller_strand);
return NoRemainingSubgoalsResult::Success;
}
}
}
None => {
debug!("answer is not available (or not new)");
// This table ned nowhere of interest
// Now we yield with `QuantumExceeded`
self.unwind_stack();
return NoRemainingSubgoalsResult::RootSearchFail(RootSearchFail::QuantumExceeded);
}
};
}
/// A "refinement" strand is used in coinduction. When the root
/// table on the stack publishes an answer has delayed subgoals,
/// we create a new strand that will attempt to prove out those
/// delayed subgoals (the root answer here is not *special* except
/// in so far as that there is nothing above it, and hence we know
/// that the delayed subgoals (which resulted in some cycle) must
/// be referring to a table that now has completed).
///
/// Note that it is important for this to be a *refinement* strand
/// -- meaning that the answer with delayed subgoals has been
/// published. This is necessary because sometimes the strand must
/// build on that very answer that it is refining. See Delayed
/// Trivial Self Cycle, Variant 3.
fn create_refinement_strand(
&self,
context: &impl ContextOps<C>,
table: TableIndex,
answer: &Answer<C>,
table_goal: C::Goal,
) -> Option<CanonicalStrand<C>> {
// If there are no delayed subgoals, then there is no need for
// a refinement strand.
if !C::has_delayed_subgoals(&answer.subst) {
return None;
}
let num_universes = C::num_universes(&self.tables[table].table_goal);
let (table, subst, constraints, delayed_subgoals) =
context.instantiate_answer_subst(num_universes, &answer.subst);
// FIXME: it would be nice if these delayed subgoals didn't get added to the answer
// at all. However, we can't compare the delayed subgoals with the table goal until
// we call `canonicalize_answer_subst` in `pursue_answer`. However, at this point,
// it's a bit late since `pursue_answer` doesn't know about the table goal. This could
// be refactored a bit.
let filtered_delayed_subgoals = delayed_subgoals
.into_iter()
.filter(|delayed_subgoal| {
*C::goal_from_goal_in_environment(delayed_subgoal) != table_goal
})
.map(Literal::Positive)
.collect();
let strand = Strand {
infer: table,
ex_clause: ExClause {
subst,
ambiguous: answer.ambiguous,
constraints,
subgoals: filtered_delayed_subgoals,
delayed_subgoals: Vec::new(),
answer_time: TimeStamp::default(),
floundered_subgoals: Vec::new(),
},
selected_subgoal: None,
last_pursued_time: TimeStamp::default(),
};
Some(Self::canonicalize_strand(strand))
}
fn on_subgoal_selection_flounder(&mut self, strand: Strand<C>) -> RootSearchFail {
debug!("all subgoals floundered");
// We were unable to select a subgoal for this strand
// because all of them had floundered or because any one
// that we dependended on negatively floundered
// We discard this strand because it led nowhere of interest
drop(strand);
loop {
// This table is marked as floundered
let table = self.stack.top().table;
debug!("Marking table {:?} as floundered!", table);
self.tables[table].mark_floundered();
let mut strand = match self.stack.pop_and_take_caller_strand() {
Some(s) => s,
None => {
// That was the root table, so we are done.
return RootSearchFail::Floundered;
}
};
if self.propagate_floundered_subgoal(&mut strand) {
// This strand will never lead anywhere of interest.
// Drop it and continue around the loop.
drop(strand);
} else {
// We want to maybe pursue this strand later
let table = self.stack.top().table;
let canonical_strand = Self::canonicalize_strand(strand);
self.tables[table].enqueue_strand(canonical_strand);
// Now we yield with `QuantumExceeded`
self.unwind_stack();
return RootSearchFail::QuantumExceeded;
}
}
}
fn on_no_strands_left(&mut self) -> Result<(), RootSearchFail> {
debug!("no more strands available (or all cycles)");
// No more strands left to try! This is either because all
// strands have failed or because all strands encountered a
// cycle.
let table = self.stack.top().table;
if self.tables[table].strands_mut().count() == 0 {
// All strands for the table T on the top of the stack
// have **failed**. Hence we can pop it off the stack and
// check what this means for the table T' that was just
// below T on the stack (if any).
debug!("no more strands available");
let caller_strand = match self.stack.pop_and_borrow_caller_strand() {
Some(s) => s,
None => {
// T was the root table, so we are done.
debug!("no more solutions");
return Err(RootSearchFail::NoMoreSolutions);
}
};
// This subgoal selection for the strand is finished, so take it
let caller_selected_subgoal = caller_strand.selected_subgoal.take().unwrap();
return match caller_strand.ex_clause.subgoals[caller_selected_subgoal.subgoal_index] {
// T' wanted an answer from T, but none is
// forthcoming. Therefore, the active strand from T'
// has failed and can be discarded.
Literal::Positive(_) => {
debug!("discarding strand because positive literal");
self.stack.top().active_strand.take();
self.unwind_stack();
Err(RootSearchFail::QuantumExceeded)
}
// T' wanted there to be no answer from T, but none is forthcoming.
Literal::Negative(_) => {
debug!("subgoal was proven because negative literal");
// There is no solution for this strand. But, this
// is what we want, so can remove this subgoal and
// keep going.
caller_strand
.ex_clause
.subgoals
.remove(caller_selected_subgoal.subgoal_index);
// This strand is still active, so continue
Ok(())
}
};
}
let clock = self.stack.top().clock;
let cyclic_minimums = self.stack.top().cyclic_minimums;
if cyclic_minimums.positive >= clock && cyclic_minimums.negative >= clock {
debug!("cycle with no new answers");
if cyclic_minimums.negative < TimeStamp::MAX {
// This is a negative cycle.
self.unwind_stack();
return Err(RootSearchFail::NegativeCycle);
}
// If all the things that we recursively depend on have
// positive dependencies on things below us in the stack,
// then no more answers are forthcoming. We can clear all
// the strands for those things recursively.
let table = self.stack.top().table;
let cyclic_strands = self.tables[table].take_strands();
self.clear_strands_after_cycle(cyclic_strands);
// Now we yield with `QuantumExceeded`
self.unwind_stack();
return Err(RootSearchFail::QuantumExceeded);
} else {
debug!("table part of a cycle");
// This table resulted in a positive cycle, so we have
// to check what this means for the subgoal containing
// this strand
let caller_strand = match self.stack.pop_and_borrow_caller_strand() {
Some(s) => s,
None => {
panic!("nothing on the stack but cyclic result");
}
};
// We can't take this because we might need it later to clear the cycle
let caller_selected_subgoal = caller_strand.selected_subgoal.as_ref().unwrap();
match caller_strand.ex_clause.subgoals[caller_selected_subgoal.subgoal_index] {
Literal::Positive(_) => {
self.stack
.top()
.cyclic_minimums
.take_minimums(&cyclic_minimums);
}
Literal::Negative(_) => {
// We depend on `not(subgoal)`. For us to continue,
// `subgoal` must be completely evaluated. Therefore,
// we depend (negatively) on the minimum link of
// `subgoal` as a whole -- it doesn't matter whether
// it's pos or neg.
let mins = Minimums {
positive: self.stack.top().clock,
negative: cyclic_minimums.minimum_of_pos_and_neg(),
};
self.stack.top().cyclic_minimums.take_minimums(&mins);
}
}
// We can't pursue this strand anymore, so push it back onto the table
let active_strand = self.stack.top().active_strand.take().unwrap();
let table = self.stack.top().table;
let canonical_active_strand = Self::canonicalize_strand(active_strand);
self.tables[table].enqueue_strand(canonical_active_strand);
// The strand isn't active, but the table is, so just continue
return Ok(());
}
}
/// Unwinds the entire stack, returning all active strands back to
/// their tables (this time at the end of the queue).
fn unwind_stack(&mut self) {
loop {
match self.stack.pop_and_take_caller_strand() {
Some(active_strand) => {
let table = self.stack.top().table;
let canonical_active_strand = Self::canonicalize_strand(active_strand);
self.tables[table].enqueue_strand(canonical_active_strand);
}
None => return,
}
}
}
pub(crate) fn answer(&self, table: TableIndex, answer: AnswerIndex) -> &Answer<C> {
self.tables[table].answer(answer).unwrap()
}
fn canonicalize_strand(strand: Strand<C>) -> CanonicalStrand<C> {
let Strand {
mut infer,
ex_clause,
selected_subgoal,
last_pursued_time,
} = strand;
Self::canonicalize_strand_from(&mut infer, &ex_clause, selected_subgoal, last_pursued_time)
}
fn canonicalize_strand_from(
infer: &mut dyn InferenceTable<C>,
ex_clause: &ExClause<C>,
selected_subgoal: Option<SelectedSubgoal<C>>,
last_pursued_time: TimeStamp,
) -> CanonicalStrand<C> {
let canonical_ex_clause = infer.canonicalize_ex_clause(&ex_clause);
CanonicalStrand {
canonical_ex_clause,
selected_subgoal,
last_pursued_time,
}
}
/// Invoked after we have determined that every strand in `table`
/// encounters a cycle; `strands` is the set of strands (which
/// have been moved out of the table). This method then
/// recursively clears the active strands from the tables
/// referenced in `strands`, since all of them must encounter
/// cycles too.
fn clear_strands_after_cycle(&mut self, strands: impl IntoIterator<Item = CanonicalStrand<C>>) {
for strand in strands {
let CanonicalStrand {
canonical_ex_clause,
selected_subgoal,
last_pursued_time: _,
} = strand;
let selected_subgoal = selected_subgoal.unwrap_or_else(|| {
panic!(
"clear_strands_after_cycle invoked on strand in table \
without a selected subgoal: {:?}",
canonical_ex_clause,
)
});
let strand_table = selected_subgoal.subgoal_table;
let strands = self.tables[strand_table].take_strands();
self.clear_strands_after_cycle(strands);
}
}
fn select_subgoal(
&mut self,
context: &impl ContextOps<C>,
strand: &mut Strand<C>,
) -> SubGoalSelection {
loop {
while strand.selected_subgoal.is_none() {
if strand.ex_clause.subgoals.len() == 0 {
if strand.ex_clause.floundered_subgoals.is_empty() {
return SubGoalSelection::NoRemainingSubgoals;
}
self.reconsider_floundered_subgoals(&mut strand.ex_clause);
if strand.ex_clause.subgoals.is_empty() {
assert!(!strand.ex_clause.floundered_subgoals.is_empty());
return SubGoalSelection::Floundered;
}
continue;
}
let subgoal_index = C::next_subgoal_index(&strand.ex_clause);
// Get or create table for this subgoal.
match self.get_or_create_table_for_subgoal(
context,
&mut strand.infer,
&strand.ex_clause.subgoals[subgoal_index],
) {
Some((subgoal_table, universe_map)) => {
strand.selected_subgoal = Some(SelectedSubgoal {
subgoal_index,
subgoal_table,
universe_map,
answer_index: AnswerIndex::ZERO,
});
}
None => {
// If we failed to create a table for the subgoal,
// that is because we have a floundered negative
// literal.
self.flounder_subgoal(&mut strand.ex_clause, subgoal_index);
}
}
}
let selected_subgoal_table = strand.selected_subgoal.as_ref().unwrap().subgoal_table;
if self.tables[selected_subgoal_table].is_floundered() {
if self.propagate_floundered_subgoal(strand) {
// This strand will never lead anywhere of interest.
return SubGoalSelection::Floundered;
} else {
// This subgoal has floundered and has been marked.
// We previously would immediately mark the table as