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//! Span-aware shrink passes.
//!
//! These passes consult [`Shrinker::current_spans`] to operate on
//! structured sub-sequences of the choice list rather than individual
//! nodes.
use super::ordering::shrink_ordering;
use super::{ShrinkRun, Shrinker};
use crate::native::core::sort_key;
impl<'a> Shrinker<'a> {
/// Delete every contiguous non-overlapping discarded span in one pass.
///
/// Useful for rejection-sampling data left behind by filtered
/// strategies — that whole region can usually be cut in a single
/// attempt rather than element-by-element.
///
/// Returns `true` if either (a) there was nothing to discard, or
/// (b) the deletion attempts succeeded. Returns `false` when the
/// shrinker has discarded data that can't be removed (a follow-up
/// pass shouldn't try this work again on the same target).
pub(crate) fn remove_discarded(&mut self) -> bool {
loop {
// Gather the outermost discarded spans in source order. A span
// nested inside an already-collected discarded region is skipped
// because the outer deletion subsumes it.
let mut discarded: Vec<(usize, usize)> = Vec::new();
for span in self.current_spans.iter() {
if span.end > span.start
&& span.discarded
&& discarded
.last()
.is_none_or(|&(_, prev_end)| span.start >= prev_end)
{
discarded.push((span.start, span.end));
}
}
if discarded.is_empty() {
return true;
}
let mut attempt = self.current_nodes.clone();
for &(u, v) in discarded.iter().rev() {
attempt.drain(u..v);
}
if !self.consider(&attempt) {
return false;
}
// If `consider` accepted but the actual run produced no new
// discards, the next loop iteration will find an empty
// `discarded` and return.
}
}
/// For every span in `current_spans`, try replacing its choices with their
/// kind-simplest values. Forced choices stay put.
///
/// When the attempted replacement isn't interesting but the test run
/// still produced a valid result, a second attempt is made using the
/// realised span content from the run — this lets recursive
/// structures whose simplest form is shape-dependent (e.g. an
/// inner span that becomes shorter under simplest values) still
/// converge.
pub(crate) fn try_trivial_spans(&mut self) {
let mut i = 0;
while i < self.current_spans.len() {
// Capture the shrink epoch before any mutation so we can
// detect whether the first attempt improved the shrink
// target. `improvements` only bumps on a strict shrink, so
// its delta is exactly "did we shrink?".
let epoch_before = self.improvements;
let span = self.current_spans[i].clone();
if span.end > self.current_nodes.len() {
i += 1;
continue;
}
let already_trivial = self.current_spans.trivial(i, &self.current_nodes);
if already_trivial {
i += 1;
continue;
}
let mut attempt: Vec<_> = self.current_nodes.clone();
for node in attempt[span.start..span.end].iter_mut() {
if node.was_forced {
continue;
}
let simplest = node.kind.simplest();
if node.value != simplest {
*node = node.with_value(simplest);
}
}
// Manually invoke the closure so we keep hold of the actual
// realised nodes and spans even when the attempt isn't an
// improvement — we retry with the realised span content
// below.
let (is_interesting, actual_nodes, actual_spans) =
(self.test_fn)(ShrinkRun::Full(&attempt));
if is_interesting && sort_key(&actual_nodes) < sort_key(&self.current_nodes) {
self.accept_improvement(actual_nodes, actual_spans);
i += 1;
continue;
}
// First attempt didn't improve. If the run produced a valid
// (or interesting-but-not-smaller) result that still records
// a span at this index, splice its realised content back into
// the original sequence and try once more.
//
// `if let` chains stabilised after MSRV 1.86, so this is
// spelled out as nested conditions instead.
if self.improvements == epoch_before {
if let Some(new_span) = actual_spans.get(i) {
if new_span.start <= new_span.end && new_span.end <= actual_nodes.len() {
let mut spliced = self.current_nodes[..span.start].to_vec();
spliced.extend_from_slice(&actual_nodes[new_span.start..new_span.end]);
spliced.extend_from_slice(&self.current_nodes[span.end..]);
self.consider(&spliced);
}
}
}
i += 1;
}
}
/// Replace each span with one of its same-label descendants.
///
/// This is the pass that lets recursive strategies collapse a tree
/// onto one of its subtrees in a single step, instead of having to
/// chew through each layer individually.
///
/// For every pair `(ancestor, descendant)` of same-label spans where
/// the descendant is strictly contained in the ancestor and is
/// strictly shorter, we splice the descendant's nodes in place of the
/// ancestor's and ask the predicate whether that's still interesting.
pub(crate) fn pass_to_descendant(&mut self) {
// Snapshot (start, end, label) tuples up front. Each consider()
// may rebuild current_spans, which would invalidate live indices
// — re-reading from the snapshot after every consider would mean
// recomputing the per-label index every time. Instead we iterate
// all candidates from the initial snapshot and let each
// consider() bail naturally on a stale ancestor — the
// negative-result cache in `consider` covers cross-invocation
// deduplication.
let spans: Vec<(usize, usize, String)> = self
.current_spans
.iter()
.map(|s| (s.start, s.end, s.label.clone()))
.collect();
// Group span indices by label.
let mut by_label: std::collections::BTreeMap<&str, Vec<usize>> =
std::collections::BTreeMap::new();
for (idx, (_, _, label)) in spans.iter().enumerate() {
by_label.entry(label.as_str()).or_default().push(idx);
}
for (_label, indices) in by_label {
if indices.len() < 2 {
continue;
}
for ai in 0..indices.len() {
let ancestor_idx = indices[ai];
let (a_start, a_end, _) = spans[ancestor_idx].clone();
let ancestor_len = a_end.saturating_sub(a_start);
if ancestor_len == 0 {
continue;
}
for &descendant_idx in &indices[ai + 1..] {
let (d_start, d_end, _) = spans[descendant_idx].clone();
// Past the ancestor's range: no further descendants
// because spans are ordered by `start`.
if d_start >= a_end {
break;
}
let descendant_len = d_end.saturating_sub(d_start);
if descendant_len == 0 || descendant_len >= ancestor_len {
continue;
}
// Sanity: indices must still be in range of the current
// node list — if a prior consider() shortened it past
// the ancestor's end, skip.
if a_end > self.current_nodes.len() {
continue;
}
// Spans are tree-structured: any span starting inside
// an ancestor must also end inside it. Guard for
// future deviations from that invariant.
debug_assert!(d_start >= a_start && d_end <= a_end);
let mut attempt = self.current_nodes[..a_start].to_vec();
attempt.extend_from_slice(&self.current_nodes[d_start..d_end]);
attempt.extend_from_slice(&self.current_nodes[a_end..]);
self.consider(&attempt);
}
}
}
}
/// Reorder same-label sibling spans into a more-sorted permutation.
///
/// For each parent span, for each label that appears more than once
/// among its direct children, run an [`shrink_ordering`] over the
/// children indices, keyed by the sort key of the child's realised
/// node slice.
///
/// Permutation by index keeps the multiset of children intact; the
/// only thing that changes is *which* slice ends up at which start
/// position. This is the pass that ensures `test_not_equal(x, y)`
/// collapses to a canonical `(x="", y="0")` rather than the
/// symmetric alternative.
pub(crate) fn reorder_spans(&mut self) {
let parents: Vec<Option<usize>> = {
// Build the set of parent indices that have direct children
// (including the implicit root, parent == None).
let mut seen: std::collections::BTreeSet<Option<usize>> =
std::collections::BTreeSet::new();
for span in self.current_spans.iter() {
seen.insert(span.parent);
}
seen.into_iter().collect()
};
for parent in parents {
// Group this parent's children by label, with owned label
// strings so the BTreeMap doesn't keep a borrow on
// `self.current_spans` while the closure later asks
// `self.consider` for a borrow.
let mut by_label: std::collections::BTreeMap<String, Vec<usize>> =
std::collections::BTreeMap::new();
for (idx, span) in self.current_spans.iter().enumerate() {
if span.parent == parent {
by_label.entry(span.label.clone()).or_default().push(idx);
}
}
for (_label, child_indices) in by_label {
if child_indices.len() <= 1 {
continue;
}
let endpoints: Vec<(usize, usize)> = child_indices
.iter()
.map(|&i| {
let s = &self.current_spans[i];
(s.start, s.end)
})
.collect();
// Sanity: endpoints must all fit in current_nodes.
let nodes_len = self.current_nodes.len();
if endpoints.iter().any(|&(_, e)| e > nodes_len) {
continue;
}
// Sibling spans are always non-overlapping and in source
// order under the span recorder's invariants. A
// debug_assert documents the precondition; we don't try
// to recover from a violation at runtime.
debug_assert!({
let mut sorted_eps = endpoints.clone();
sorted_eps.sort();
sorted_eps.windows(2).all(|w| w[0].1 <= w[1].0)
});
let n = child_indices.len();
let snapshot_nodes = self.current_nodes.clone();
// The keys for sorting are the sort_keys of each child's
// realised node slice. `NodesSortKey` is a `Copy` view
// over `snapshot_nodes` — the snapshot lives until after
// `shrink_ordering` returns, so cached refs stay valid.
let cached_keys: Vec<crate::native::core::NodesSortKey<'_>> = endpoints
.iter()
.map(|&(s, e)| sort_key(&snapshot_nodes[s..e]))
.collect();
// Snapshot is needed to translate a permutation back into a
// full node list: prefix + ordered slices + suffix. We
// splice into the *snapshot* not the live current_nodes,
// because each accept call may modify current_nodes.
shrink_ordering::<crate::native::core::NodesSortKey<'_>, _, _>(
n,
|i| cached_keys[i],
|permutation| {
// Build the candidate by interleaving the
// permuted slices with the unchanged regions
// between sibling endpoints. shrink_ordering
// only ever calls accept with length-`n`
// permutations.
debug_assert_eq!(permutation.len(), n);
let mut attempt: Vec<_> = Vec::with_capacity(snapshot_nodes.len());
attempt.extend_from_slice(&snapshot_nodes[..endpoints[0].0]);
for (k, &(_, target_end)) in endpoints.iter().enumerate() {
let src_idx = permutation[k];
let (src_start, src_end) = endpoints[src_idx];
attempt.extend_from_slice(&snapshot_nodes[src_start..src_end]);
// Gap to next sibling (or to suffix).
if k + 1 < endpoints.len() {
attempt.extend_from_slice(
&snapshot_nodes[target_end..endpoints[k + 1].0],
);
} else {
attempt.extend_from_slice(&snapshot_nodes[target_end..]);
}
}
self.consider(&attempt)
},
);
}
}
}
}
#[cfg(test)]
#[path = "../../../tests/embedded/native/shrinker_remove_discarded_tests.rs"]
mod remove_discarded_tests;
#[cfg(test)]
#[path = "../../../tests/embedded/native/shrinker_pass_to_descendant_tests.rs"]
mod pass_to_descendant_tests;
#[cfg(test)]
#[path = "../../../tests/embedded/native/shrinker_reorder_spans_tests.rs"]
mod reorder_spans_tests;
#[cfg(test)]
#[path = "../../../tests/embedded/native/shrinker_try_trivial_spans_tests.rs"]
mod try_trivial_spans_tests;