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//! Engine decorator that performs **head skipping** – an extremely optimized search for
//! the first matching member name in a query starting with a self-looping state.
//! This happens in queries starting with a descendant selector.
use std::sync::Arc;
use crate::{
automaton::{Automaton, State},
classification::{
mask::Mask as _,
memmem::Memmem as _,
quotes::{InnerIter as _, QuoteClassifiedIterator, ResumedQuoteClassifier},
simd::{dispatch_simd, Simd},
structural::{BracketType, Structural, StructuralIterator as _},
ResumeClassifierBlockState, ResumeClassifierState,
},
debug,
depth::Depth,
engine::EngineError,
input::{
error::{InputError, InputErrorConvertible as _},
Input, InputBlockIterator,
},
result::Recorder,
string_pattern::StringPattern,
FallibleIterator as _, MaskType, BLOCK_SIZE,
};
/// Trait that needs to be implemented by an [`Engine`](`super::Engine`) to use this submodule.
pub(super) trait CanHeadSkip<'i, 'r, I, R, V>
where
I: Input + 'i,
R: Recorder<I::Block<'i, BLOCK_SIZE>>,
V: Simd,
{
/// Function called when head-skipping finds a member name at which normal query execution
/// should resume.
///
/// The [`HeadSkip::run_head_skipping`] function will call this implementation
/// whenever it finds a member name matching the first transition in the query.
/// The structural `classifier` passed is guaranteed to have classified the
/// `next_event` and nothing past that. It is guaranteed that
/// `next_event` is [`Structural::Opening`].
///
/// When called, the engine must start with in the automaton state as given in `state`
/// and execute the query until a matching [`Structural::Closing`] character is encountered,
/// using `classifier` for classification and `result` for reporting query results. The `classifier`
/// must *not* be used to classify anything past the matching [`Structural::Closing`] character.
fn run_on_subtree(
&mut self,
next_event: Structural,
state: State,
structural_classifier: V::StructuralClassifier<'i, I::BlockIterator<'i, 'r, R, BLOCK_SIZE>>,
) -> Result<ResumeState<'i, I::BlockIterator<'i, 'r, R, BLOCK_SIZE>, V, MaskType>, EngineError>;
fn recorder(&mut self) -> &'r R;
}
pub(super) struct ResumeState<'i, I, V, M>(
pub(super) ResumeClassifierState<'i, I, V::QuotesClassifier<'i, I>, M, BLOCK_SIZE>,
)
where
I: InputBlockIterator<'i, BLOCK_SIZE>,
V: Simd;
/// Configuration of the head-skipping decorator.
pub(super) struct HeadSkip<'b, I, V, const N: usize> {
bytes: &'b I,
state: State,
is_accepting: bool,
member_name: Arc<StringPattern>,
simd: V,
}
impl<'b, I: Input, V: Simd> HeadSkip<'b, I, V, BLOCK_SIZE> {
/// Create a new instance of the head-skipping decorator over a given input
/// and for a compiled query [`Automaton`].
///
/// # Returns
/// If head-skipping is possible for the query represented by `automaton`,
/// returns [`Some`] with a configured instance of [`HeadSkip`].
/// If head-skipping is not possible, returns [`None`].
///
/// ## Details
/// Head-skipping is possible if the query automaton starts
/// with a state with a wildcard self-loop and a single member-labelled transition forward.
/// Syntactically, if the [`fallback_state`](`crate::query::automaton::StateTable::fallback_state`)
/// of the [`initial_state`](`crate::query::automaton::StateTable::initial_state`) is the same as the
/// [`initial_state`](`crate::query::automaton::StateTable::initial_state`), and its
/// [`transitions`](`crate::query::automaton::StateTable::transitions`) are a single-element list.
///
/// This means that we can search for the label of the forward transition in the entire document,
/// disregarding any additional structure – during execution we would always loop
/// around in the initial state until encountering the desired member name. This search can be done
/// extremely quickly with [`classification::memmem`](crate::classification::memmem).
///
/// In all other cases, head-skipping is not supported.
pub(super) fn new(bytes: &'b I, automaton: &Automaton, simd: V) -> Option<Self> {
let initial_state = automaton.initial_state();
let fallback_state = automaton[initial_state].fallback_state();
let transitions = automaton[initial_state].member_transitions();
if fallback_state == initial_state
&& transitions.len() == 1
&& automaton[initial_state].array_transitions().is_empty()
{
let (member_name, target_state) = &transitions[0];
debug!("Automaton starts with a descendant search, using memmem heuristic.");
return Some(Self {
bytes,
state: *target_state,
is_accepting: automaton.is_accepting(*target_state),
member_name: member_name.clone(),
simd,
});
}
None
}
/// Run a preconfigured [`HeadSkip`] using the given `engine` and reporting
/// to the `result`.
pub(super) fn run_head_skipping<'r, E, R>(&self, engine: &mut E) -> Result<(), EngineError>
where
'b: 'r,
E: CanHeadSkip<'b, 'r, I, R, V>,
R: Recorder<I::Block<'b, BLOCK_SIZE>> + 'r,
{
dispatch_simd!(self.simd; self, engine =>
fn<'b, 'r, I, V, E, R>(head_skip: &HeadSkip<'b, I, V, BLOCK_SIZE>, engine: &mut E) -> Result<(), EngineError>
where
'b: 'r,
E: CanHeadSkip<'b, 'r, I, R, V>,
R: Recorder<I::Block<'b, BLOCK_SIZE>> + 'r,
I: Input,
V: Simd
{
let mut input_iter = head_skip.bytes.iter_blocks(engine.recorder());
let mut idx = 0;
let mut first_block = None;
loop {
let mut memmem = head_skip.simd.memmem(head_skip.bytes, &mut input_iter);
debug!("Starting memmem search from {idx}");
if let Some((starting_quote_idx, last_block)) = memmem.find_label(first_block, idx, head_skip.member_name.as_ref())? {
drop(memmem);
first_block = Some(last_block);
idx = starting_quote_idx;
debug!("Needle found at {idx}");
let seek_start_idx = idx + head_skip.member_name.quoted().len();
match head_skip.bytes.seek_non_whitespace_forward(seek_start_idx).e()? {
Some((colon_idx, b':')) => {
let (next_idx, next_c) = head_skip
.bytes
.seek_non_whitespace_forward(colon_idx + 1).e()?
.ok_or(EngineError::MissingItem())?;
let ResumedQuoteClassifier {
classifier: quote_classifier,
first_block: quote_classified_first_block,
} = head_skip.simd.resume_quote_classification(input_iter, first_block);
// Temporarily set the index within the current block to zero.
// This makes sense for the move below.
let mut classifier_state = ResumeClassifierState {
iter: quote_classifier,
block: quote_classified_first_block
.map(|b| ResumeClassifierBlockState { block: b, idx: 0 }),
are_colons_on: false,
are_commas_on: head_skip.is_accepting,
};
debug!("Actual match with colon at {colon_idx}");
debug!("Next significant character at {next_idx}");
debug!("Classifier is at {}", classifier_state.get_idx());
debug!("We will forward to {colon_idx} first, then to {next_idx}",);
// Now we want to move the entire iterator state so that the current block is quote-classified,
// and correctly points to the place the engine would expect had it found the matching key
// in the regular loop. If the value is atomic, we handle it ourselves. If the value is complex,
// the engine wants to start one byte *after* the opening character. However, the match report
// has to happen before we advance one more byte, or else the opening character might be lost
// in the output (if it happens at a block boundary).
if next_c == b'{' || next_c == b'[' {
forward_to(&mut classifier_state, next_idx)?;
if head_skip.is_accepting {
engine.recorder().record_match(
next_idx,
Depth::ZERO,
crate::result::MatchedNodeType::Complex,
)?;
}
forward_to(&mut classifier_state, next_idx + 1)?;
} else {
forward_to(&mut classifier_state, next_idx)?;
}
// We now have the block where we want and we ran quote classification, but during the `forward_to`
// call we lose all the flow-through quote information that usually is passed from one block to the next.
// We need to manually verify the soundness of the classification. Fortunately:
// 1. we know that resume_idx is either the start of a value, or one byte after an opening -
// in a valid JSON this character can be within quotes if and only if it is itself a quote;
// 2. the only way the mask can be wrong is if it is flipped - marks chars within quotes
// as outside and vice versa - so it suffices to flip it if it is wrong.
if let Some(block) = classifier_state.block.as_mut() {
let should_be_quoted = block.block.block[block.idx] == b'"';
if block.block.within_quotes_mask.is_lit(block.idx) != should_be_quoted {
debug!("Mask needs flipping!");
block.block.within_quotes_mask = !block.block.within_quotes_mask;
classifier_state.iter.flip_quotes_bit();
}
}
classifier_state = match next_c {
b'{' | b'[' => {
debug!("resuming");
let classifier = head_skip.simd.resume_structural_classification(classifier_state);
engine
.run_on_subtree(
Structural::Opening(
if next_c == b'{' {
BracketType::Curly
} else {
BracketType::Square
},
next_idx,
),
head_skip.state,
classifier,
)?
.0
}
_ if head_skip.is_accepting => {
engine.recorder().record_match(
next_idx,
Depth::ZERO,
crate::result::MatchedNodeType::Atomic,
)?;
let mut classifier = head_skip.simd.resume_structural_classification(classifier_state);
let next_structural = classifier.next()?;
match next_structural {
Some(s) => engine.recorder().record_value_terminator(s.idx(), Depth::ZERO)?,
None => return Err(EngineError::MissingClosingCharacter()),
}
classifier.stop()
}
_ => classifier_state,
};
debug!("Quote classified up to {}", classifier_state.get_idx());
idx = classifier_state.get_idx();
first_block = classifier_state.block.map(|b| b.block.block);
input_iter = classifier_state.iter.into_inner();
}
_ => idx += 1,
}
} else {
debug!("No memmem matches, exiting");
break;
}
}
return Ok(());
/// Move the state forward to `index`.
///
/// # Errors
/// If the offset crosses block boundaries, then a new block is read from the underlying
/// [`Input`](crate::input::Input) implementation, which can fail.
///
/// # Panics
/// If the `index` is not ahead of the current position of the state ([`get_idx`](ResumeClassifierState::get_idx)).
#[inline(always)]
#[allow(clippy::panic_in_result_fn, reason = "Err here is only raised for input errors, assertions check programming bugs")]
fn forward_to<'i, I, Q, M, const N: usize>(state: &mut ResumeClassifierState<'i, I, Q, M, N>, index: usize) -> Result<(), InputError>
where
I: InputBlockIterator<'i, N>,
Q: QuoteClassifiedIterator<'i, I, M, N>,
{
let current_block_start = state.iter.get_offset();
let current_block_idx = state.block.as_ref().map_or(0, |b| b.idx);
let current_idx = current_block_start + current_block_idx;
debug!(
"Calling forward_to({index}) when the inner iter offset is {current_block_start} and block idx is {current_block_idx:?}"
);
// We want to move by this much forward.
assert!(index > current_idx, "must forward by at least one byte");
let delta = index - current_idx;
// First we virtually pretend to move *backward*, setting the index of the current block to zero,
// and adjust the delta to cover that distance. This makes calculations simpler.
// Then we need to skip zero or more blocks and set our self.block to the last one we visit.
let remaining = delta + current_block_idx;
let blocks_to_skip = remaining / N;
let remainder = remaining % N;
match state.block.as_mut() {
Some(b) if blocks_to_skip == 0 => {
b.idx = remaining;
}
Some(_) => {
state.block = state
.iter
.offset(blocks_to_skip as isize)?
.map(|b| ResumeClassifierBlockState {
block: b,
idx: remainder,
});
}
None => {
state.block = state
.iter
.offset((blocks_to_skip + 1) as isize)?
.map(|b| ResumeClassifierBlockState {
block: b,
idx: remainder,
});
}
}
debug!("forward_to({index}) results in idx moved to {}", state.get_idx());
Ok(())
}
})
}
}