use core::{cmp, fmt, mem, u16, usize};
use alloc::{boxed::Box, string::String, vec, vec::Vec};
use crate::{
packed::{api::MatchKind, ext::Pointer},
PatternID,
};
/// A non-empty collection of non-empty patterns to search for.
///
/// This collection of patterns is what is passed around to both execute
/// searches and to construct the searchers themselves. Namely, this permits
/// searches to avoid copying all of the patterns, and allows us to keep only
/// one copy throughout all packed searchers.
///
/// Note that this collection is not a set. The same pattern can appear more
/// than once.
#[derive(Clone, Debug)]
pub(crate) struct Patterns {
/// The match semantics supported by this collection of patterns.
///
/// The match semantics determines the order of the iterator over patterns.
/// For leftmost-first, patterns are provided in the same order as were
/// provided by the caller. For leftmost-longest, patterns are provided in
/// descending order of length, with ties broken by the order in which they
/// were provided by the caller.
kind: MatchKind,
/// The collection of patterns, indexed by their identifier.
by_id: Vec<Vec<u8>>,
/// The order of patterns defined for iteration, given by pattern
/// identifiers. The order of `by_id` and `order` is always the same for
/// leftmost-first semantics, but may be different for leftmost-longest
/// semantics.
order: Vec<PatternID>,
/// The length of the smallest pattern, in bytes.
minimum_len: usize,
/// The total number of pattern bytes across the entire collection. This
/// is used for reporting total heap usage in constant time.
total_pattern_bytes: usize,
}
// BREADCRUMBS: I think we want to experiment with a different bucket
// representation. Basically, each bucket is just a Range<usize> to a single
// contiguous allocation? Maybe length-prefixed patterns or something? The
// idea is to try to get rid of the pointer chasing in verification. I don't
// know that that is the issue, but I suspect it is.
impl Patterns {
/// Create a new collection of patterns for the given match semantics. The
/// ID of each pattern is the index of the pattern at which it occurs in
/// the `by_id` slice.
///
/// If any of the patterns in the slice given are empty, then this panics.
/// Similarly, if the number of patterns given is zero, then this also
/// panics.
pub(crate) fn new() -> Patterns {
Patterns {
kind: MatchKind::default(),
by_id: vec![],
order: vec![],
minimum_len: usize::MAX,
total_pattern_bytes: 0,
}
}
/// Add a pattern to this collection.
///
/// This panics if the pattern given is empty.
pub(crate) fn add(&mut self, bytes: &[u8]) {
assert!(!bytes.is_empty());
assert!(self.by_id.len() <= u16::MAX as usize);
let id = PatternID::new(self.by_id.len()).unwrap();
self.order.push(id);
self.by_id.push(bytes.to_vec());
self.minimum_len = cmp::min(self.minimum_len, bytes.len());
self.total_pattern_bytes += bytes.len();
}
/// Set the match kind semantics for this collection of patterns.
///
/// If the kind is not set, then the default is leftmost-first.
pub(crate) fn set_match_kind(&mut self, kind: MatchKind) {
self.kind = kind;
match self.kind {
MatchKind::LeftmostFirst => {
self.order.sort();
}
MatchKind::LeftmostLongest => {
let (order, by_id) = (&mut self.order, &mut self.by_id);
order.sort_by(|&id1, &id2| {
by_id[id1].len().cmp(&by_id[id2].len()).reverse()
});
}
}
}
/// Return the number of patterns in this collection.
///
/// This is guaranteed to be greater than zero.
pub(crate) fn len(&self) -> usize {
self.by_id.len()
}
/// Returns true if and only if this collection of patterns is empty.
pub(crate) fn is_empty(&self) -> bool {
self.len() == 0
}
/// Returns the approximate total amount of heap used by these patterns, in
/// units of bytes.
pub(crate) fn memory_usage(&self) -> usize {
self.order.len() * mem::size_of::<PatternID>()
+ self.by_id.len() * mem::size_of::<Vec<u8>>()
+ self.total_pattern_bytes
}
/// Clears all heap memory associated with this collection of patterns and
/// resets all state such that it is a valid empty collection.
pub(crate) fn reset(&mut self) {
self.kind = MatchKind::default();
self.by_id.clear();
self.order.clear();
self.minimum_len = usize::MAX;
}
/// Returns the length, in bytes, of the smallest pattern.
///
/// This is guaranteed to be at least one.
pub(crate) fn minimum_len(&self) -> usize {
self.minimum_len
}
/// Returns the match semantics used by these patterns.
pub(crate) fn match_kind(&self) -> &MatchKind {
&self.kind
}
/// Return the pattern with the given identifier. If such a pattern does
/// not exist, then this panics.
pub(crate) fn get(&self, id: PatternID) -> Pattern<'_> {
Pattern(&self.by_id[id])
}
/// Return the pattern with the given identifier without performing bounds
/// checks.
///
/// # Safety
///
/// Callers must ensure that a pattern with the given identifier exists
/// before using this method.
pub(crate) unsafe fn get_unchecked(&self, id: PatternID) -> Pattern<'_> {
Pattern(self.by_id.get_unchecked(id.as_usize()))
}
/// Return an iterator over all the patterns in this collection, in the
/// order in which they should be matched.
///
/// Specifically, in a naive multi-pattern matcher, the following is
/// guaranteed to satisfy the match semantics of this collection of
/// patterns:
///
/// ```ignore
/// for i in 0..haystack.len():
/// for p in patterns.iter():
/// if haystack[i..].starts_with(p.bytes()):
/// return Match(p.id(), i, i + p.bytes().len())
/// ```
///
/// Namely, among the patterns in a collection, if they are matched in
/// the order provided by this iterator, then the result is guaranteed
/// to satisfy the correct match semantics. (Either leftmost-first or
/// leftmost-longest.)
pub(crate) fn iter(&self) -> PatternIter<'_> {
PatternIter { patterns: self, i: 0 }
}
}
/// An iterator over the patterns in the `Patterns` collection.
///
/// The order of the patterns provided by this iterator is consistent with the
/// match semantics of the originating collection of patterns.
///
/// The lifetime `'p` corresponds to the lifetime of the collection of patterns
/// this is iterating over.
#[derive(Debug)]
pub(crate) struct PatternIter<'p> {
patterns: &'p Patterns,
i: usize,
}
impl<'p> Iterator for PatternIter<'p> {
type Item = (PatternID, Pattern<'p>);
fn next(&mut self) -> Option<(PatternID, Pattern<'p>)> {
if self.i >= self.patterns.len() {
return None;
}
let id = self.patterns.order[self.i];
let p = self.patterns.get(id);
self.i += 1;
Some((id, p))
}
}
/// A pattern that is used in packed searching.
#[derive(Clone)]
pub(crate) struct Pattern<'a>(&'a [u8]);
impl<'a> fmt::Debug for Pattern<'a> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Pattern")
.field("lit", &String::from_utf8_lossy(&self.0))
.finish()
}
}
impl<'p> Pattern<'p> {
/// Returns the length of this pattern, in bytes.
pub(crate) fn len(&self) -> usize {
self.0.len()
}
/// Returns the bytes of this pattern.
pub(crate) fn bytes(&self) -> &[u8] {
&self.0
}
/// Returns the first `len` low nybbles from this pattern. If this pattern
/// is shorter than `len`, then this panics.
pub(crate) fn low_nybbles(&self, len: usize) -> Box<[u8]> {
let mut nybs = vec![0; len].into_boxed_slice();
for (i, byte) in self.bytes().iter().take(len).enumerate() {
nybs[i] = byte & 0xF;
}
nybs
}
/// Returns true if this pattern is a prefix of the given bytes.
#[inline(always)]
pub(crate) fn is_prefix(&self, bytes: &[u8]) -> bool {
is_prefix(bytes, self.bytes())
}
/// Returns true if this pattern is a prefix of the haystack given by the
/// raw `start` and `end` pointers.
///
/// # Safety
///
/// * It must be the case that `start < end` and that the distance between
/// them is at least equal to `V::BYTES`. That is, it must always be valid
/// to do at least an unaligned load of `V` at `start`.
/// * Both `start` and `end` must be valid for reads.
/// * Both `start` and `end` must point to an initialized value.
/// * Both `start` and `end` must point to the same allocated object and
/// must either be in bounds or at most one byte past the end of the
/// allocated object.
/// * Both `start` and `end` must be _derived from_ a pointer to the same
/// object.
/// * The distance between `start` and `end` must not overflow `isize`.
/// * The distance being in bounds must not rely on "wrapping around" the
/// address space.
#[inline(always)]
pub(crate) unsafe fn is_prefix_raw(
&self,
start: *const u8,
end: *const u8,
) -> bool {
let patlen = self.bytes().len();
let haylen = end.distance(start);
if patlen > haylen {
return false;
}
// SAFETY: We've checked that the haystack has length at least equal
// to this pattern. All other safety concerns are the responsibility
// of the caller.
is_equal_raw(start, self.bytes().as_ptr(), patlen)
}
}
/// Returns true if and only if `needle` is a prefix of `haystack`.
///
/// This uses a latency optimized variant of `memcmp` internally which *might*
/// make this faster for very short strings.
///
/// # Inlining
///
/// This routine is marked `inline(always)`. If you want to call this function
/// in a way that is not always inlined, you'll need to wrap a call to it in
/// another function that is marked as `inline(never)` or just `inline`.
#[inline(always)]
fn is_prefix(haystack: &[u8], needle: &[u8]) -> bool {
if needle.len() > haystack.len() {
return false;
}
// SAFETY: Our pointers are derived directly from borrowed slices which
// uphold all of our safety guarantees except for length. We account for
// length with the check above.
unsafe { is_equal_raw(haystack.as_ptr(), needle.as_ptr(), needle.len()) }
}
/// Compare corresponding bytes in `x` and `y` for equality.
///
/// That is, this returns true if and only if `x.len() == y.len()` and
/// `x[i] == y[i]` for all `0 <= i < x.len()`.
///
/// Note that this isn't used. We only use it in tests as a convenient way
/// of testing `is_equal_raw`.
///
/// # Inlining
///
/// This routine is marked `inline(always)`. If you want to call this function
/// in a way that is not always inlined, you'll need to wrap a call to it in
/// another function that is marked as `inline(never)` or just `inline`.
///
/// # Motivation
///
/// Why not use slice equality instead? Well, slice equality usually results in
/// a call out to the current platform's `libc` which might not be inlineable
/// or have other overhead. This routine isn't guaranteed to be a win, but it
/// might be in some cases.
#[cfg(test)]
#[inline(always)]
fn is_equal(x: &[u8], y: &[u8]) -> bool {
if x.len() != y.len() {
return false;
}
// SAFETY: Our pointers are derived directly from borrowed slices which
// uphold all of our safety guarantees except for length. We account for
// length with the check above.
unsafe { is_equal_raw(x.as_ptr(), y.as_ptr(), x.len()) }
}
/// Compare `n` bytes at the given pointers for equality.
///
/// This returns true if and only if `*x.add(i) == *y.add(i)` for all
/// `0 <= i < n`.
///
/// # Inlining
///
/// This routine is marked `inline(always)`. If you want to call this function
/// in a way that is not always inlined, you'll need to wrap a call to it in
/// another function that is marked as `inline(never)` or just `inline`.
///
/// # Motivation
///
/// Why not use slice equality instead? Well, slice equality usually results in
/// a call out to the current platform's `libc` which might not be inlineable
/// or have other overhead. This routine isn't guaranteed to be a win, but it
/// might be in some cases.
///
/// # Safety
///
/// * Both `x` and `y` must be valid for reads of up to `n` bytes.
/// * Both `x` and `y` must point to an initialized value.
/// * Both `x` and `y` must each point to an allocated object and
/// must either be in bounds or at most one byte past the end of the
/// allocated object. `x` and `y` do not need to point to the same allocated
/// object, but they may.
/// * Both `x` and `y` must be _derived from_ a pointer to their respective
/// allocated objects.
/// * The distance between `x` and `x+n` must not overflow `isize`. Similarly
/// for `y` and `y+n`.
/// * The distance being in bounds must not rely on "wrapping around" the
/// address space.
#[inline(always)]
unsafe fn is_equal_raw(mut x: *const u8, mut y: *const u8, n: usize) -> bool {
// If we don't have enough bytes to do 4-byte at a time loads, then
// handle each possible length specially. Note that I used to have a
// byte-at-a-time loop here and that turned out to be quite a bit slower
// for the memmem/pathological/defeat-simple-vector-alphabet benchmark.
if n < 4 {
return match n {
0 => true,
1 => x.read() == y.read(),
2 => {
x.cast::<u16>().read_unaligned()
== y.cast::<u16>().read_unaligned()
}
// I also tried copy_nonoverlapping here and it looks like the
// codegen is the same.
3 => x.cast::<[u8; 3]>().read() == y.cast::<[u8; 3]>().read(),
_ => unreachable!(),
};
}
// When we have 4 or more bytes to compare, then proceed in chunks of 4 at
// a time using unaligned loads.
//
// Also, why do 4 byte loads instead of, say, 8 byte loads? The reason is
// that this particular version of memcmp is likely to be called with tiny
// needles. That means that if we do 8 byte loads, then a higher proportion
// of memcmp calls will use the slower variant above. With that said, this
// is a hypothesis and is only loosely supported by benchmarks. There's
// likely some improvement that could be made here. The main thing here
// though is to optimize for latency, not throughput.
// SAFETY: The caller is responsible for ensuring the pointers we get are
// valid and readable for at least `n` bytes. We also do unaligned loads,
// so there's no need to ensure we're aligned. (This is justified by this
// routine being specifically for short strings.)
let xend = x.add(n.wrapping_sub(4));
let yend = y.add(n.wrapping_sub(4));
while x < xend {
let vx = x.cast::<u32>().read_unaligned();
let vy = y.cast::<u32>().read_unaligned();
if vx != vy {
return false;
}
x = x.add(4);
y = y.add(4);
}
let vx = xend.cast::<u32>().read_unaligned();
let vy = yend.cast::<u32>().read_unaligned();
vx == vy
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn equals_different_lengths() {
assert!(!is_equal(b"", b"a"));
assert!(!is_equal(b"a", b""));
assert!(!is_equal(b"ab", b"a"));
assert!(!is_equal(b"a", b"ab"));
}
#[test]
fn equals_mismatch() {
let one_mismatch = [
(&b"a"[..], &b"x"[..]),
(&b"ab"[..], &b"ax"[..]),
(&b"abc"[..], &b"abx"[..]),
(&b"abcd"[..], &b"abcx"[..]),
(&b"abcde"[..], &b"abcdx"[..]),
(&b"abcdef"[..], &b"abcdex"[..]),
(&b"abcdefg"[..], &b"abcdefx"[..]),
(&b"abcdefgh"[..], &b"abcdefgx"[..]),
(&b"abcdefghi"[..], &b"abcdefghx"[..]),
(&b"abcdefghij"[..], &b"abcdefghix"[..]),
(&b"abcdefghijk"[..], &b"abcdefghijx"[..]),
(&b"abcdefghijkl"[..], &b"abcdefghijkx"[..]),
(&b"abcdefghijklm"[..], &b"abcdefghijklx"[..]),
(&b"abcdefghijklmn"[..], &b"abcdefghijklmx"[..]),
];
for (x, y) in one_mismatch {
assert_eq!(x.len(), y.len(), "lengths should match");
assert!(!is_equal(x, y));
assert!(!is_equal(y, x));
}
}
#[test]
fn equals_yes() {
assert!(is_equal(b"", b""));
assert!(is_equal(b"a", b"a"));
assert!(is_equal(b"ab", b"ab"));
assert!(is_equal(b"abc", b"abc"));
assert!(is_equal(b"abcd", b"abcd"));
assert!(is_equal(b"abcde", b"abcde"));
assert!(is_equal(b"abcdef", b"abcdef"));
assert!(is_equal(b"abcdefg", b"abcdefg"));
assert!(is_equal(b"abcdefgh", b"abcdefgh"));
assert!(is_equal(b"abcdefghi", b"abcdefghi"));
}
#[test]
fn prefix() {
assert!(is_prefix(b"", b""));
assert!(is_prefix(b"a", b""));
assert!(is_prefix(b"ab", b""));
assert!(is_prefix(b"foo", b"foo"));
assert!(is_prefix(b"foobar", b"foo"));
assert!(!is_prefix(b"foo", b"fob"));
assert!(!is_prefix(b"foobar", b"fob"));
}
}
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