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/*!
Converts ranges of Unicode scalar values to equivalent ranges of UTF-8 bytes.
This is sub-module is useful for constructing byte based automatons that need
to embed UTF-8 decoding. The most common use of this module is in conjunction
with the [`hir::ClassUnicodeRange`](crate::hir::ClassUnicodeRange) type.
See the documentation on the `Utf8Sequences` iterator for more details and
an example.
# Wait, what is this?
This is simplest to explain with an example. Let's say you wanted to test
whether a particular byte sequence was a Cyrillic character. One possible
scalar value range is `[0400-04FF]`. The set of allowed bytes for this
range can be expressed as a sequence of byte ranges:
```text
[D0-D3][80-BF]
```
This is simple enough: simply encode the boundaries, `0400` encodes to
`D0 80` and `04FF` encodes to `D3 BF`, and create ranges from each
corresponding pair of bytes: `D0` to `D3` and `80` to `BF`.
However, what if you wanted to add the Cyrillic Supplementary characters to
your range? Your range might then become `[0400-052F]`. The same procedure
as above doesn't quite work because `052F` encodes to `D4 AF`. The byte ranges
you'd get from the previous transformation would be `[D0-D4][80-AF]`. However,
this isn't quite correct because this range doesn't capture many characters,
for example, `04FF` (because its last byte, `BF` isn't in the range `80-AF`).
Instead, you need multiple sequences of byte ranges:
```text
[D0-D3][80-BF] # matches codepoints 0400-04FF
[D4][80-AF] # matches codepoints 0500-052F
```
This gets even more complicated if you want bigger ranges, particularly if
they naively contain surrogate codepoints. For example, the sequence of byte
ranges for the basic multilingual plane (`[0000-FFFF]`) look like this:
```text
[0-7F]
[C2-DF][80-BF]
[E0][A0-BF][80-BF]
[E1-EC][80-BF][80-BF]
[ED][80-9F][80-BF]
[EE-EF][80-BF][80-BF]
```
Note that the byte ranges above will *not* match any erroneous encoding of
UTF-8, including encodings of surrogate codepoints.
And, of course, for all of Unicode (`[000000-10FFFF]`):
```text
[0-7F]
[C2-DF][80-BF]
[E0][A0-BF][80-BF]
[E1-EC][80-BF][80-BF]
[ED][80-9F][80-BF]
[EE-EF][80-BF][80-BF]
[F0][90-BF][80-BF][80-BF]
[F1-F3][80-BF][80-BF][80-BF]
[F4][80-8F][80-BF][80-BF]
```
This module automates the process of creating these byte ranges from ranges of
Unicode scalar values.
# Lineage
I got the idea and general implementation strategy from Russ Cox in his
[article on regexps](https://web.archive.org/web/20160404141123/https://swtch.com/~rsc/regexp/regexp3.html) and RE2.
Russ Cox got it from Ken Thompson's `grep` (no source, folk lore?).
I also got the idea from
which uses it for executing automata on their term index.
*/
use core::{char, fmt, iter::FusedIterator, slice};
use alloc::{vec, vec::Vec};
const MAX_UTF8_BYTES: usize = 4;
/// Utf8Sequence represents a sequence of byte ranges.
///
/// To match a Utf8Sequence, a candidate byte sequence must match each
/// successive range.
///
/// For example, if there are two ranges, `[C2-DF][80-BF]`, then the byte
/// sequence `\xDD\x61` would not match because `0x61 < 0x80`.
#[derive(Copy, Clone, Eq, PartialEq, PartialOrd, Ord)]
pub enum Utf8Sequence {
/// One byte range.
One(Utf8Range),
/// Two successive byte ranges.
Two([Utf8Range; 2]),
/// Three successive byte ranges.
Three([Utf8Range; 3]),
/// Four successive byte ranges.
Four([Utf8Range; 4]),
}
impl Utf8Sequence {
/// Creates a new UTF-8 sequence from the encoded bytes of a scalar value
/// range.
///
/// This assumes that `start` and `end` have the same length.
fn from_encoded_range(start: &[u8], end: &[u8]) -> Self {
assert_eq!(start.len(), end.len());
match start.len() {
2 => Utf8Sequence::Two([
Utf8Range::new(start[0], end[0]),
Utf8Range::new(start[1], end[1]),
]),
3 => Utf8Sequence::Three([
Utf8Range::new(start[0], end[0]),
Utf8Range::new(start[1], end[1]),
Utf8Range::new(start[2], end[2]),
]),
4 => Utf8Sequence::Four([
Utf8Range::new(start[0], end[0]),
Utf8Range::new(start[1], end[1]),
Utf8Range::new(start[2], end[2]),
Utf8Range::new(start[3], end[3]),
]),
n => unreachable!("invalid encoded length: {}", n),
}
}
/// Returns the underlying sequence of byte ranges as a slice.
pub fn as_slice(&self) -> &[Utf8Range] {
use self::Utf8Sequence::*;
match *self {
One(ref r) => slice::from_ref(r),
Two(ref r) => &r[..],
Three(ref r) => &r[..],
Four(ref r) => &r[..],
}
}
/// Returns the number of byte ranges in this sequence.
///
/// The length is guaranteed to be in the closed interval `[1, 4]`.
pub fn len(&self) -> usize {
self.as_slice().len()
}
/// Reverses the ranges in this sequence.
///
/// For example, if this corresponds to the following sequence:
///
/// ```text
/// [D0-D3][80-BF]
/// ```
///
/// Then after reversal, it will be
///
/// ```text
/// [80-BF][D0-D3]
/// ```
///
/// This is useful when one is constructing a UTF-8 automaton to match
/// character classes in reverse.
pub fn reverse(&mut self) {
match *self {
Utf8Sequence::One(_) => {}
Utf8Sequence::Two(ref mut x) => x.reverse(),
Utf8Sequence::Three(ref mut x) => x.reverse(),
Utf8Sequence::Four(ref mut x) => x.reverse(),
}
}
/// Returns true if and only if a prefix of `bytes` matches this sequence
/// of byte ranges.
pub fn matches(&self, bytes: &[u8]) -> bool {
if bytes.len() < self.len() {
return false;
}
for (&b, r) in bytes.iter().zip(self) {
if !r.matches(b) {
return false;
}
}
true
}
}
impl<'a> IntoIterator for &'a Utf8Sequence {
type IntoIter = slice::Iter<'a, Utf8Range>;
type Item = &'a Utf8Range;
fn into_iter(self) -> Self::IntoIter {
self.as_slice().iter()
}
}
impl fmt::Debug for Utf8Sequence {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
use self::Utf8Sequence::*;
match *self {
One(ref r) => write!(f, "{:?}", r),
Two(ref r) => write!(f, "{:?}{:?}", r[0], r[1]),
Three(ref r) => write!(f, "{:?}{:?}{:?}", r[0], r[1], r[2]),
Four(ref r) => {
write!(f, "{:?}{:?}{:?}{:?}", r[0], r[1], r[2], r[3])
}
}
}
}
/// A single inclusive range of UTF-8 bytes.
#[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord)]
pub struct Utf8Range {
/// Start of byte range (inclusive).
pub start: u8,
/// End of byte range (inclusive).
pub end: u8,
}
impl Utf8Range {
fn new(start: u8, end: u8) -> Self {
Utf8Range { start, end }
}
/// Returns true if and only if the given byte is in this range.
pub fn matches(&self, b: u8) -> bool {
self.start <= b && b <= self.end
}
}
impl fmt::Debug for Utf8Range {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
if self.start == self.end {
write!(f, "[{:X}]", self.start)
} else {
write!(f, "[{:X}-{:X}]", self.start, self.end)
}
}
}
/// An iterator over ranges of matching UTF-8 byte sequences.
///
/// The iteration represents an alternation of comprehensive byte sequences
/// that match precisely the set of UTF-8 encoded scalar values.
///
/// A byte sequence corresponds to one of the scalar values in the range given
/// if and only if it completely matches exactly one of the sequences of byte
/// ranges produced by this iterator.
///
/// Each sequence of byte ranges matches a unique set of bytes. That is, no two
/// sequences will match the same bytes.
///
/// # Example
///
/// This shows how to match an arbitrary byte sequence against a range of
/// scalar values.
///
/// ```rust
/// use regex_syntax::utf8::{Utf8Sequences, Utf8Sequence};
///
/// fn matches(seqs: &[Utf8Sequence], bytes: &[u8]) -> bool {
/// for range in seqs {
/// if range.matches(bytes) {
/// return true;
/// }
/// }
/// false
/// }
///
/// // Test the basic multilingual plane.
/// let seqs: Vec<_> = Utf8Sequences::new('\u{0}', '\u{FFFF}').collect();
///
/// // UTF-8 encoding of 'a'.
/// assert!(matches(&seqs, &[0x61]));
/// // UTF-8 encoding of '☃' (`\u{2603}`).
/// assert!(matches(&seqs, &[0xE2, 0x98, 0x83]));
/// // UTF-8 encoding of `\u{10348}` (outside the BMP).
/// assert!(!matches(&seqs, &[0xF0, 0x90, 0x8D, 0x88]));
/// // Tries to match against a UTF-8 encoding of a surrogate codepoint,
/// // which is invalid UTF-8, and therefore fails, despite the fact that
/// // the corresponding codepoint (0xD800) falls in the range given.
/// assert!(!matches(&seqs, &[0xED, 0xA0, 0x80]));
/// // And fails against plain old invalid UTF-8.
/// assert!(!matches(&seqs, &[0xFF, 0xFF]));
/// ```
///
/// If this example seems circuitous, that's because it is! It's meant to be
/// illustrative. In practice, you could just try to decode your byte sequence
/// and compare it with the scalar value range directly. However, this is not
/// always possible (for example, in a byte based automaton).
#[derive(Debug)]
pub struct Utf8Sequences {
range_stack: Vec<ScalarRange>,
}
impl Utf8Sequences {
/// Create a new iterator over UTF-8 byte ranges for the scalar value range
/// given.
pub fn new(start: char, end: char) -> Self {
let mut it = Utf8Sequences { range_stack: vec![] };
it.push(u32::from(start), u32::from(end));
it
}
/// reset resets the scalar value range.
/// Any existing state is cleared, but resources may be reused.
///
/// N.B. Benchmarks say that this method is dubious.
#[doc(hidden)]
pub fn reset(&mut self, start: char, end: char) {
self.range_stack.clear();
self.push(u32::from(start), u32::from(end));
}
fn push(&mut self, start: u32, end: u32) {
self.range_stack.push(ScalarRange { start, end });
}
}
struct ScalarRange {
start: u32,
end: u32,
}
impl fmt::Debug for ScalarRange {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "ScalarRange({:X}, {:X})", self.start, self.end)
}
}
impl Iterator for Utf8Sequences {
type Item = Utf8Sequence;
fn next(&mut self) -> Option<Self::Item> {
'TOP: while let Some(mut r) = self.range_stack.pop() {
'INNER: loop {
if let Some((r1, r2)) = r.split() {
self.push(r2.start, r2.end);
r.start = r1.start;
r.end = r1.end;
continue 'INNER;
}
if !r.is_valid() {
continue 'TOP;
}
for i in 1..MAX_UTF8_BYTES {
let max = max_scalar_value(i);
if r.start <= max && max < r.end {
self.push(max + 1, r.end);
r.end = max;
continue 'INNER;
}
}
if let Some(ascii_range) = r.as_ascii() {
return Some(Utf8Sequence::One(ascii_range));
}
for i in 1..MAX_UTF8_BYTES {
let m = (1 << (6 * i)) - 1;
if (r.start & !m) != (r.end & !m) {
if (r.start & m) != 0 {
self.push((r.start | m) + 1, r.end);
r.end = r.start | m;
continue 'INNER;
}
if (r.end & m) != m {
self.push(r.end & !m, r.end);
r.end = (r.end & !m) - 1;
continue 'INNER;
}
}
}
let mut start = [0; MAX_UTF8_BYTES];
let mut end = [0; MAX_UTF8_BYTES];
let n = r.encode(&mut start, &mut end);
return Some(Utf8Sequence::from_encoded_range(
&start[0..n],
&end[0..n],
));
}
}
None
}
}
impl FusedIterator for Utf8Sequences {}
impl ScalarRange {
/// split splits this range if it overlaps with a surrogate codepoint.
///
/// Either or both ranges may be invalid.
fn split(&self) -> Option<(ScalarRange, ScalarRange)> {
if self.start < 0xE000 && self.end > 0xD7FF {
Some((
ScalarRange { start: self.start, end: 0xD7FF },
ScalarRange { start: 0xE000, end: self.end },
))
} else {
None
}
}
/// is_valid returns true if and only if start <= end.
fn is_valid(&self) -> bool {
self.start <= self.end
}
/// as_ascii returns this range as a Utf8Range if and only if all scalar
/// values in this range can be encoded as a single byte.
fn as_ascii(&self) -> Option<Utf8Range> {
if self.is_ascii() {
let start = u8::try_from(self.start).unwrap();
let end = u8::try_from(self.end).unwrap();
Some(Utf8Range::new(start, end))
} else {
None
}
}
/// is_ascii returns true if the range is ASCII only (i.e., takes a single
/// byte to encode any scalar value).
fn is_ascii(&self) -> bool {
self.is_valid() && self.end <= 0x7f
}
/// encode writes the UTF-8 encoding of the start and end of this range
/// to the corresponding destination slices, and returns the number of
/// bytes written.
///
/// The slices should have room for at least `MAX_UTF8_BYTES`.
fn encode(&self, start: &mut [u8], end: &mut [u8]) -> usize {
let cs = char::from_u32(self.start).unwrap();
let ce = char::from_u32(self.end).unwrap();
let ss = cs.encode_utf8(start);
let se = ce.encode_utf8(end);
assert_eq!(ss.len(), se.len());
ss.len()
}
}
fn max_scalar_value(nbytes: usize) -> u32 {
match nbytes {
1 => 0x007F,
2 => 0x07FF,
3 => 0xFFFF,
4 => 0x0010_FFFF,
_ => unreachable!("invalid UTF-8 byte sequence size"),
}
}
#[cfg(test)]
mod tests {
use core::char;
use alloc::{vec, vec::Vec};
use crate::utf8::{Utf8Range, Utf8Sequences};
fn rutf8(s: u8, e: u8) -> Utf8Range {
Utf8Range::new(s, e)
}
fn never_accepts_surrogate_codepoints(start: char, end: char) {
for cp in 0xD800..0xE000 {
let buf = encode_surrogate(cp);
for r in Utf8Sequences::new(start, end) {
if r.matches(&buf) {
panic!(
"Sequence ({:X}, {:X}) contains range {:?}, \
which matches surrogate code point {:X} \
with encoded bytes {:?}",
u32::from(start),
u32::from(end),
r,
cp,
buf,
);
}
}
}
}
#[test]
fn codepoints_no_surrogates() {
never_accepts_surrogate_codepoints('\u{0}', '\u{FFFF}');
never_accepts_surrogate_codepoints('\u{0}', '\u{10FFFF}');
never_accepts_surrogate_codepoints('\u{0}', '\u{10FFFE}');
never_accepts_surrogate_codepoints('\u{80}', '\u{10FFFF}');
never_accepts_surrogate_codepoints('\u{D7FF}', '\u{E000}');
}
#[test]
fn single_codepoint_one_sequence() {
// Tests that every range of scalar values that contains a single
// scalar value is recognized by one sequence of byte ranges.
for i in 0x0..=0x0010_FFFF {
let c = match char::from_u32(i) {
None => continue,
Some(c) => c,
};
let seqs: Vec<_> = Utf8Sequences::new(c, c).collect();
assert_eq!(seqs.len(), 1);
}
}
#[test]
fn bmp() {
use crate::utf8::Utf8Sequence::*;
let seqs = Utf8Sequences::new('\u{0}', '\u{FFFF}').collect::<Vec<_>>();
assert_eq!(
seqs,
vec![
One(rutf8(0x0, 0x7F)),
Two([rutf8(0xC2, 0xDF), rutf8(0x80, 0xBF)]),
Three([
rutf8(0xE0, 0xE0),
rutf8(0xA0, 0xBF),
rutf8(0x80, 0xBF)
]),
Three([
rutf8(0xE1, 0xEC),
rutf8(0x80, 0xBF),
rutf8(0x80, 0xBF)
]),
Three([
rutf8(0xED, 0xED),
rutf8(0x80, 0x9F),
rutf8(0x80, 0xBF)
]),
Three([
rutf8(0xEE, 0xEF),
rutf8(0x80, 0xBF),
rutf8(0x80, 0xBF)
]),
]
);
}
#[test]
fn reverse() {
use crate::utf8::Utf8Sequence::*;
let mut s = One(rutf8(0xA, 0xB));
s.reverse();
assert_eq!(s.as_slice(), &[rutf8(0xA, 0xB)]);
let mut s = Two([rutf8(0xA, 0xB), rutf8(0xB, 0xC)]);
s.reverse();
assert_eq!(s.as_slice(), &[rutf8(0xB, 0xC), rutf8(0xA, 0xB)]);
let mut s = Three([rutf8(0xA, 0xB), rutf8(0xB, 0xC), rutf8(0xC, 0xD)]);
s.reverse();
assert_eq!(
s.as_slice(),
&[rutf8(0xC, 0xD), rutf8(0xB, 0xC), rutf8(0xA, 0xB)]
);
let mut s = Four([
rutf8(0xA, 0xB),
rutf8(0xB, 0xC),
rutf8(0xC, 0xD),
rutf8(0xD, 0xE),
]);
s.reverse();
assert_eq!(
s.as_slice(),
&[
rutf8(0xD, 0xE),
rutf8(0xC, 0xD),
rutf8(0xB, 0xC),
rutf8(0xA, 0xB)
]
);
}
fn encode_surrogate(cp: u32) -> [u8; 3] {
const TAG_CONT: u8 = 0b1000_0000;
const TAG_THREE_B: u8 = 0b1110_0000;
assert!(0xD800 <= cp && cp < 0xE000);
let mut dst = [0; 3];
dst[0] = u8::try_from(cp >> 12 & 0x0F).unwrap() | TAG_THREE_B;
dst[1] = u8::try_from(cp >> 6 & 0x3F).unwrap() | TAG_CONT;
dst[2] = u8::try_from(cp & 0x3F).unwrap() | TAG_CONT;
dst
}
}