utf8_ranges/

lib.rs

/*!
Crate `utf8-ranges` converts ranges of Unicode scalar values to equivalent
ranges of UTF-8 bytes. This is useful for constructing byte based automatons
that need to embed UTF-8 decoding.

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:

```ignore
[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:

```ignore
[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:

```ignore
[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]`):

```ignore
[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 crate automates the process of creating these byte ranges from ranges of
Unicode scalar values.

# Why would I ever use this?

You probably won't ever need this. In 99% of cases, you just decode the byte
sequence into a Unicode scalar value and compare scalar values directly.
However, this explicit decoding step isn't always possible. For example, the
construction of some finite state machines may benefit from converting ranges
of scalar values into UTF-8 decoder automata (e.g., for character classes in
regular expressions).

# 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
[Lucene](https://github.com/apache/lucene-solr/blob/ae93f4e7ac6a3908046391de35d4f50a0d3c59ca/lucene/core/src/java/org/apache/lucene/util/automaton/UTF32ToUTF8.java),
which uses it for executing automata on their term index.
*/

#![deny(missing_docs)]

#[cfg(test)]
extern crate quickcheck;

use std::char;
use std::fmt;
use std::slice;

use char_utf8::encode_utf8;

const MAX_UTF8_BYTES: usize = 4;

mod char_utf8;

/// 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)]
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()
    }

    /// 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().into_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, PartialEq, Eq)]
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: start, end: 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 utf8_ranges::{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).
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(start as u32, end as u32);
        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(start as u32, end as u32);
    }

    fn push(&mut self, start: u32, end: u32) {
        self.range_stack.push(ScalarRange { start: start, end: 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 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() {
            Some(Utf8Range::new(self.start as u8, self.end as u8))
        } 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.
    ///
    /// 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 n = encode_utf8(cs, start).unwrap();
        let m = encode_utf8(ce, end).unwrap();
        assert_eq!(n, m);
        n
    }
}

fn max_scalar_value(nbytes: usize) -> u32 {
    match nbytes {
        1 => 0x007F,
        2 => 0x07FF,
        3 => 0xFFFF,
        4 => 0x10FFFF,
        _ => unreachable!("invalid UTF-8 byte sequence size"),
    }
}

#[cfg(test)]
mod tests {
    use std::char;

    use quickcheck::{TestResult, quickcheck};

    use {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 {:?}",
                        start as u32, end as u32, 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..(0x10FFFF + 1) {
            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 qc_codepoints_no_surrogate() {
        fn p(s: char, e: char) -> TestResult {
            if s > e {
                return TestResult::discard();
            }
            never_accepts_surrogate_codepoints(s, e);
            TestResult::passed()
        }
        quickcheck(p as fn(char, char) -> TestResult);
    }

    #[test]
    fn bmp() {
        use 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)]),
        ]);
    }

    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] = (cp >> 12 & 0x0F) as u8 | TAG_THREE_B;
        dst[1] = (cp >> 6 & 0x3F) as u8 | TAG_CONT;
        dst[2] = (cp & 0x3F) as u8 | TAG_CONT;
        dst
    }
}