irgx 2.1.2

//! What a search hands back: [`Match`], [`Captures`], and the three iterators.
//!
//! Offsets here are byte offsets into the searched `&str`, and that is not a
//! translation - it is the same coordinate system the engine reports in. Rust
//! `str` is UTF-8 and sliced by byte index, exactly like the engine's spans, so
//! `&text[m.range()]` is the matched text by construction. The one thing that
//! needs care is that slicing a `str` at a non-boundary panics, so every span is
//! checked for boundary alignment before it becomes a `Match`; see
//! [`crate::Error::NotCharBoundary`].

use std::iter::FusedIterator;
use std::ops::{Index, Range};

use crate::pattern::{Regex, expect};

/// One match's group spans, index 0 first. `None` is a group the match did not
/// enter, which the C ABI reports as `(-1, -1)` and which is a different fact
/// from a group that matched empty.
pub(crate) type GroupSpans = Box<[Option<(usize, usize)>]>;

/// One match: a byte range in the text that was searched.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct Match<'t> {
    text: &'t str,
    start: usize,
    end: usize,
}

impl<'t> Match<'t> {
    /// Both offsets are checked UTF-8 boundaries in `text` before this is
    /// called, which is what lets [`Match::as_str`] slice without panicking.
    pub(crate) fn new(text: &'t str, start: usize, end: usize) -> Self {
        debug_assert!(text.is_char_boundary(start) && text.is_char_boundary(end));
        Self { text, start, end }
    }

    /// The byte offset the match starts at.
    #[must_use]
    pub fn start(&self) -> usize {
        self.start
    }

    /// The byte offset just past the match.
    #[must_use]
    pub fn end(&self) -> usize {
        self.end
    }

    /// The match as a byte range, for slicing the text you searched.
    #[must_use]
    pub fn range(&self) -> Range<usize> {
        self.start..self.end
    }

    /// Whether the match is zero-width. Nullable patterns produce these, and
    /// they are real matches.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.start == self.end
    }

    /// The match length in bytes.
    #[must_use]
    pub fn len(&self) -> usize {
        self.end - self.start
    }

    /// The matched text.
    #[must_use]
    pub fn as_str(&self) -> &'t str {
        &self.text[self.start..self.end]
    }

    /// The matched bytes. Always available, including for a pattern compiled
    /// with `unicode(false)` over text where the boundaries would not align.
    #[must_use]
    pub fn as_bytes(&self) -> &'t [u8] {
        &self.text.as_bytes()[self.start..self.end]
    }
}

impl<'t> From<Match<'t>> for &'t str {
    fn from(found: Match<'t>) -> Self {
        found.as_str()
    }
}

impl From<Match<'_>> for Range<usize> {
    fn from(found: Match<'_>) -> Self {
        found.range()
    }
}

/// Thin the engine's byte-granular sequence to the one the `regex` crate would
/// have reported, so swapping this crate in does not change which matches a
/// nullable pattern yields.
///
/// The engine hands back every empty match at every byte offset - the sequence
/// Python's `re` reports, and the one a C host receives. `regex` shows fewer,
/// by two rules taken from its `Searcher`:
///
/// * an empty match starting exactly where the previous match ended is skipped,
///   and "previous" counts one that was itself skipped, so `prev_end` advances
///   on both paths;
/// * after an empty match the scan resumes at the next CHARACTER boundary, so
///   an empty match inside a multi-byte character is never reached. `l*` over
///   `"héllo"` reports nothing at byte 2, the continuation byte of the `é`.
///   (`regex::bytes::Regex` steps one byte instead and does report it; this
///   crate's surface is `&str`, so it follows `regex::Regex`.)
///
/// Both rules only ever REMOVE spans, which is what makes applying them
/// afterwards sound: the positions `regex` would visit are a subset of the ones
/// the engine already searched, and at a shared position both find the same
/// leftmost match.
pub(crate) fn crate_sequence(spans: Vec<(usize, usize)>, text: &str) -> Vec<(usize, usize)> {
    // Nothing to remove unless some match is empty, which is the overwhelmingly
    // common case - and the check is cheaper than the copy it avoids.
    if !spans.iter().any(|&(s, e)| s == e) {
        return spans;
    }
    let mut out = Vec::with_capacity(spans.len());
    let (mut prev_end, mut resume) = (usize::MAX, 0);
    for (start, end) in spans {
        if start < resume {
            continue; // inside a character the crate's scan stepped over
        }
        resume = if start == end {
            start + char_width(text, start)
        } else {
            end
        };
        if start != end || start != prev_end {
            out.push((start, end));
        }
        prev_end = end;
    }
    out
}

/// How far the crate's scan advances past an empty match at `at`: the width of
/// the character starting there, and never less than one byte, so that a
/// malformed or out-of-range offset cannot stall the walk.
fn char_width(text: &str, at: usize) -> usize {
    text[at..].chars().next().map_or(1, char::len_utf8)
}

/// Every match in one text, in the sequence the `regex` crate reports.
///
/// Eager rather than lazy, because the engine reports the whole sequence in one
/// call and that is deliberate: where the matches are, and `word(true)`
/// filtering, are its rules, and re-deriving them from a resumable cursor is
/// exactly how a binding gets nullable patterns wrong. Which of them this crate
/// shows is the separate question `crate_sequence` above answers. The upside is
/// that this iterator knows its length, runs backwards, and borrows only the
/// text.
#[derive(Clone, Debug)]
pub struct Matches<'t> {
    text: &'t str,
    spans: std::vec::IntoIter<(usize, usize)>,
}

impl<'t> Matches<'t> {
    pub(crate) fn new(text: &'t str, spans: Vec<(usize, usize)>) -> Self {
        Self {
            text,
            spans: spans.into_iter(),
        }
    }
}

impl<'t> Iterator for Matches<'t> {
    type Item = Match<'t>;

    fn next(&mut self) -> Option<Self::Item> {
        let (start, end) = self.spans.next()?;
        Some(Match::new(self.text, start, end))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.spans.size_hint()
    }
}

impl DoubleEndedIterator for Matches<'_> {
    fn next_back(&mut self) -> Option<Self::Item> {
        let (start, end) = self.spans.next_back()?;
        Some(Match::new(self.text, start, end))
    }
}

impl ExactSizeIterator for Matches<'_> {}
impl FusedIterator for Matches<'_> {}

/// The capture groups of one match.
///
/// Index 0 is the whole match; index `k` is group `k`. A group the match did not
/// enter is `None`, which is a different fact from a group that matched empty -
/// `(a)|(b)` produces one of each, every time.
#[derive(Clone, Debug)]
pub struct Captures<'r, 't> {
    re: &'r Regex,
    text: &'t str,
    spans: GroupSpans,
}

impl<'r, 't> Captures<'r, 't> {
    pub(crate) fn new(re: &'r Regex, text: &'t str, spans: GroupSpans) -> Self {
        Self { re, text, spans }
    }

    /// How many groups this match reports, counting the whole match. Always at
    /// least 1.
    //
    // No `is_empty`: the count includes the whole match, so it is never zero and
    // a method that can only answer `false` would be noise.
    #[must_use]
    #[allow(clippy::len_without_is_empty)]
    pub fn len(&self) -> usize {
        self.spans.len()
    }

    /// Group `index`, or `None` when there is no such group or the match did not
    /// enter it.
    #[must_use]
    pub fn get(&self, index: usize) -> Option<Match<'t>> {
        let (start, end) = (*self.spans.get(index)?)?;
        Some(Match::new(self.text, start, end))
    }

    /// The group named `name`, or `None` when the pattern declares no such name
    /// or the match did not enter it.
    #[must_use]
    pub fn name(&self, name: &str) -> Option<Match<'t>> {
        self.get(self.re.group_index(name)?)
    }

    /// Every group in order, starting with the whole match.
    pub fn iter(&self) -> impl ExactSizeIterator<Item = Option<Match<'t>>> {
        let text = self.text;
        self.spans
            .iter()
            .map(move |span| span.map(|(start, end)| Match::new(text, start, end)))
    }

    /// Append `template` to `dst`, with `$1` / `${name}` references filled in.
    ///
    /// The syntax is the `regex` crate's, not `re`'s: `$name` takes the longest
    /// run of `[0-9A-Za-z_]`, `${name}` delimits it explicitly, and `$$` is a
    /// literal `$`. A reference to a group that does not exist or that the match
    /// did not enter expands to nothing.
    pub fn expand(&self, template: &str, dst: &mut String) {
        crate::replace::expand(self, template, dst);
    }
}

/// Panics when there is no such group, or when the match did not enter it. Use
/// [`Captures::get`] when absence is a possibility you want to handle.
impl<'t> Index<usize> for Captures<'_, 't> {
    type Output = str;

    fn index(&self, index: usize) -> &str {
        match self.get(index) {
            Some(found) => found.as_str(),
            // The two reasons are different bugs in the caller, so the message
            // says which one happened rather than making them guess.
            None if index < self.spans.len() => {
                panic!("group {index} did not participate in this match; use get({index})")
            },
            None => panic!(
                "no group at index {index}: `{}` declares {}",
                self.re,
                self.spans.len() - 1
            ),
        }
    }
}

/// Panics when the pattern declares no such name, or when the match did not
/// enter that group. Use [`Captures::name`] when absence is a possibility.
impl<'t> Index<&str> for Captures<'_, 't> {
    type Output = str;

    fn index(&self, name: &str) -> &str {
        if let Some(found) = self.name(name) {
            return found.as_str();
        }
        // The two reasons are different bugs in the caller, so the message says
        // which one happened rather than making them guess.
        assert!(
            self.re.group_index(name).is_none(),
            "group `{name}` did not participate in this match; use name(\"{name}\")"
        );
        panic!(
            "no group named `{name}`: `{}` declares none by that name",
            self.re
        )
    }
}

/// The capture groups of every match in one text.
///
/// The spans come from one `find_all`; each match's group detail is filled as
/// the iterator reaches it, so walking this purely to count costs no capture
/// passes beyond the ones you consume.
#[derive(Clone, Debug)]
pub struct CaptureMatches<'r, 't> {
    re: &'r Regex,
    text: &'t str,
    spans: std::vec::IntoIter<(usize, usize)>,
}

impl<'r, 't> CaptureMatches<'r, 't> {
    pub(crate) fn new(re: &'r Regex, text: &'t str, spans: Vec<(usize, usize)>) -> Self {
        Self {
            re,
            text,
            spans: spans.into_iter(),
        }
    }
}

impl<'r, 't> Iterator for CaptureMatches<'r, 't> {
    type Item = Captures<'r, 't>;

    /// # Panics
    ///
    /// If the engine faults while reading one match's groups. There is nowhere
    /// for an error to go from inside `Iterator::next`, and the fault this could
    /// report - a capture arm the engine refuses - is already reported by
    /// [`Regex::try_captures_iter`] before iteration starts.
    fn next(&mut self) -> Option<Self::Item> {
        let (start, end) = self.spans.next()?;
        let spans = expect(self.re.captures_at(self.text, start, end));
        Some(Captures::new(self.re, self.text, spans))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.spans.size_hint()
    }
}

impl ExactSizeIterator for CaptureMatches<'_, '_> {}
impl FusedIterator for CaptureMatches<'_, '_> {}

/// The pieces of a text between its matches.
#[derive(Clone, Debug)]
pub struct Split<'t> {
    text: &'t str,
    matches: Matches<'t>,
    cut: usize,
    /// How many pieces may still be produced. `usize::MAX` for an unlimited
    /// split; 0 means this iterator is finished.
    left: usize,
}

impl<'t> Split<'t> {
    pub(crate) fn new(text: &'t str, matches: Matches<'t>, limit: usize) -> Self {
        Self {
            text,
            matches,
            cut: 0,
            left: limit,
        }
    }
}

impl<'t> Iterator for Split<'t> {
    type Item = &'t str;

    fn next(&mut self) -> Option<Self::Item> {
        if self.left == 0 {
            return None;
        }
        self.left -= 1;
        // The last piece a limit allows is the whole remainder, matches and all;
        // so is the piece after the final match.
        if self.left == 0 {
            return Some(&self.text[self.cut..]);
        }
        match self.matches.next() {
            Some(found) => {
                let piece = &self.text[self.cut..found.start()];
                self.cut = found.end();
                Some(piece)
            },
            None => {
                self.left = 0;
                Some(&self.text[self.cut..])
            },
        }
    }
}

impl FusedIterator for Split<'_> {}

#[cfg(test)]
mod thinning {
    //! The thinning rule on its own, without the engine.
    //!
    //! Input is the widest sequence - every empty match at every byte - which is
    //! what the C ABI reports and what Python's `re` shows. Expected output comes
    //! from the `regex` crate at test time. So this pins the transformation
    //! itself: given what the engine says, does this crate show what Rust shows?
    //! It is the same claim `tests/sequence.rs` makes end to end, minus the FFI,
    //! which means it still fails loudly if the rule regresses while the vendored
    //! archive is stale or a target has no engine at all.
    use super::crate_sequence;

    /// Every empty match, at every byte offset, merged with the real matches -
    /// the sequence the engine hands the bindings.
    fn widest(pattern: &str, text: &str) -> Vec<(usize, usize)> {
        let re = regex::Regex::new(pattern).unwrap();
        (0..=text.len())
            .filter_map(|at| {
                let found = re.find_at(text, at)?;
                (found.start() == at).then_some((found.start(), found.end()))
            })
            .collect()
    }

    fn thinned(pattern: &str, text: &str) -> Vec<(usize, usize)> {
        crate_sequence(widest(pattern, text), text)
    }

    fn crate_says(pattern: &str, text: &str) -> Vec<(usize, usize)> {
        regex::Regex::new(pattern)
            .unwrap()
            .find_iter(text)
            .map(|found| (found.start(), found.end()))
            .collect()
    }

    #[test]
    fn thinning_the_widest_sequence_reproduces_the_regex_crate() {
        for pattern in [
            "a*", "b*", "x*", "", "a?", "l*", "a*b*", "(?:ab)*", "a{0,2}",
        ] {
            for text in [
                "", "a", "abc", "abcb", "aaa", "bbb", "aXaXa", "bab", "héllo",
            ] {
                assert_eq!(
                    thinned(pattern, text),
                    crate_says(pattern, text),
                    "pattern {pattern:?} over text {text:?}"
                );
            }
        }
    }

    /// Thinning only ever removes, never invents or reorders. That is what makes
    /// it sound to apply after the search instead of during it.
    #[test]
    fn thinning_is_a_subsequence_of_what_it_was_given() {
        for pattern in ["a*", "", "l*", "a?"] {
            for text in ["", "abc", "héllo", "bab"] {
                let (before, after) = (widest(pattern, text), thinned(pattern, text));
                let mut left = before.iter();
                assert!(
                    after.iter().all(|span| left.any(|had| had == span)),
                    "{pattern:?} over {text:?}: {after:?} is not a subsequence of {before:?}"
                );
            }
        }
    }

    /// A sequence with no empty match is returned untouched - the common case,
    /// and the one the early return is for.
    #[test]
    fn a_sequence_without_empty_matches_is_unchanged() {
        let spans = vec![(0, 1), (1, 2), (4, 9)];
        assert_eq!(crate_sequence(spans.clone(), "abcdefghij"), spans);
    }
}