path: root/grep/src/literals.rs

/*!
The literals module is responsible for extracting *inner* literals out of the
AST of a regular expression. Normally this is the job of the regex engine
itself, but the regex engine doesn't look for inner literals. Since we're doing
line based searching, we can use them, so we need to do it ourselves.

Note that this implementation is incredibly suspicious. We need something more
principled.
*/
use std::cmp;

use regex::bytes::RegexBuilder;
use syntax::hir::{self, Hir, HirKind};
use syntax::hir::literal::{Literal, Literals};

#[derive(Clone, Debug)]
pub struct LiteralSets {
    prefixes: Literals,
    suffixes: Literals,
    required: Literals,
}

impl LiteralSets {
    pub fn create(expr: &Hir) -> Self {
        let mut required = Literals::empty();
        union_required(expr, &mut required);
        LiteralSets {
            prefixes: Literals::prefixes(expr),
            suffixes: Literals::suffixes(expr),
            required: required,
        }
    }

    pub fn to_regex_builder(&self) -> Option<RegexBuilder> {
        if self.prefixes.all_complete() && !self.prefixes.is_empty() {
            debug!("literal prefixes detected: {:?}", self.prefixes);
            // When this is true, the regex engine will do a literal scan.
            return None;
        }

        // Out of inner required literals, prefixes and suffixes, which one
        // is the longest? We pick the longest to do fast literal scan under
        // the assumption that a longer literal will have a lower false
        // positive rate.
        let pre_lcp = self.prefixes.longest_common_prefix();
        let pre_lcs = self.prefixes.longest_common_suffix();
        let suf_lcp = self.suffixes.longest_common_prefix();
        let suf_lcs = self.suffixes.longest_common_suffix();

        let req_lits = self.required.literals();
        let req = match req_lits.iter().max_by_key(|lit| lit.len()) {
            None => &[],
            Some(req) => &***req,
        };

        let mut lit = pre_lcp;
        if pre_lcs.len() > lit.len() {
            lit = pre_lcs;
        }
        if suf_lcp.len() > lit.len() {
            lit = suf_lcp;
        }
        if suf_lcs.len() > lit.len() {
            lit = suf_lcs;
        }
        if req_lits.len() == 1 && req.len() > lit.len() {
            lit = req;
        }

        // Special case: if we detected an alternation of inner required
        // literals and its longest literal is bigger than the longest
        // prefix/suffix, then choose the alternation. In practice, this
        // helps with case insensitive matching, which can generate lots of
        // inner required literals.
        let any_empty = req_lits.iter().any(|lit| lit.is_empty());
        if req.len() > lit.len() && req_lits.len() > 1 && !any_empty {
            debug!("required literals found: {:?}", req_lits);
            let alts: Vec<String> =
                req_lits.into_iter().map(|x| bytes_to_regex(x)).collect();
            let mut builder = RegexBuilder::new(&alts.join("|"));
            builder.unicode(false);
            Some(builder)
        } else if lit.is_empty() {
            None
        } else {
            debug!("required literal found: {:?}", show(lit));
            let mut builder = RegexBuilder::new(&bytes_to_regex(&lit));
            builder.unicode(false);
            Some(builder)
        }
    }
}

fn union_required(expr: &Hir, lits: &mut Literals) {
    match *expr.kind() {
        HirKind::Literal(hir::Literal::Unicode(c)) => {
            let mut buf = [0u8; 4];
            lits.cross_add(c.encode_utf8(&mut buf).as_bytes());
        }
        HirKind::Literal(hir::Literal::Byte(b)) => {
            lits.cross_add(&[b]);
        }
        HirKind::Class(hir::Class::Unicode(ref cls)) => {
            if count_unicode_class(cls) >= 5 || !lits.add_char_class(cls) {
                lits.cut();
            }
        }
        HirKind::Class(hir::Class::Bytes(ref cls)) => {
            if count_byte_class(cls) >= 5 || !lits.add_byte_class(cls) {
                lits.cut();
            }
        }
        HirKind::Group(hir::Group { ref hir, .. }) => {
            union_required(&**hir, lits);
        }
        HirKind::Repetition(ref x) => {
            match x.kind {
                hir::RepetitionKind::ZeroOrOne => lits.cut(),
                hir::RepetitionKind::ZeroOrMore => lits.cut(),
                hir::RepetitionKind::OneOrMore => {
                    union_required(&x.hir, lits);
                    lits.cut();
                }
                hir::RepetitionKind::Range(ref rng) => {
                    let (min, max) = match *rng {
                        hir::RepetitionRange::Exactly(m) => (m, Some(m)),
                        hir::RepetitionRange::AtLeast(m) => (m, None),
                        hir::RepetitionRange::Bounded(m, n) => (m, Some(n)),
                    };
                    repeat_range_literals(
                        &x.hir, min, max, x.greedy, lits, union_required);
                }
            }
        }
        HirKind::Concat(ref es) if es.is_empty() => {}
        HirKind::Concat(ref es) if es.len() == 1 => {
            union_required(&es[0], lits)
        }
        HirKind::Concat(ref es) => {
            for e in es {
                let mut lits2 = lits.to_empty();
                union_required(e, &mut lits2);
                if lits2.is_empty() {
                    lits.cut();
                    continue;
                }
                if lits2.contains_empty() {
                    lits.cut();
                }
                if !lits.cross_product(&lits2) {
                    // If this expression couldn't yield any literal that
                    // could be extended, then we need to quit. Since we're
                    // short-circuiting, we also need to freeze every member.
                    lits.cut();
                    break;
                }
            }
        }
        HirKind::Alternation(ref es) => {
            alternate_literals(es, lits, union_required);
        }
        _ => lits.cut(),
    }
}

fn repeat_range_literals<F: FnMut(&Hir, &mut Literals)>(
    e: &Hir,
    min: u32,
    max: Option<u32>,
    _greedy: bool,
    lits: &mut Literals,
    mut f: F,
) {
    if min == 0 {
        // This is a bit conservative. If `max` is set, then we could
        // treat this as a finite set of alternations. For now, we
        // just treat it as `e*`.
        lits.cut();
    } else {
        let n = cmp::min(lits.limit_size(), min as usize);
        // We only extract literals from a single repetition, even though
        // we could do more. e.g., `a{3}` will have `a` extracted instead of
        // `aaa`. The reason is that inner literal extraction can't be unioned
        // across repetitions. e.g., extracting `foofoofoo` from `(\w+foo){3}`
        // is wrong.
        f(e, lits);
        if n < min as usize {
            lits.cut();
        }
        if max.map_or(true, |max| min < max) {
            lits.cut();
        }
    }
}

fn alternate_literals<F: FnMut(&Hir, &mut Literals)>(
    es: &[Hir],
    lits: &mut Literals,
    mut f: F,
) {
    let mut lits2 = lits.to_empty();
    for e in es {
        let mut lits3 = lits.to_empty();
        lits3.set_limit_size(lits.limit_size() / 5);
        f(e, &mut lits3);
        if lits3.is_empty() || !lits2.union(lits3) {
            // If we couldn't find suffixes for *any* of the
            // alternates, then the entire alternation has to be thrown
            // away and any existing members must be frozen. Similarly,
            // if the union couldn't complete, stop and freeze.
            lits.cut();
            return;
        }
    }
    // All we do at the moment is look for prefixes and suffixes. If both
    // are empty, then we report nothing. We should be able to do better than
    // this, but we'll need something more expressive than just a "set of
    // literals."
    let lcp = lits2.longest_common_prefix();
    let lcs = lits2.longest_common_suffix();
    if !lcp.is_empty() {
        lits.cross_add(lcp);
    }
    lits.cut();
    if !lcs.is_empty() {
        lits.add(Literal::empty());
        lits.add(Literal::new(lcs.to_vec()));
    }
}

/// Return the number of characters in the given class.
fn count_unicode_class(cls: &hir::ClassUnicode) -> u32 {