use alloc::{vec, vec::Vec};

use regex_syntax::hir::Hir;

use crate::{meta::regex::RegexInfo, util::search::MatchKind};

/// Pull out an alternation of literals from the given sequence of HIR

/// expressions.

///

/// There are numerous ways for this to fail. Generally, this only applies

/// to regexes of the form 'foo|bar|baz|...|quux'. It can also fail if there

/// are "too few" alternates, in which case, the regex engine is likely faster.

///

/// And currently, this only returns something when 'hirs.len() == 1'.

pub(crate) fn alternation_literals(

    info: &RegexInfo,

    hirs: &[&Hir],

) -> Option<Vec<Vec<u8>>> {

    use regex_syntax::hir::{HirKind, Literal};

    // Might as well skip the work below if we know we can't build an

    // Aho-Corasick searcher.

    if !cfg!(feature = "perf-literal-multisubstring") {

        return None;

    }

    // This is pretty hacky, but basically, if `is_alternation_literal` is

    // true, then we can make several assumptions about the structure of our

    // HIR. This is what justifies the `unreachable!` statements below.

    if hirs.len() != 1

        || !info.props()[0].look_set().is_empty()

        || info.props()[0].explicit_captures_len() > 0

        || !info.props()[0].is_alternation_literal()

        || info.config().get_match_kind() != MatchKind::LeftmostFirst

    {

        return None;

    }

    let hir = &hirs[0];

    let alts = match *hir.kind() {

        HirKind::Alternation(ref alts) => alts,

        _ => return None, // one literal isn't worth it

    };

    let mut lits = vec![];

    for alt in alts {

        let mut lit = vec![];

        match *alt.kind() {

            HirKind::Literal(Literal(ref bytes)) => {

                lit.extend_from_slice(bytes)

            }

            HirKind::Concat(ref exprs) => {

                for e in exprs {

                    match *e.kind() {

                        HirKind::Literal(Literal(ref bytes)) => {

                            lit.extend_from_slice(bytes);

                        }

                        _ => unreachable!("expected literal, got {e:?}"),

                    }

                }

            }

            _ => unreachable!("expected literal or concat, got {alt:?}"),

        }

        lits.push(lit);

    }

    // Why do this? Well, when the number of literals is small, it's likely

    // that we'll use the lazy DFA which is in turn likely to be faster than

    // Aho-Corasick in such cases. Primarily because Aho-Corasick doesn't have

    // a "lazy DFA" but either a contiguous NFA or a full DFA. We rarely use

    // the latter because it is so hungry (in time and space), and the former

    // is decently fast, but not as fast as a well oiled lazy DFA.

    //

    // However, once the number starts getting large, the lazy DFA is likely

    // to start thrashing because of the modest default cache size. When

    // exactly does this happen? Dunno. But at whatever point that is (we make

    // a guess below based on ad hoc benchmarking), we'll want to cut over to

    // Aho-Corasick, where even the contiguous NFA is likely to do much better.

    if lits.len() < 3000 {

        debug!("skipping Aho-Corasick because there are too few literals");

        return None;

    }

    Some(lits)

}