use super::{Cursor, OtherHighlights, Parse, ParseError, Result};
use crate::regex::{GreedyBehavior, Pattern, LookDirection, LookPolarity};

const SYMBOLS: &[u8] = b"{}[]()*+-?|.\\ ";
fn is_symbol(x: u8) -> bool {
    SYMBOLS.contains(&x)
}

impl Parse for Pattern {
    fn parse(b: &mut Cursor<'_>) -> super::Result<Self> {
        let begin = b.loc();
        let result = parse0(b);
        if result.is_ok() {
            b.highlight_from(begin, OtherHighlights::Regex);
        }
        result
    }
}

fn parse0(s: &mut Cursor<'_>) -> Result<Pattern> {
    parse_alt(s)
}

fn parse_alt(s: &mut Cursor<'_>) -> Result<Pattern> {
    let mut seqs = vec![];
    loop {
        let seq = parse_seq(s)?;
        seqs.push(seq);
        let begin = s.loc();
        if s.has() && s.peek() == b'|' {
            s.adv();
            s.highlight_from(begin, OtherHighlights::RegexSymbol);
        } else {
            break;
        }
    }

    Ok(match seqs.len() {
        0 => Pattern::Nothing,
        1 => seqs.into_iter().next().unwrap(),
        _ => Pattern::Alt(seqs),
    })
}

fn parse_seq(s: &mut Cursor<'_>) -> Result<Pattern> {
    let mut reps = vec![];
    loop {
        let rep = parse_rep(s)?;
        if rep != Pattern::Nothing {
            reps.push(rep);
        } else {
            break;
        }
    }

    Ok(match reps.len() {
        0 => Pattern::Nothing,
        1 => reps.into_iter().next().unwrap(),
        _ => Pattern::Concat(reps),
    })
}

fn parse_rep(s: &mut Cursor<'_>) -> Result<Pattern> {
    let atom = parse_atom(s)?;

    if atom == Pattern::Nothing {
        return Ok(atom);
    }

    if !s.has() {
        return Ok(atom);
    }

    let begin = s.loc();

    let rep = match s.peek() {
        b'*' => Some((0, None)),
        b'+' => Some((1, None)),
        b'?' => Some((0, Some(1))),
        _ => None,
    };

    if let Some((min_rep, max_rep)) = rep {
        s.adv();
        s.highlight_from(begin, OtherHighlights::RegexSymbol);

        let greed = if s.has() && s.peek() == b'?' {
            s.adv();
            GreedyBehavior::NonGreedy
        } else {
            GreedyBehavior::Greedy
        };

        Ok(Pattern::Rep(Box::new(atom), min_rep, max_rep, greed))
    } else {
        Ok(atom)
    }
}

fn parse_atom(s: &mut super::Cursor<'_>) -> Result<Pattern> {
    if !s.has() {
        return Ok(Pattern::Nothing);
    }

    let begin = s.loc();

    match s.peek() {
        b'[' => {
            s.adv();
            s.highlight_from(begin, OtherHighlights::RegexSymbol);
            let mut ranges = Vec::new();
            loop {
                if !s.has() {
                    return Err(ParseError::Eof);
                }

                let begin = s.loc();
                let tok = s.adv();

                if tok == b']' {
                    if ranges.is_empty() {
                        todo!("error handling for empty alternative list");
                    }
                    s.highlight_from(begin, OtherHighlights::RegexSymbol);
                    return Ok(Pattern::Alt(ranges));
                }

                let begin = s.loc();
                if s.has() && s.peek() == b'-' {
                    s.adv();
                    s.highlight_from(begin, OtherHighlights::RegexSymbol);

                    if !s.has() {
                        return Err(ParseError::Eof);
                    }
                    let tok2 = s.peek();

                    if tok2 == b']' {
                        ranges.push(Pattern::Byte(tok));
                        ranges.push(Pattern::Byte(b'-'));
                    } else if is_symbol(tok2) {
                        return Err(ParseError::Unknown(tok2));
                    } else {
                        s.adv();
                        ranges.push(Pattern::Range(tok, tok2));
                    }
                } else {
                    ranges.push(Pattern::Byte(tok));
                }
            }
        }
        b'(' => {
            s.adv();

            let mut assertion = None;
            if s.buf.starts_with(b"?=") {
                s.advance(2);
                assertion = Some((LookDirection::Ahead, LookPolarity::Positive));
            } else if s.buf.starts_with(b"?!") {
                s.advance(2);
                assertion = Some((LookDirection::Ahead, LookPolarity::Negative));
            } else if s.buf.starts_with(b"?<=") {
                s.advance(3);
                assertion = Some((LookDirection::Behind, LookPolarity::Positive));
            } else if s.buf.starts_with(b"?<!") {
                s.advance(3);
                assertion = Some((LookDirection::Behind, LookPolarity::Negative));
            }

            s.highlight_from(begin, OtherHighlights::RegexSymbol);
            let inner = parse0(s)?;
            if !s.has() {
                return Err(ParseError::Eof);
            }
            let begin = s.loc();
            if s.adv() != b')' {
                return Err(ParseError::Expected(')'));
            }
            s.highlight_from(begin, OtherHighlights::RegexSymbol);

            if let Some((dir, pol)) = assertion {
                Ok(Pattern::Assertion(dir, pol, Box::new(inner)))
            } else {
                Ok(Pattern::Submatch(Box::new(inner)))
            }
        }
        b'.' => {
            s.adv();
            s.highlight_from(begin, OtherHighlights::RegexSymbol);
            Ok(Pattern::Range(0, 127))
        }
        b'\\' => {
            s.adv();
            if s.has() {
                let escaped = s.adv();
                s.highlight_from(begin, OtherHighlights::RegexSymbol);

                if is_symbol(escaped) {
                    Ok(Pattern::Byte(escaped))
                } else {
                    // TODO interpret \w and others
                    Err(ParseError::Unknown(escaped))
                }
            } else {
                Err(ParseError::Eof)
            }
        }
        x if is_symbol(x) => Ok(Pattern::Nothing),
        ch => {
            s.adv();
            Ok(Pattern::Byte(ch))
        }
    }
}