use core::slice;
use rustc_lexer::{self as lex, LiteralKind, Token, TokenKind};

/// A token pattern used for searching and matching by the [`Cursor`].
///
/// In the event that a pattern is a multi-token sequence, earlier tokens will be consumed
/// even if the pattern ultimately isn't matched. e.g. With the sequence `:*` matching
/// `DoubleColon` will consume the first `:` and then fail to match, leaving the cursor at
/// the `*`.
#[derive(Clone, Copy)]
pub enum Pat<'a> {
    /// Matches any number of comments and doc comments.
    AnyComment,
    Ident(&'a str),
    CaptureIdent,
    LitStr,
    CaptureLitStr,
    Bang,
    CloseBrace,
    CloseBracket,
    CloseParen,
    Comma,
    DoubleColon,
    Eq,
    Lifetime,
    Lt,
    Gt,
    OpenBrace,
    OpenBracket,
    OpenParen,
    Pound,
    Semi,
}

#[derive(Clone, Copy)]
pub struct Capture {
    pub pos: u32,
    pub len: u32,
}
impl Capture {
    pub const EMPTY: Self = Self { pos: 0, len: 0 };
}

/// A unidirectional cursor over a token stream that is lexed on demand.
pub struct Cursor<'txt> {
    next_token: Token,
    pos: u32,
    inner: lex::Cursor<'txt>,
    text: &'txt str,
}
impl<'txt> Cursor<'txt> {
    #[must_use]
    pub fn new(text: &'txt str) -> Self {
        let mut inner = lex::Cursor::new(text, lex::FrontmatterAllowed::Yes);
        Self {
            next_token: inner.advance_token(),
            pos: 0,
            inner,
            text,
        }
    }

    /// Gets the text of the captured token assuming it came from this cursor.
    #[must_use]
    pub fn get_text(&self, capture: Capture) -> &'txt str {
        &self.text[capture.pos as usize..(capture.pos + capture.len) as usize]
    }

    /// Gets the text that makes up the next token in the stream, or the empty string if
    /// stream is exhausted.
    #[must_use]
    pub fn peek_text(&self) -> &'txt str {
        &self.text[self.pos as usize..(self.pos + self.next_token.len) as usize]
    }

    /// Gets the length of the next token in bytes, or zero if the stream is exhausted.
    #[must_use]
    pub fn peek_len(&self) -> u32 {
        self.next_token.len
    }

    /// Gets the next token in the stream, or [`TokenKind::Eof`] if the stream is
    /// exhausted.
    #[must_use]
    pub fn peek(&self) -> TokenKind {
        self.next_token.kind
    }

    /// Gets the offset of the next token in the source string, or the string's length if
    /// the stream is exhausted.
    #[must_use]
    pub fn pos(&self) -> u32 {
        self.pos
    }

    /// Gets whether the cursor has exhausted its input.
    #[must_use]
    pub fn at_end(&self) -> bool {
        self.next_token.kind == TokenKind::Eof
    }

    /// Advances the cursor to the next token. If the stream is exhausted this will set
    /// the next token to [`TokenKind::Eof`].
    pub fn step(&mut self) {
        // `next_token.len` is zero for the eof marker.
        self.pos += self.next_token.len;
        self.next_token = self.inner.advance_token();
    }

    /// Consumes tokens until the given pattern is either fully matched of fails to match.
    /// Returns whether the pattern was fully matched.
    ///
    /// For each capture made by the pattern one item will be taken from the capture
    /// sequence with the result placed inside.
    fn match_impl(&mut self, pat: Pat<'_>, captures: &mut slice::IterMut<'_, Capture>) -> bool {
        loop {
            match (pat, self.next_token.kind) {
                #[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/6697
                (_, TokenKind::Whitespace)
                | (
                    Pat::AnyComment,
                    TokenKind::BlockComment { terminated: true, .. } | TokenKind::LineComment { .. },
                ) => self.step(),
                (Pat::AnyComment, _) => return true,
                (Pat::Bang, TokenKind::Bang)
                | (Pat::CloseBrace, TokenKind::CloseBrace)
                | (Pat::CloseBracket, TokenKind::CloseBracket)
                | (Pat::CloseParen, TokenKind::CloseParen)
                | (Pat::Comma, TokenKind::Comma)
                | (Pat::Eq, TokenKind::Eq)
                | (Pat::Lifetime, TokenKind::Lifetime { .. })
                | (Pat::Lt, TokenKind::Lt)
                | (Pat::Gt, TokenKind::Gt)
                | (Pat::OpenBrace, TokenKind::OpenBrace)
                | (Pat::OpenBracket, TokenKind::OpenBracket)
                | (Pat::OpenParen, TokenKind::OpenParen)
                | (Pat::Pound, TokenKind::Pound)
                | (Pat::Semi, TokenKind::Semi)
                | (
                    Pat::LitStr,
                    TokenKind::Literal {
                        kind: LiteralKind::Str { terminated: true } | LiteralKind::RawStr { .. },
                        ..
                    },
                ) => {
                    self.step();
                    return true;
                },
                (Pat::Ident(x), TokenKind::Ident) if x == self.peek_text() => {
                    self.step();
                    return true;
                },
                (Pat::DoubleColon, TokenKind::Colon) => {
                    self.step();
                    if !self.at_end() && matches!(self.next_token.kind, TokenKind::Colon) {
                        self.step();
                        return true;
                    }
                    return false;
                },
                #[rustfmt::skip]
                (
                    Pat::CaptureLitStr,
                    TokenKind::Literal {
                        kind:
                            LiteralKind::Str { terminated: true }
                            | LiteralKind::RawStr { n_hashes: Some(_) },
                        ..
                    },
                )
                | (Pat::CaptureIdent, TokenKind::Ident) => {
                    *captures.next().unwrap() = Capture { pos: self.pos, len: self.next_token.len };
                    self.step();
                    return true;
                },
                _ => return false,
            }
        }
    }

    /// Consumes all tokens until the specified identifier is found and returns its
    /// position. Returns `None` if the identifier could not be found.
    ///
    /// The cursor will be positioned immediately after the identifier, or at the end if
    /// it is not.
    pub fn find_ident(&mut self, ident: &str) -> Option<u32> {
        loop {
            match self.next_token.kind {
                TokenKind::Ident if self.peek_text() == ident => {
                    let pos = self.pos;
                    self.step();
                    return Some(pos);
                },
                TokenKind::Eof => return None,
                _ => self.step(),
            }
        }
    }

    /// Consumes all tokens until the next identifier is found and captures it. Returns
    /// `None` if no identifier could be found.
    ///
    /// The cursor will be positioned immediately after the identifier, or at the end if
    /// it is not.
    pub fn find_any_ident(&mut self) -> Option<Capture> {
        loop {
            match self.next_token.kind {
                TokenKind::Ident => {
                    let res = Capture {
                        pos: self.pos,
                        len: self.next_token.len,
                    };
                    self.step();
                    return Some(res);
                },
                TokenKind::Eof => return None,
                _ => self.step(),
            }
        }
    }

    /// Consume the returns the position of the next non-whitespace token if it's an
    /// identifier. Returns `None` otherwise.
    pub fn match_ident(&mut self, s: &str) -> Option<u32> {
        loop {
            match self.next_token.kind {
                TokenKind::Ident if s == self.peek_text() => {
                    let pos = self.pos;
                    self.step();
                    return Some(pos);
                },
                TokenKind::Whitespace => self.step(),
                _ => return None,
            }
        }
    }

    /// Continually attempt to match the pattern on subsequent tokens until a match is
    /// found. Returns whether the pattern was successfully matched.
    ///
    /// Not generally suitable for multi-token patterns or patterns that can match
    /// nothing.
    #[must_use]
    pub fn find_pat(&mut self, pat: Pat<'_>) -> bool {
        let mut capture = [].iter_mut();
        while !self.match_impl(pat, &mut capture) {
            self.step();
            if self.at_end() {
                return false;
            }
        }
        true
    }

    /// Attempts to match a sequence of patterns at the current position. Returns whether
    /// all patterns were successfully matched.
    ///
    /// Captures will be written to the given slice in the order they're matched. If a
    /// capture is matched, but there are no more capture slots this will panic. If the
    /// match is completed without filling all the capture slots they will be left
    /// unmodified.
    ///
    /// If the match fails the cursor will be positioned at the first failing token.
    #[must_use]
    pub fn match_all(&mut self, pats: &[Pat<'_>], captures: &mut [Capture]) -> bool {
        let mut captures = captures.iter_mut();
        pats.iter().all(|&p| self.match_impl(p, &mut captures))
    }

    /// Attempts to match a single pattern at the current position. Returns whether the
    /// pattern was successfully matched.
    ///
    /// If the pattern attempts to capture anything this will panic. If the match fails
    /// the cursor will be positioned at the first failing token.
    #[must_use]
    pub fn match_pat(&mut self, pat: Pat<'_>) -> bool {
        self.match_impl(pat, &mut [].iter_mut())
    }
}