pub struct Lexer<'a> {
    pub buf: &'a str,
}

impl<'a> Lexer<'a> {
    fn skip_whitespace(&mut self) {
        while let Some(b' ') = self.buf.as_bytes().first() {
            self.buf = &self.buf[1..];
        }
    }

    fn parse_num(&mut self) -> &'a str {
        let mut i = 0;
        while matches!(self.buf.as_bytes().get(i), Some(&b) if b.is_ascii_digit()) {
            i += 1;
        }
        if let Some(b'.') = self.buf.as_bytes().get(i) {
            i += 1;
        }
        while matches!(self.buf.as_bytes().get(i), Some(&b) if b.is_ascii_digit()) {
            i += 1;
        }
        let (Some(b'e') | Some(b'E')) = self.buf.as_bytes().get(i) else {
            let t = &self.buf[..i];
            self.buf = &self.buf[i..];
            return t;
        };
        i += 1;
        if let Some(b'-') = self.buf.as_bytes().get(i) {
            i += 1;
        }
        while matches!(self.buf.as_bytes().get(i), Some(&b) if b.is_ascii_digit()) {
            i += 1;
        }
        let t = &self.buf[..i];
        self.buf = &self.buf[i..];
        t
    }
}

impl<'a> Iterator for Lexer<'a> {
    type Item = &'a str;

    fn next(&mut self) -> Option<&'a str> {
        self.skip_whitespace();
        if self.buf.starts_with("//") {
            self.buf = &self.buf[2..];
            return Some("//");
        }
        Some(match self.buf.as_bytes().first()? {
            b'0'..=b'9' | b'.' => self.parse_num(),
            b'a'..=b'z' | b'A'..=b'Z' => {
                let mut i = 1;
                while let Some(b'a'..=b'z' | b'A'..=b'Z' | b'0'..=b'9') = self.buf.as_bytes().get(i)
                {
                    i += 1;
                }
                let t = &self.buf[..i];
                self.buf = &self.buf[i..];
                t
            }
            _ => {
                let mut i = 1;
                while !self.buf.is_char_boundary(i) {
                    i += 1;
                }
                let t = &self.buf[..i];
                self.buf = &self.buf[i..];
                t
            }
        })
    }
}