use std::{fmt, str::CharIndices};

use super::error::LispError;

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum Token<'a> {
    LeftParen,
    RightParen,
    Float(&'a str),
    Integer(&'a str),
    // Char(&'a str),
    String(&'a str),
    Name(&'a str),
    // Keyword(&'a str),
    BackQuote,
    Comma,
    CommaAt,
    Quote,
    End,
}

impl<'a> fmt::Display for Token<'a> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Token::LeftParen => write!(f, "("),
            Token::RightParen => write!(f, ")"),
            Token::Float(_) => write!(f, "float"),
            Token::Integer(_) => write!(f, "integer"),
            // Token::Char(_) => write!(f, "char"),
            Token::String(_) => write!(f, "string"),
            Token::Name(_) => write!(f, "name"),
            // Token::Keyword(_) => write!(f, "keyword"),
            Token::BackQuote => write!(f, "`"),
            Token::Comma => write!(f, ","),
            Token::CommaAt => write!(f, ",@"),
            Token::Quote => write!(f, "'"),
            Token::End => write!(f, "EOF"),
        }
    }
}

#[derive(Debug, Copy, Clone, PartialEq)]
pub struct Span {
    low: u32,
    high: u32,
}

impl Span {
    pub fn empty(pos: u32) -> Self {
        Self {
            low: pos,
            high: pos,
        }
    }
}

pub struct Lexer<'input> {
    input: &'input str,
    cur_pos: u32,
    offset: u32,
}

impl<'a> Lexer<'a> {
    pub fn new(input: &'a str, offset: u32) -> Self {
        Self {
            input,
            cur_pos: 0,
            offset,
        }
    }
    pub fn next_token(&mut self) -> Result<(Span, Token<'a>), LispError> {
        let mut chars = self.input.char_indices();

        while let Some((ind, chr)) = chars.next() {
            let low = self.cur_pos;
            let res = match chr {
                '(' => Ok((1, Token::LeftParen)),
                ')' => Ok((1, Token::RightParen)),
                '\'' => Ok((1, Token::Quote)),
                '`' => Ok((1, Token::BackQuote)),
                ',' => match chars.next() {
                    Some((_, '@')) => Ok((2, Token::CommaAt)),
                    _ => Ok((1, Token::Comma)),
                },
                '#' => parse_name(&self.input[ind..]),
                '-' | '0'..='9' => parse_number(&self.input[ind..]),
                '"' => parse_string(&self.input[ind..]),
                ';' => {
                    self.cur_pos += consume_comment(ind, &mut chars) as u32;
                    continue;
                }
                _ if is_ident(chr) => parse_name(&self.input[ind..]),
                ch if ch.is_whitespace() => {
                    self.cur_pos += ch.len_utf8() as u32;
                    continue;
                }
                _ => Err(LispError::ParseError),
            };
            let (size, token) = match res {
                Ok(v) => v,
                Err(_) => return Err(LispError::ParseError),
            };
            self.cur_pos += size as u32;
            self.input = &self.input[ind + size..];
            let sp = Span {
                low,
                high: low + size as u32,
            };
            return Ok((sp, token));
        }
        self.input = &self.input[..0];
        return Ok((Span::empty(self.cur_pos), Token::End));
    }
}

fn parse_number<'a>(mut input: &'a str) -> Result<(usize, Token<'a>), LispError> {
    let mut dot = false;
    let mut minus = false;
    let mut size = 0;
    let mut chars = input.chars();

    if let Some(v) = chars.next() {
        if v == '-' {
            minus = true;
            size += 1;
            input = &input[1..];
        } else if v.is_digit(10) {
            size += 1;
        }
    }

    while let Some(chr) = chars.next() {
        if chr.is_digit(10) {
            size += 1;
        } else if chr == '.' {
            if !dot {
                dot = true;
                size += 1;
            } else {
                return Err(LispError::ParseError);
            }
        } else if !is_ident(chr) {
            break;
        } else {
            return Err(LispError::ParseError);
        }
    }

    let tok = if size == 1 && minus {
        Token::Name("-")
    } else if dot {
        Token::Float(&input[..size])
    } else {
        Token::Integer(&input[..size])
    };
    return Ok((size, tok));
}

fn parse_string<'a>(input: &'a str) -> Result<(usize, Token<'a>), LispError> {
    // count opening quote
    let mut size = 1;
    let mut chars = input.char_indices().skip(1);
    while let Some((ind, chr)) = chars.next() {
        match chr {
            '\\' => {
                let _ = chars.next();
            }
            '"' => {
                size += ind;
                break;
            }
            _ => (),
        }
    }
    return Ok((size, Token::String(&input[..size])));
}

fn is_ident(ch: char) -> bool {
    match ch {
        '!' | '$' | '%' | '&' | '*' | '+' | '-' | '.' | '/' | '<' | '=' | '>' | '?' | '^' | '_'
        | '|' | '#' => true,
        _ if ch.is_alphanumeric() => true,
        _ => false,
    }
}

fn consume_comment(start: usize, chars: &mut CharIndices) -> usize {
    let mut last = start;

    for (ind, ch) in chars {
        last = ind;
        if ch == '\n' {
            break;
        }
    }

    last - start + 1
}

fn parse_name<'a>(input: &'a str) -> Result<(usize, Token<'a>), LispError> {
    for (ind, chr) in input.char_indices() {
        if !is_ident(chr) {
            return Ok((ind, Token::Name(&input[..ind])));
        }
    }
    return Ok((input.len(), Token::Name(input)));
}

#[cfg(test)]
mod tests {
    use super::*;

    fn sp(low: u32, high: u32) -> Span {
        Span { low, high }
    }

    fn tokens(input: &str) -> Vec<(Span, Token)> {
        let mut lexer = Lexer::new(input, 0);
        let mut tokens = Vec::new();
        loop {
            match lexer.next_token().unwrap() {
                (_, Token::End) => break,
                t => tokens.push(t),
            }
        }
        tokens
    }

    #[test]
    fn string_parsing() {
        let input = r#""hello there""#;
        let parsed = parse_string(input).unwrap();
        assert_eq!(parsed.0, 13);
        assert_eq!(parsed.1, Token::String(r#""hello there""#));
    }

    #[test]
    fn integer_parsing() {
        let input = "12345";
        let parsed = parse_number(input).unwrap();
        assert_eq!(parsed.0, 5);
        assert_eq!(parsed.1, Token::Integer("12345"));
    }

    #[test]
    fn float_parsing() {
        let input = "12.345";
        let parsed = parse_number(input).unwrap();
        assert_eq!(parsed.0, 6);
        assert_eq!(parsed.1, Token::Float("12.345"));
    }

    #[test]
    fn lexer() {
        assert_eq!(
            tokens("1 2 3"),
            [
                (sp(0, 1), Token::Integer("1")),
                (sp(2, 3), Token::Integer("2")),
                (sp(4, 5), Token::Integer("3"))
            ]
        );

        assert_eq!(
            tokens("1 foo 3"),
            [
                (sp(0, 1), Token::Integer("1")),
                (sp(2, 5), Token::Name("foo")),
                (sp(6, 7), Token::Integer("3"))
            ]
        );

        assert_eq!(tokens("foo"), [(sp(0, 3), Token::Name("foo")),]);
        assert_eq!(tokens("#t"), [(sp(0, 2), Token::Name("#t")),]);

        assert_eq!(
            tokens("1 \"foo\" 3"),
            [
                (sp(0, 1), Token::Integer("1")),
                (sp(2, 7), Token::String(r#""foo""#)),
                (sp(8, 9), Token::Integer("3"))
            ]
        );

        assert_eq!(
            tokens("(* 1 (+ 2 3))"),
            [
                (sp(0, 1), Token::LeftParen),
                (sp(1, 2), Token::Name("*")),
                (sp(3, 4), Token::Integer("1")),
                (sp(5, 6), Token::LeftParen),
                (sp(6, 7), Token::Name("+")),
                (sp(8, 9), Token::Integer("2")),
                (sp(10, 11), Token::Integer("3")),
                (sp(11, 12), Token::RightParen),
                (sp(12, 13), Token::RightParen),
            ]
        );

        assert_eq!(tokens("; foo"), []);
        assert_eq!(tokens("1; foo"), [(sp(0, 1), Token::Integer("1"))]);
        assert_eq!(tokens(">="), [(sp(0, 2), Token::Name(">="))]);
        assert_eq!(tokens("&&"), [(sp(0, 2), Token::Name("&&"))]);
    }
}