#[derive(PartialEq, Eq, Clone, Debug)]
pub enum LTIRToken {
    Symbol(String),
    Char(char),
    Num(i64),

    // SingleQuote(String),
    // DoubleQuote(String),
    // TripleQuote(String),
    Lambda,
    MapsTo,
    AssignType(String),
    AssignValue,

    ExprOpen,
    ExprClose,

    BlockOpen,
    BlockClose,
    StatementSep,
}

#[derive(PartialEq, Eq, Clone, Debug)]
pub enum LexError {
    InvalidDigit,
    InvalidChar,
}

#[derive(PartialEq, Eq, Clone, Debug)]
pub enum LexerState {
    Any,
    TypeTerm(String),
    Sym(String),
    Num(i64),
    Char(Option<char>),
}

impl LexerState {
    fn into_token(self) -> Option<LTIRToken> {
        match self {
            LexerState::Any => None,
            LexerState::TypeTerm(s) => Some(LTIRToken::AssignType(s)),
            LexerState::Sym(s) => Some(LTIRToken::Symbol(s)),
            LexerState::Num(n) => Some(LTIRToken::Num(n)),
            LexerState::Char(c) => Some(LTIRToken::Char(c?)),
        }
    }
}

pub struct LTIRLexer<It>
where
    It: std::iter::Iterator<Item = char>,
{
    chars: std::iter::Peekable<It>,
    position: usize
}

impl<It> LTIRLexer<It>
where
    It: Iterator<Item = char>,
{
    pub fn into_inner(self) -> std::iter::Peekable<It> {
        self.chars
    }
}

impl<It> From<It> for LTIRLexer<It>
where
    It: Iterator<Item = char>,
{
    fn from(chars: It) -> Self {
        LTIRLexer {
            chars: chars.peekable(),
            position: 0,
        }
    }
}


#[derive(Clone, Debug)]
pub struct InputRegionTag {
    begin: usize,
    end: usize
}

impl InputRegionTag {
    pub fn max( a: InputRegionTag, b: InputRegionTag ) -> InputRegionTag {
        InputRegionTag {
            begin: usize::min( a.begin, b.begin ),
            end: usize::max( a.end, b.end )
        }
    }
}

impl<It> Iterator for LTIRLexer<It>
where
    It: Iterator<Item = char>,
{
    type Item = (InputRegionTag, Result<LTIRToken, LexError>);

    fn next(&mut self) -> Option<Self::Item> {
        let mut state = LexerState::Any;
        let mut region = InputRegionTag{
            begin: self.position,
            end: self.position
        };

        while let Some(c) = self.chars.peek() {
            match &mut state {
                // determine token type
                LexerState::Any => match c {
                    'λ' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::Lambda)));
                    }
                    '↦' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::MapsTo)));
                    }
                    '(' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::ExprOpen)));
                    }
                    ')' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::ExprClose)));
                    }
                    '{' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::BlockOpen)));
                    }
                    '}' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::BlockClose)));
                    }
                    ':' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        state = LexerState::TypeTerm(String::new());
                    }
                    '=' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::AssignValue)));
                    }
                    ';' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        return Some((region, Ok(LTIRToken::StatementSep)));
                    }
                    '\'' => {
                        self.chars.next();
                        self.position += 1;
                        region.end += 1;
                        state = LexerState::Char(None);
                    }
                    c => {
                        if c.is_whitespace() {
                            self.chars.next();
                            self.position += 1;
                            region.begin += 1;
                            region.end += 1;
                        } else if c.is_digit(10) {
                            state = LexerState::Num(0);
                        } else {
                            state = LexerState::Sym(String::new());
                        }
                    }
                },

                LexerState::Char(val) => {
                    self.position += 2;
                    region.end += 2;
                    *val = Some(match self.chars.next() {
                        Some('\\') => {
                            self.position += 1;
                            region.end += 1;
                            match self.chars.next() {
                                Some('0') => '\0',
                                Some('n') => '\n',
                                Some('t') => '\t',
                                Some(c) => c,
                                None => {
                                    return Some((region, Err(LexError::InvalidChar)));
                                }
                            }
                        },
                        Some(c) => c,
                        None => {
                            return Some((region, Err(LexError::InvalidChar)));
                        }
                    });

                    match self.chars.next() {
                        Some('\'') => {
                            if let Some(token) = state.clone().into_token() {
                                return Some((region, Ok(token)));
                            }
                        }
                        _ => {
                            return Some((region, Err(LexError::InvalidChar)));
                        }
                    }
                }

                LexerState::TypeTerm(s) => {
                    if *c == '=' || *c == '↦' || *c == ';' {
                        if let Some(token) = state.clone().into_token() {
                            return Some((region, Ok(token)));
                        }
                    } else {
                        if let Some(c) = self.chars.next() {
                            self.position += 1;
                            region.end += 1;
                            s.push(c);
                        }
                    }
                }

                _ => {
                    if c.is_whitespace()
                        || *c == '('
                        || *c == ')'
                        || *c == '{'
                        || *c == '}'
                        || *c == ';'
                        || *c == '='
                        || *c == ':'
                        || *c == '↦'
                    {
                        // finish the current token

                        if let Some(token) = state.clone().into_token() {
                            return Some((region, Ok(token)));
                        }
                    } else {
                        // append to the current token

                        let c = self.chars.next().unwrap();
                        self.position += 1;
                        region.end += 1;

                        match &mut state {
                            LexerState::Sym(s) => {
                                s.push(c);
                            }

                            LexerState::Num(n) => {
                                if let Some(d) = c.to_digit(10) {
                                    *n = (*n) * 10 + d as i64;
                                } else {
                                    return Some((region, Err(LexError::InvalidDigit)));
                                }
                            }

                            _ => {}
                        }
                    }
                }
            }
        }

        if let Some(token) = state.into_token() {
            Some((region, Ok(token)))
        } else {
            None
        }
    }
}

mod tests {
    #[test]
    fn test_lexer() {
        let mut lexer = crate::lexer::LTIRLexer::from(
            "let var1:ℕ=123;
                let square =λx.* x x;

                let sqrt = λx:ℝ~machine::Float64~machine::Word.(f64-sqrt x);
                let magnitude =
                     λx:ℝ
                    .λy:ℝ
                    .sqrt (+ (* x x) (* y y));
                "
            .chars(),
        );

        for (range, token) in lexer {
            eprintln!("[{:?}] {:?}", range, token);
        }
    }
}