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

    // SingleQuote(String),
    // DoubleQuote(String),
    // TripleQuote(String),

    Lambda,
    AssignType,
    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,
    Sym( String ),
    Num( i64 ),
    Char( Option<char> )
}

impl LexerState {
    fn into_token(self) -> Option< LTIRToken > {
        match self {
            LexerState::Any => None,
            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>,
}

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()
        }
    }
}

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

    fn next(&mut self) -> Option<Self::Item> {
        let mut state = LexerState::Any;

        while let Some(c) = self.chars.peek() {
            match &mut state {

                // determine token type
                LexerState::Any => {
                    match c {
                        'λ' => { self.chars.next(); return Some(Ok(LTIRToken::Lambda)); },
                        '(' => { self.chars.next(); return Some(Ok(LTIRToken::ExprOpen)); },
                        ')' => { self.chars.next(); return Some(Ok(LTIRToken::ExprClose)); },
                        '{' => { self.chars.next(); return Some(Ok(LTIRToken::BlockOpen)); },
                        '}' => { self.chars.next(); return Some(Ok(LTIRToken::BlockClose)); },
                        ':' => { self.chars.next(); return Some(Ok(LTIRToken::AssignType)); },
                        '=' => { self.chars.next(); return Some(Ok(LTIRToken::AssignValue)); },
                        ';' => { self.chars.next(); return Some(Ok(LTIRToken::StatementSep)); },
                        '\'' => { self.chars.next(); state = LexerState::Char(None); },
                        c => {
                            if c.is_whitespace() {
                                self.chars.next();
                            } else if c.is_digit(10) {
                                state = LexerState::Num( 0 );
                            } else {
                                state = LexerState::Sym( String::new() );
                            }
                        }
                    }
                }

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

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

                _ => {

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

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

                        let c = self.chars.next().unwrap();

                        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(Err(LexError::InvalidDigit));
                                }
                            }

                            _ => {}
                        }
                    }
                }
            }
        }

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