use std::cell::Cell;
use std::str;
use std::char;

#[derive(Debug, PartialEq)]
enum TokenType<'a> {
    ILLEGAL,
    EOF,
    IDENT(&'a str),
    INT(i64),
    ASSIGN,
    PLUS,
    COMMA,
    SEMICOLON,
    LPAREN,
    RPAREN,
    LBRACE,
    RBRACE,
    FUNCTION,
    LET,
}

struct Token<'a> {
    token_type: TokenType<'a>,
    literal: &'a str,
}

struct Lexer<'a> {
    input: &'a [u8], // assume the text input is all ascii
    curr_pos: Cell<usize>,
    read_pos: Cell<usize>,
    curr_char: Cell<char>,
}

impl<'a> Lexer<'a> {
    fn next_token(&self) -> Token {
        match self.curr_char.get() {
            '='         => Token { token_type: TokenType::ASSIGN,    literal: "=" },
            ';'         => Token { token_type: TokenType::SEMICOLON, literal: ";" },
            '('         => Token { token_type: TokenType::LPAREN,    literal: "(" },
            ')'         => Token { token_type: TokenType::RPAREN,    literal: ")" },
            '{'         => Token { token_type: TokenType::LBRACE,    literal: "{" },
            '}'         => Token { token_type: TokenType::RBRACE,    literal: "}" },
            ','         => Token { token_type: TokenType::COMMA,     literal: "," },
            '+'         => Token { token_type: TokenType::PLUS,      literal: "+" },
            '\0'        => Token { token_type: TokenType::EOF,       literal: "\0" },
            _           => unimplemented!(),
        }
    }
}