You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: calculator.rs
    path of file: ./repos/native-windows-gui/native-windows-gui/examples
    
    the code of the file until where you have to start completion: /*!
    A calculator that use the grid layout of NWG.
*/

extern crate native_windows_gui as nwg;
use nwg::NativeUi;


#[derive(Debug)]
enum Token {
    Number(i32),
    Plus,
    Minus,
    Mult,
    Div
}


#[derive(Default)]
pub struct Calculator {
    window: nwg::Window,

    layout: nwg::GridLayout,

    input: nwg::TextInput,

    btn0: nwg::Button,
    btn1: nwg::Button,
    btn2: nwg::Button,
    btn3: nwg::Button,
    btn4: nwg::Button,
    btn5: nwg::Button,
    btn6: nwg::Button,
    btn7: nwg::Button,
    btn8: nwg::Button,
    btn9: nwg::Button,

    btn_plus: nwg::Button,
    btn_minus: nwg::Button,
    btn_mult: nwg::Button,
    btn_divide: nwg::Button,
    btn_process: nwg::Button,
    btn_clear: nwg::Button,
}

impl Calculator {

    fn number(&self, button: &nwg::Button) {
        let text = self.input.text();
        self.input.set_text(&format!("{}{}", text, button.text()));
    }

    fn clear(&self) {
        self.input.set_text("");
    }

    fn compute(&self) {
        use Token::*;
        static SYMBOLS: &'static [char] = &['+', '-', '*', '/'];

        let eq = self.input.text();
        if eq.len() == 0 {
            return;
        }

        l
    
    fn compute(&self) {
        use Token::*;
        static SYMBOLS: &'static [char] = &['+', '-', '*', '/'];

        let eq = self.input.text();
        if eq.len() == 0 {
            return;
        }

        let mut tokens: Vec<Token> = Vec::with_capacity(5);
        let mut last = 0;

        for (i, chr) in eq.char_indices() {
            if SYMBOLS.iter().any(|&s| s == chr) {
                let left = &eq[last..i];
                match left.parse::<i32>() {
                    Ok(i) => tokens.push(Token::Number(i)),
                    _ => {
                        nwg::error_message("Error", "Invalid equation!");
                        self.input.set_text("");
                        return
                    }
                }

                let tk = match chr {
                    '+' => Plus,
                    '-' => Minus,
                    '*' => Mult,
                    '/' => Div,
                    _ => unreachable!()
                };

                tokens.push(tk);

                last = i+1;
            }
        }

        let right = &eq[last..];
        match right.parse::<i32>() {
            Ok(i) => tokens.push(Token::Number(i)),
            _ =>  {
                nwg::error_message("Error", "Invalid equation!");
                self.input.set_text("");
                return
            }
        }

        let mut i = 1;
        let mut result = match &tokens[0] { Token::Number(n) => *n, _ => unreachable!() };
        while i < tokens.len() {
            match [&tokens[i], &tokens[i+1]] {
                [Plus, Number(n)] => { result += n; },
                [Minus, Number(n)] => { result -= n;},
                [Mult, Number(n)] => { result *= n; },
                [Div, Number(n)] => { result /= n; },
                _ => unreachable!()
            }
            i += 2;
        }

        self.input.set_text(&result.to_string());
    }