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: full_term.rs
    path of file: ./repos/cackle/src/ui
    
    the code of the file until where you have to start completion: //! A fullscreen terminal user interface.

use crate::checker::Checker;

    
fn update_counter(counter: &mut usize, key_code: KeyCode, len: usize) {
    match key_code {
        KeyCode::Up => *counter = (*counter + len - 1) % len,
        KeyCode::Down => *counter = (*counter + len + 1) % len,
        _ => panic!("Invalid call to update_counter"),
    }
}