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: mod.rs
    path of file: ./repos/unrust/src/engine/sound
    
    the code of the file until where you have to start completion: mod channel;
mod generator;

use std::cell::RefCell;
use std::rc::Rc;

use engine::{AssetError, AssetSystem};
use futures::Future;
use std::collections::BTreeSet;
use std::collections::HashMap;
use uni_snd::SoundDriver;

use self::generator::Generator;

const CHANNEL_COUNT: usize = 4;

#[derive(Debug, Clone, Copy, Ord, PartialOrd, Eq, PartialEq)]
pub struct SoundHandle(usize);

pub struct SoundSystem {
    cache: HashMap<String, SoundHandle>,

    loading: Rc<RefCell<BTreeSet<SoundHandle>>>,
    pending_play: Vec<SoundPlayEvent>,

    next_handle: usize,
    driver: Rc<RefCell<SoundDriver<SoundEvent>>>,
    asys: Box<AssetSystem>,
}

impl SoundSystem {
    pub fn new(asys: Box<AssetSystem>) -> Self {
        let mut driver = SoundDriver::new(Box::new(Generator::new(CHAN
    
    pub fn load_sound(&mut self, filepath: &str) -> SoundHandle {
        let filepath = filepath.to_owned();
        let buffer = self.cache.get(&filepath);
        match buffer {
            None => {
                let id = self.next_handle;
                self.next_handle += 1;
                let f = self.asys.new_file(&filepath);
                let driver = self.driver.clone();

                let load_f = f.and_then({
                    let filepath = filepath.clone();
                    let loading = self.loading.clone();
                    move |mut fdata| {
                        driver.borrow_mut().send_event(SoundEvent::LoadBuffer(
                            id,
                            fdata.read_binary()?,
                            filepath,
                        ));

                        loading.borrow_mut().remove(&SoundHandle(id));

                        Ok(())
                    }
                });

                self.asys
                    .execute(Box::new(load_f.map_err(|e| AssetError::FileIoError(e))));

                self.cache.insert(filepath.clone(), SoundHandle(id));
                self.loading.borrow_mut().insert(SoundHandle(id));

                SoundHandle(id)
            }
            Some(buf) => *buf,
        }
    }