```ruby
class UsersController < ApplicationController
  def new
  end

  def create
  end

  def edit
  end

  def update
  end

  def show
  end

  def index
  end

  def destroy
  end
end
```

Implement actions for users controller
user
```ruby
class UsersController < ApplicationController

  def index
    @users = User.all
  end

  def new
    @user = User.new
  end

  def create
    user = User.find_by(username: user_params[:username])
    unless user
      @user = User.create(user_params)
      session[:user_id] = @user.id
      redirect_to root_path
    else
      flash[:notice] = "A user with that name or email already exists"
      redirect_to new_user_path
    end
  end

  def edit
    find_user
  end

  def update
    find_user
    @user.assign_attributes(user_params)
    if @user.save
      redirect_to @user
    else
      render :edit
    end
  end

  def show
    find_user
  end

  def destroy
    find_user
    @user.destroy if @user
    redirect_to root_path
  end

  private

  def user_params
    params.require(:user).permit(:username, :email, :password, :country_id, :native_language_id,
      :study_language_id, :avatar_url, :points, :level_id)
  end

  def find_user
    @user = User.find_by(id: params[:id])
  end
end
```
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