r size is less than the current batch size:<ol>
<li>call <code>build_buffer</code> event</li>
<li>call <code>filter_buffer</code> event</li>
<li>call <code>after_build_buffer</code> event</li>
</ol>
</li>
<li>Call <a href="/mrl/environment.html#Environment.sample_batch"><code>Environment.sample_batch</code></a><ol>
<li>create new <a href="/mrl/callback_core.html#BatchState"><code>BatchState</code></a></li>
<li>call <code>before_batch</code> event</li>
<li>call <code>sample_batch</code> event</li>
<li>call <