private function normalizeL1(array &$sample)
{
$norm1 = 0;
foreach ($sample as $feature) {
$norm1 += abs($feature);
}
if (0 == $norm1) {
$count = count($sample);
$sample = array_fill(0, $count, 1.0 / $count);
} else {
foreach ($sample as &$feature) {
$feature = $feature / $norm1;
}
}
}