function agglomerate($labels, $grid)
{
$clusters = $labels;
while (count($clusters) > 1) {
$distances = array(array(1, 0, $grid[1][0]));
for ($i = 2; $i < count($grid); $i++) {
for ($j = 0; $j < $i; $j++) {
$distances[] = array($i, $j, $grid[$i][$j]);
}
}
usort($distances, function ($a, $b) {
return $b[2] < $a[2];
});
list($j, $i, $d) = $distances[0];
# merge i⇐j
$c = new Cluster($d);
list($clusters, $grid) = $c->add($clusters, $grid, $i, $j);
$clusters[$i] = $c;
}
return $clusters[min(0, count($clusters) - 1)];
}