largest_label = 0;
for x in 0 to n_columns {
	for y in 0 to n_rows {
		if occupied[x,y] {
			left = occupied[x-1,y];
			above = occupied[x,y-1];
			if (left == 0) and (above == 0) { # As-yet-isolated node
				largest_label = largest_label + 1;
				label[x,y] = largest_label;
			}
			else {
				if (left != 0) {
					if (above != 0) # Connected to leftward and upward node
						union(left,above);
					label[x,y] = find(above); # Connected to leftward node only
				}
				else {
					label[x,y] = find(above); # Connected to upward node only
				}
			}
		}
	}
}