def add_vertices(self, vertices: Iterable['Vertex']) -> Sequence[int]:
"""
Add new `vertices` only, if no vertex with identical ``(x, y, z)`` coordinates already exist, else the index of
the existing vertex is returned as index of the added vertices.
Args:
vertices: list of vertices, vertex as ``(x, y, z)`` tuple or :class:`~ezdxf.math.Vector` objects
Returns:
tuple: indices of the `vertices` added to the :attr:`~MeshBuilder.vertices` list
"""
indices = []
precision = self.precision
for vertex in vertices:
vertex = Vector(vertex)
key = vertex.round(precision)
try:
index, count = self.ledger[key]
except KeyError: # new key
index = len(self.vertices)
self.vertices.append(vertex)
self.ledger[key] = (index, 1)
else: # update key entry
# calculate new average location
average = (self.vertices[index] * count) + vertex
count += 1
# update vertex location
self.vertices[index] = average / count
# update ledger
self.ledger[key] = (index, count)
indices.append(index)
return tuple(indices)
