def write_ply(mesh, filename, binary=True):
    """
    Output the mesh object as an ASCII .ply file, or as a little endian binary file.
    """
    header = "comment This file was generated by the SurfacePy libraryn"
    header += "element vertex {0}n".format(len(mesh.vertices))
    header += "property float xnproperty float ynproperty float zn"
    header += "element face {0}n".format(len(mesh.triangles))
    header += "property list uchar int vertex_indicesn"
    header += "end_headern"
    with open(filename, 'wb') as file:
        if binary==True:
            file.write(__pack_string("plynformat binary_little_endian 1.0n"))
        else:
            file.write(__pack_string("plynformat ascii 1.0n"))
        file.write(__pack_string(header))
        if binary==True:
            for vert in mesh.vertices:
                file.write(struct.pack('<fff', vert.x, vert.y, vert.z))
            for tri in mesh.triangles:
                file.write(struct.pack('<Biii', ord('3'), tri.i1, tri.i3, tri.i2))
        else:
            for vert in mesh.vertices:
                file.write(__pack_string("{0}n".format(vert)))
            for tri in mesh.triangles:
                file.write(__pack_string("3 {0} {1} {2}n".format(tri.i1, tri.i3, tri.i2)))


def __pack_string(str):
    """
    Returns a bytes object of a string, in little-endian format.
    """
    chars = [c.encode('utf-8') for c in str]
    fmt = "<{0}".format(''.join(['c' for i in range(0, len(chars))]))
    return __pack(fmt, chars)



def __pack(fmt, args):
    return struct.pack(fmt, *args)