CGAL users discussion list

["Shi Yan" <>]

hello folks,

did you have any experience of writing a half-edge data structure with
undo/redo feature?

i'm wondering if the command pattern is the only way to achieve undo and redo.

the command pattern packs each operation and its parameters as a
command and pushes the commands into a stack as executing them.

once the user needs to restore the data, the commands will be popped
up and executed in backwards.

aside from the function parameters, i also need to pack the half-edge
handle on which the operation is applied   with the command.

however after some operations, the half-edge handle could be invalid.
for example:

suppose i have an edge  E with endpoints A and B                     A
<--------E--------- B

the half-edge handle E points to A.

now i want to move A a little bit. so i create a "move command" which
contains the half-edge handle E pointing to A.

and then, i split this E in to two edges by a "split command"
                           A<---------K<------------B


now suppose i want to restore all my previous operations.

i first reconstruct the edge BA by connecting KA and BK   :
        A <-------------U-------------- B

and now i need to move point A back to the original position, but i
cannot do it, because i lost the half-edge handle E.

so how i'm suppose to do this?

thank you.