Opened 8 years ago
Last modified 6 years ago
#249 assigned enhancement
Bidirectional Bfs and Dijkstra
Reported by: | kpeter | Owned by: | kpeter |
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Priority: | major | Milestone: | |
Component: | core | Version: | hg main |
Keywords: | Cc: | ||
Revision id: |
Description
It would be nice to have bidirectional Bfs and Dijkstra implementation in LEMON for the s-t shortest path problem.
Theoretically it is simple: the algorithm should be started "in parallel" from the source node s on the original graph and from the target node t on the reversed graph. As soon as we have a node u for which both direction have set the final dist and pred values, we can finish searching and construct the s->t path form the s->u path and the opposite of the t->u path. It could be faster by about a factor of two due to smaller search spaces.
However it is not clear for me how difficult it would be to implement it.
Change History (4)
comment:1 in reply to: ↑ description ; follow-up: ↓ 2 Changed 8 years ago by alpar
comment:2 in reply to: ↑ 1 Changed 8 years ago by kpeter
Replying to alpar:
It is a bit more difficult, even theoretically (it may be that the path s->u->t is not the shortest one).
Oops, I'm sorry. You are right. I've heard (or studied?) this method in this way, but it's wrong.
E.g. G = ({s,u,v,w,t},{su,ut,sv,vw,wt}), c(su)=c(ut)=80, c(sv)=c(w,t)=10, c(vw)=100. Here the shortest path is <sv,vw,wt>, not <su,ut>.
comment:3 Changed 8 years ago by kpeter
- Milestone LEMON 1.2 release deleted
comment:4 Changed 6 years ago by kpeter
- Owner changed from alpar to kpeter
- Status changed from new to assigned
Tamas Bibok implemented bidirectional Dijkstra and A* (A-star) algorithms for LEMON. These codes and his BSc thesis (in Hungarian) can be found in this repository:
http://lime.cs.elte.hu/~kpeter/hg/hgwebdir.cgi/lemon-astar/
Replying to kpeter:
It is a bit more difficult, even theoretically (it may be that the path s->u->t is not the shortest one).