How to detect edge loops from set of faces

Given a set of selected faces (triangular only) from single mesh, how can we detect following: a) edge loops that selected faces create. b) fast grouping of faces based on edge loop they are contained in.

This method is required for doing face extrusion when more than one face is selected and y开发者_JS百科ou want to extrude multiple regions but using average normals as direction instead of extruding individual faces.

Here's a paper that talks about detecting edge loops in a mesh, and partitioning: Punctuated Simplification of Man-Made Objects

Hope this helps.