Bundler searches an NP-complete problem
'bundle install' may take a very long time...
The dependency resolution algorithm used in bundler (up to at least version 1.1) attempts an exhaustive search (using backtracking or backjumping) of what is an NP-complete problem. In most cases, a solution is found quickly. But in some real-world cases, the solution is not found quickly, and bundler appears to have hung.