It looks like both CVRP::solve() and VRPTW::solve() functions have grown and became more (too) complex, but we can remedy this with a couple steps.

It seems both functions are identical, except for two details:

1. CVRP::solve() starts with a part that falls back to simpler TSP problem.
2. CVRP::solve() operates on RawRoute and RawSolution types, while VRPTW::solve() operates on TWSolution and TWRoute respectively.

Regarding 1. Could this check be extracted to Input::get_problem() function, so it returns the simpler problem instance?

Regarding 2. We could have a template function parametrised with Route and Solution types that would handle both cases. This would replace two functions with one. A further improvement could be gained by extracting handling threads to a separate class.

I can make some commits on top of this branch to show what I have in mind.

You're right, the reason for having nearly similar duplicated implementation in the first place is that they were simple enough to not bother going through an additional abstraction layer. It would be a nice refactor to change that now.

Regarding 1. Could this check be extracted to Input::get_problem() function, so it returns the simpler problem instance?

Definitely would make sense. We'd simply have to add a TSP(const Input& input) ctor.

Agree on using templates for the core solving code. Another thing to consider here is that we use 4 different parameter sets for all cvrp/vrptw X homogeneous/heterogeneous options.

Also happy to have the threads code live somewhere else if it makes the code more expressive and easy to read.

I can make some commits on top of this branch to show what I have in mind.

Since this PR was originally a fix and we're talking about a more thorough refactor, I'd be in favor of merging as is now and moving the refactor part to another issue, what do you think?

Since this PR was originally a fix and we're talking about a more thorough refactor, I'd be in favor of merging as is now and moving the refactor part to another issue, what do you think?

Makes sense.