Increase maximum migration distance

[XHawk87]

Is your feature request related to a problem? Please describe.
Currently, the maximum migration distance for a server is arbitrarily limited by the maximum city working radius. This is currently blocking: longturn/rules-of-chaos#17

Describe the solution you'd like
Increase the maximum migration distance to the highest value it can be set to without causing errors.

Describe alternatives you've considered
https://youtu.be/ubPWaDWcOLU?feature=shared

Additional context

freeciv21/common/game.h

Line 492 in a9b77fd

#define GAME_MAX_MGR_DISTANCE (1 + CITY_MAP_MAX_RADIUS)

[lmoureaux]

I don't think there's gonna be any error, but the performance impact should be kept in mind: a large migration distance would mean walking through the whole map for every city (effectively quadratic complexity in the map size).

[XHawk87]

That's true. However, we don't know exactly what the performance impact would be without testing it. It's also something that could be balanced in other ways, higher citymindist, smaller map etc. I think people should have the freedom to do stupid things if they want to, and our role should be to provide appropriate warnings so they know it's their own fault.

I don't think there's gonna be any error

This is a value that gets squared, so at least make sure it doesn't cause an overflow.

[lmoureaux]

I don't see it getting squared... The loop goes outward and stops once the distance becomes is too large, so it's capped by the map size.

[XHawk87]

You're right, I was thinking about the city map max radius. That algorithm looks pretty inefficient in general.

[XHawk87]

If we did need to improve the efficiency, this seems like low-hanging frut:

freeciv21/server/cityturn.cpp

Line 4217 in a9b77fd

mgr_dist = static_cast<int>(sqrt(static_cast<double> MAX(

There are only a handful of possible values for the squared city radius, so we could get those from a lookup table rather than calculating a costly square root. This is probably premature optimisation though.