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.

[Corbeau-1]

Ahem... This already exists. I don't know the text server command, but it's in server settings, Sociological, "Maximum distance citizens can migrate". The number is added to the target city's radius. For example, in Sim06 it is set to zero so population can migrate only if its source city is inside target city radius. If it was 1, they could migrate if, say, target city radius was 2 and the source city was 3 (or less) tiles away.

[XHawk87]

Yes, but the server places a cap on how high you can make this setting. This task is to increase or remove the cap.