Motivation / Problem

Subsidies are hard-coded to last only one year, with a maximum distance of 70 tiles (Manhattan distance). While this incentivizes local, short-distance routes which ordinarily don't pay much, even with the subsidy multiplier the short duration of the subsidy often isn't worth the player's time and effort.

Description

This PR adds a Settings > Subsidy duration to allow players to select the subsidy length, in years.

Slightly longer subsidies would be more powerful and beneficial to players. Additionally, allowing much longer subsidies would allow new gameplay modes, such as making unsubsidized NewGRF cargos unprofitable and forcing players to compete for long-term subsidies created by the game or a GS.

The setting defaults to 1, the current hard-coded duration. The minimum value is 0, which disables subsidies, and the maximum is 5000; a nice round number which doesn't overflow the uint16 storing the subsidy duration in months. The longest games I've seen in screenshots are between 500 and 1000 years, so this should be plenty of space for an effectively indefinite subsidy. (Note: the maximum was originally 256 when I created the PR, but I realized this wasn't long enough.)

I know that adding settings needs strong justification, so I considered several alternatives, none of which felt right:

• Inverse correlation with company score, using the same stages as the company HQ — introducing a Mario Kart item-like negative feedback loop to boost new and struggling companies while being less effective for larger companies. I actually started with this and it was cool, but would need major changes to news and the Subsidies GUI to display different information for each player.
• Inverse correlation with the selected subsidy payment multiplier, e.g. a 1.5x multiplier for 5 years or a 4x multiplier for 1 year. But this breaks existing gameplay styles and customization.
• The above, but chosen randomly for each subsidy and using a new "Random" option for the multiplier setting. It's a messy reuse of an existing setting. Bleh.

So I added a setting.

Note that while subsidies are randomly calculated each month without regard to how many subsidies currently exist, new subsidy offers will not accumulate forever since towns and industries with more than 42% of cargo transported are ineligible for new subsidies.

I also plan to look at increasing the maximum distance of the subsidy based on town and industry density to fix subsidies not being possible on low-density maps, but that is a separate project entirely.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker