I’ve just finished reworking the old state-machine based AI system that I threw in to the game last year just to get something working. Sol Trader now boasts a full STRIPS-based planning AI. This works by starting a character off with some basic needs to fulfil: the need to socialise, rest, work, self-improve, etc. It then uses pathfinding techniques to work out a series of steps to get those needs fulfilled, such as buying a cheap good and selling it for more somewhere else, hitting the bar after work, or hanging around a jumpgate looking for easy prey. Here’s how it works.

The new planning system at work: a few of your relatives sleep at a tiny motel

Making a plan

Let’s say Anthony, an AI character, is tired and has a strong need to rest. To fulfil that need, the game allows the character to rest at home, but also to stay a night at a friend’s house. Here are some rules from the actual planner in the game written in a semi-formalised manner:

  • in order to REST we can RELAX_AT_HOME if we are IN_LOCATION(MY_HOME) at cost of 0
  • in order to REST we can STAY_A_NIGHT if we are AT_HOME_OF(CLOSE FRIEND) at cost of 50

To stay the night at a friend’s house, Anthony would need to move to their house, so we need some more rules to cover this:

  • in order to be IN_LOCATION(LOCATION) we can MOVE_TO(LOCATION) if we are IN_CITY(CONTAINING LOCATION) at cost of 10

Let’s assume that we are already in the city in question. The planner starts off at the need (REST) and works backwards until it find this IN_CITY state. It then forms a chain of actions to complete to get the need fulfilled:


The game will always chose the lowest cost option. If Anthony is in his home already, or in the same city, then he would just go home to rest, rather than to a friend’s house. However, let’s assume he is in a different city. It’s too far for Anthony to head home to his house, so the lowest cost option would then be to trespass on the hospitality of a friend who lives in that city.

Planning works extremely well for Sol Trader

There’s no one-size-fits-all when it comes to good game AI. The effectiveness of AI techniques varies dramatically depending on the type of game being designed. However, planning is a great fit for Sol Trader: it’s had a dramatic effect of the feel of the game.

Now the AI is now intelligently making decision based on relative needs, the game has the following new features, all of which were easily added:

  • Characters now visit friends and colleagues, hiring ships if they don’t have their own transport
  • Traders buy and sell goods intelligently, by buying cheap, travelling to another planet and selling high. It’s great fun watching the ships take off and land, knowing they have a cargo full of cheap goods bound for some far-flung planet, which will actually be more expensive in their destination.
Plenty of characters making travel enquiries at a Martian spaceport
  • Navy ships now take off and escort famous characters.
  • Unwise navy characters might take the law into their own hands, destroying ships close to them that contain characters they know to be immoral.
  • All ships defend themselves when attacked, depending on their piloting skills and their wisdom
  • Characters will run to a planet for repairs should their ship get too damaged.
  • I’ve also got the bare bones of the Pirate AI in, so you need to watch out when travelling around the various systems now: I’ve already been attacked once whilst trying to test something else. They’re not that smart yet, and will attack even in major population centres, but I’ll fix that in an upcoming release.
Watch that trigger finger: ships will now attack and defend themselves

It was also very easy to put in a conversation option which asks what a character is thinking about. This returns some text detailing the character’s top need, which shows what they’re most likely to do next:

Pretty forward thinking of Alysa to want to improve her life... at the age of 1

I’ve posted the start of a reference guide to the forums for modders.

This new build is now available from the forum if you have purchased insider access (if you haven’t there’s still time!) If you are already a Kickstarter backer and you haven’t received your copy, or you’re a member of the press, do get in touch.