Caravan trade

Caravans are unit type 27. CONTROL.C handles their arrival and offers trade or help with a wonder. rpath() in CAPTURE.C calculates the delivery reward and updates route records.

Each city has three route slots in routes. A slot stores a destination city index, with −1 for empty. Destroying a city clears routes that refer to it. Routes link cities, rather than storing a fixed income in the save.

During city_process(), the city first records its trade after an initial corruption calculation. It then visits its route slots in order. A foreign route adds (destination_trade + current_trade + 4) / 8; a domestic route uses a divisor of 16. Each addition changes current_trade for the next slot. Corruption is recalculated before the national spending split.

For the one-time delivery reward, rpath() first calculates:

reward = (map_dist(source, destination) + 10) * (source_trade + destination_trade) / 24

It halves the result when a region comparison succeeds, and halves it again for a domestic destination. It then reduces the reward by a third for each of Railroad and Flight known by the destination’s owner. The result is added to both treasury and research, and the Caravan is removed.

The region comparison contains a quirk: the source lookup uses the source city’s x coordinate for both x and y. It therefore does not reliably test whether the two cities share a continent.