Production and growth

civ_economy() in ECONOMY.C calls city_process(city, 0L) for each city. CITYMGT.C contains both this turn processing and the interactive city screen. The mode argument chooses which work to perform.

Food and growth

The turn pass first resolves the food box left by the previous turn. If it is negative, a supported Settler is removed if one can be found. Otherwise the city loses one population point. A city reduced to zero is removed. The food box is then set to zero.

The growth threshold is (size + 1) * cost_scale. For the human, cost_scale is 10. For other owners it starts at 16 - 2 * difficulty, with a further reduction of two from AD 1 when the human leads the power ranking and difficulty exceeds Warlord. The same scale applies to production.

At the threshold the city grows. Its food box becomes zero without a Granary, or new_size * 5 + 5 with one. If growth would take it above size 10 without an Aqueduct, and rule bit 4 is enabled, the size increase is cancelled after resetting the food box.

Later in the pass, tile food is added and consumption subtracted: two per citizen, plus one per supported Settler under Anarchy or Despotism, or two under later governments. That balance is resolved on the next city pass.

Shields and completion

Worked tiles supply shields. Factory and power bonuses are applied, then unit support is subtracted. Existing disorder sets the shield contribution to zero. The remainder goes into shield_box.

A non-negative production selects a unit in utypes[28]; a negative value selects improvements[-production]. Completion requires the table cost times cost_scale.

Unit completion subtracts the cost and normally leaves the excess in the box. Settlers can reduce population; a size-one city on Chieftain postpones Settler completion. Buildings set their improvement bit, wonders record their city in marvel_site, and spaceship parts update the ship. The human completion path clears the remaining shields for these projects. AI building completion can retain the excess and choose another project.

Buying production

For the human, let r be missing shields, clamped to 0–999. Unit purchases cost r * r / 20 + 2 * r. Improvements cost 2 * r, doubled when the improvement ID exceeds 21, which includes spaceship parts and wonders. An empty shield box doubles the price again. Integer division truncates.

For example, a 40-shield unit with one shield stored costs 39 * 39 / 20 + 78 = 154 gold. With none stored it costs 320 gold.

AI purchases follow separate conditions. They add a chosen number of shields and charge two gold per shield; treasury fractions limit how much is bought. They do not use the human unit-price formula.