Unit Orders and Attack Evaluation

When ai_turn() in CONTROL.C reaches a computer player’s unit, it calls ai_order(player, unit). The result is the command a human would type, so the AI’s units go through the same command handlers as the human’s. Any non-zero result also clears the unit’s destination. After five orders in one turn the unit is made to wait.

Barbarians (civilization 0) use a separate routine, unit_ai_move(), described at the end of this chapter.

Distances quoted as map_dist() or town_gap use the game’s distance score, which includes half the shorter coordinate difference. They are not exact travel costs.

Special units

Bombers return to base ('h') when out of fuel. Fighters without a destination also return when they have used half their movement. Otherwise they chase the nearest human Bomber they can see. If the Bomber is out of reach, the Fighter flies to the nearest of its own cities within range of it.

Nuclear units pick a city to destroy. A candidate city has no SDI Defense and is larger than size 4. Its owner must not be at peace with the AI and must have been marked for nuclear exchange (0x080). In addition, the AI must be weaker than the owner, hold a grievance, or face an owner with no nuclear weapons that it does not outclass two to one. The score is half the city’s size, plus one for each tile of its 3 × 3 block occupied by units of such an enemy, minus two for each tile with the AI’s own units and 99 for each tile with anyone else’s. The AI needs one of its own cities at map_dist() <= 16 from the target. If the best score is at least 10 and the AI has more than one Nuclear unit, the unit is moved directly onto a free tile beside the target and attacks it. A strike on the human sets last_met_turn to −2, which lowers the price of the next audience.

Diplomats without a destination head for the nearest city if it is foreign, and are disbanded inside the AI’s own territory.

City sites and huts

World generation stores a site value of 8–15 for every possible city tile in page 2 at (x + 80, y) (0 elsewhere). Settlers, attackers and defenders on a tile with a site value look at the eight neighbours. A neighbour with a hut attracts any unit except Settlers. If the best value is at least 8, at least 15 - distance to the nearest city, and no enemy is adjacent, then:

• Settlers found a city ('b'), or step towards a better neighbour first;
• other units record the site (site_add(), kind 0, priority 2) for settlers to find later.

On the first turn of an EARTH game, the threshold is treated as met, so settlers placed at their tribe’s historical start found a city at once. (The flag for an EARTH game is named beginner_help in the source.) Above Chieftain, an AI settler near a human city may also found a city there when the AI knows fewer advances than the human. This uses the unmodified copy of the site value at (x + 80, y + 50), with a threshold of 9 and 14 - distance.

Defenders

A defender alone in a city fortifies. In a larger stack, the strongest defender also fortifies. The city needs size / divisor + 1 defence, where the divisor is 4 (3 if enemies are adjacent), one less for the capital and one more on a continent in plan 0. While the stack’s defence is no more than that, every defender fortifies, and while it is less, the city is listed as a defence target. Every eighth turn a surplus defender’s fortification is cleared so that it can be sent elsewhere.

Away from a city, a defender with no destination fortifies when it is within three tiles of one of its own cities with no other unit beside it, or when enemies block it. An attacker alone in a city waits there and lists the city as a defence target.

Ships

A transport (role 5) or warship (role 3) first counts the land units it carries. A transport with too little cargo heads for one of the AI’s coastal cities on a continent with at least 16 defence. It prefers continents planned for expansion and avoids ports where it is not needed. Arriving, it adds settle, defend and attack targets there so that units come to it.

A loaded transport looks for an adjacent landing site: land with no foreign units, on a continent where its cargo is useful (one with little defence and no expansion plan, a continent planned for the cargo’s role, or the destination). The continent must have at least five city sites. If possible it first moves along the coast to the beach with the most unclaimed land (ocean_edge()), then unloads ('u'). On a continent planned for attack it also lists the landing site as a naval target.

A ship with no destination then chooses one (Triremes excepted):

1. an enemy city, taking the nearest of every eighth city (an offset that changes each turn). While the human ranks first, the only enemy cities considered are the human’s;
2. failing that, a distant city of its own on another continent, preferring one whose plan matches the units’ roles;
3. failing that, a random land tile on a continent with none of its cities.

When the last defender of a human city is destroyed, city_captured() sends every loaded transport at sea at a map_dist() less than three times the ship’s movement allowance, belonging to a civilization without a treaty with the human, towards that city.

Settlers at work

A settler outside its city’s square, at map_dist() <= 2 from the AI’s own city, improves terrain when the rule option allows (always, in practice):

• work_tile() chooses irrigation ('i') or a mine ('m') when the terrain rule gives a worthwhile bonus and water is available;
• road ('r') on irrigated or mined grassland, plains or desert;
• railroad, once the AI knows the RailRoad advance, on a tile yielding at least two shields (one if it already has a road).

A settler on polluted land cleans it ('p'). Next comes the expansion test from the leader’s f1 trait (see Leaders and Personalities): a settler with a destination keeps going while the continent has room.

A settler beside its own city then looks for a neighbouring tile to work, and otherwise builds roads that join existing ones. Rivers need Bridge Building. With nothing to improve, it explores a random nearby 4 × 4 block holding none of its cities or settlers. If it finds none and the continent already has more than one town per eight sites, a settler in a city of size under 10 joins it ('b'), and one elsewhere goes home.

Pillage

A slow unit (fewer than two moves) near a human city (town_gap <= 3), when the AI has no treaty with the human, pillages an irrigated or mined tile it stands on ('P').

Choosing a step

When no rule above has given an order, ai_order() scores the eight neighbouring tiles and takes the best:

• A base of rand_scaled(5). For a unit on land that no one can see, the base is rand_scaled(3) minus twice the move cost (attackers) or minus the terrain defence (others).
• A tile of a friendly stack scores by the stack’s mix: attackers like stacks that need attack, defenders stacks that need defence. Other tiles add four times the terrain defence.
• An undefended foreign city scores 999 and a hut adds 20.
• When the unit is free to move, it gains 8 for an unexplored 4 × 4 block in that direction, 2 for each unseen tile around the point four steps away, and loses 2 for each occupied one. Settlers add the food of those tiles.
• Turning is penalized by 2 * d * d, where d is the change of direction from the last step (f8).

Attack odds

A neighbouring enemy stack at war (Settlers never attack) is scored with battle(..., 0), which returns the attack-to-defence ratio without fighting (see Combat):

odds = battle() * (defenders in the stack + 1) / attacker cost
odds *= 3   against a city
odds *= 3   for an attacker on a continent planned for attack
odds *= 2   for an attacker in a stack with more than twice its attack

The unit attacks if odds >= 6, adding 4 * odds to the tile’s score. Otherwise the tile scores −999. Then, a land attacker on an attack continent facing a lone fortified or veteran defender on hills or mountains outside a city may, one time in four, call buy_u(), the Diplomat’s bribery routine. With fewer than three movement thirds left, the unit does not attack at all.

Stacks of a civilization at peace are not attacked. Units other than Fighters ignore enemy aircraft outside cities.

Barbarians

unit_ai_move() handles civilization 0:

• Barbarian units in the polar rows are disbanded.
• An empty barbarian ship is disbanded. A loaded ship attacks an adjacent unit of another civilization. Otherwise it lands beside a free, unclaimed land tile within a town_gap of 8 from a city. If the human can see the landing near one of its cities, it reports “Barbarian raiding party lands near …! Citizens are alarmed.” Otherwise the ship sails for the city with the highest (unrest_chance + 50) / (distance + 1).
• A land unit alone in a city, or a defender there, fortifies. Every fourth turn a unit is disbanded if it is more than eight tiles from the nearest city or on another continent.
• Units march towards the nearest human city on their continent, unless that civilization has fewer than two cities and less than 100 gold. They attack any adjacent city that is not theirs.
• The barbarian leader (a Diplomat) stays with its troops, keeps away from enemies and is disbanded when left alone too far from a city. Capturing it pays a ransom of 100 gold (BATTLE.C).