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AI Diplomacy Between Rivals

Rivals make treaties and wars among themselves without the human, through diplomacy_event() and city_phase() in DIPLOMAC.C. The same war test also decides whether two rivals stay at peace.

Meetings

UNITS.C calls diplomacy_event(a, b, x, y) when units of two civilizations meet. Contact (0x011) is set in both directions. If neither is the human:

  • A repeated meeting is ignored three times in four ((a + b + turn_tick) & 3).
  • A pair in which either side holds a grievance does nothing.
  • Otherwise each side runs the war test against the other. A side that passes does not go to war if the target owns the Great Wall or if its own government is a Republic or Democracy.

If neither side goes to war, trade_advances() swaps one advance each way when both have something the other lacks, and a pair not already at peace signs a treaty. If either side goes to war, a pair at peace cancels its treaty and a pair meeting for the first time declares war; a pair already at war carries on. Messages (“… sign a peace treaty”, “… cancel peace treaty”, “… declare war on each other”) appear only if the human has an embassy with one of them.

The war test

alliance_ok(a, b) (the name is provisional) returns 1 when a is willing to fight b:

  1. If a is at war with the human, it leaves alone a b weaker than the human, unless b is allied with the human. The same rule applies with a and b reversed.

  2. a does not attack a civilization with more nuclear weapons.

  3. On every continent where a has towns, half of a’s attack plus its defence must exceed b’s attack there.

  4. a compares the attack it can bring against b’s strength on the continents where both have units:

    wc   = a's other wars + (1 if a and b are at peace) - a's aggression f0
x    = sum of a's attack on continents where b has defenders
den  = 1 + sum of (b's attack / 2 + b's defence) on those continents
pass = x * 4 / den >= wc + 4, or a has no towns on those continents

Other wars and an existing treaty raise the bar; an Aggressive leader lowers it by one. Clearing bit 1 of rule_options would make the test always pass, but new_game() sets all four rule bits.

Requests to the human

Once every 32 turns (on the turn where turn_tick & 31 equals four times its slot number), each AI checks its recent fighting. For each civilization i that it fought this period, while it has not fought the human and the human is at peace with i:

  • If i’s military total is more than twice its own, it asks for help (*HELPME) and offers clamp(treasury / 100, 1, 10) * 50 gold.
  • If i is stronger, but no more than twice as strong, it proposes a joint war (*CRUSADE), worded “agree to a” or “extend our” depending on whether it already has a treaty with the human.

Either way, accepting gives the AI a treaty with the human, cancels the human’s treaty with i, and gives i a grievance against the human. In the *HELPME case the code adds the gold to both treasuries rather than moving it from the AI to the human.

Ganging up on the leader

rate_civs() in TURN.C ranks the civilizations each turn by

treasury / 32 + citizens * 8 + advances + sum of unit cost over all units

and stores the rank (7 for the leader) in nation_ratings. When the human leads, has more than four cities and no nuclear weapons, and more than 200 turns have passed, it sets gangmode. Every AI with more than one city that is at peace with the human marks its treaty for cancellation (0x100); the others are put in contact with the human, and so at war. The same condition makes AIs hostile in audiences. While gangmode is set, AI cities on continents planned for expansion (plan 5) choose production as if they were at war.

Government

unit_phase() reconsiders an AI’s government every eighth turn (staggered by slot). It chooses Despotism while its troops exceed what its cities can support. Otherwise it prefers, in order:

  1. Republic, if it knows The Republic and margin_democracy is not negative (see Leaders and Personalities);
  2. Communism, if it knows Communism and has more than ten cities;
  3. Monarchy, if it knows Monarchy.

An AI does not choose Democracy, although a hostile Republic or Democracy in an audience may revert to Monarchy. After a revolution, city_phase() moves an AI from Anarchy to Despotism on the next turn divisible by four, or at once with the Pyramids.