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The FreeCiv AI

Zack IvesSteve Wolfman

CSE 590AGApril 20, 1999

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Cheesy Heuristics!

Mostly a series of rules with weights and randomness

Different options are evaluated for desirability Roulette-wheel-style ai_fuzzy function decides whether

AI will follow heuristics

Mostly a peer-model agent structure with a bit of hierarchy, mostly re-evaluating at each turn Different AI’s to control cities, tech advances, units, etc. (Though military AI is somewhat hierarchical; both

coordinator to assign roles and a unit-level AI) Each AI gets “votes” weighting decisions

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The AI Files

aihand - main AI routines aicity - city-level AI aitech - tech goal AI aitools - utility routines (choose wonders, govt) aiunit - unit-level AI; settlers - settler AI (in server) advdomestic - intra-city advisor (worker placement,

valuation of improvements) advisland, advscience, advtrade, advattitude -

blank advmilitary - offense & defense for units server/citytools - city/worker reasoning functions

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General AI Components

In aihand, several important functions: ai_manage_units - control units, assess defense

& offense & needs ai_manage_cities - control city building, spending ai_manage_taxes - set tax/science/luxury rate ai_manage_government - set government style ai_manage_tech - tech advances

No coordinating AI, but inter-module communication

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Interesting Ideas

Bodyguard if city already defended, defensive unit will try to find a

low-defense “charge” and AI will coordinate movement

Urgency and value Cities and domestic and military advisors collaboratively

determine how valuable improvements, reinforcements, or new technologies will be, and how urgently they’re needed

Tech is valued according to needs noticed throughout movement, building, …

Sacrifice If city is ready to be conquered, try to sell what you can

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What the AI Emphasizes

Wonders and caravans Diplomats

If opponent’s diplomat or spy is nearby, builds diplomat for defense

Builds up money to buy improvements If troops not serving purpose, AI brings

them home Transports try to land near destination Republic and “We love the consul day”

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Cheats

Apparently not as many as we’d think! AI has knowledge of full map and enemy unit

locations AI has no penalty for changing what’s being built At higher difficulty levels, AI not constrained by

maximum rates on tax, science, etc. Civil disorder - last-second fix for unhappy city

(though does suffer effects if unable to make city happy)

AI changes government for free (no Anarchy)

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The Unit AI - Structure

In aiunit and advmilitary Main function ai_manage_units

ai_manage_settler ai_manage_caravan ai_manage_ferryboat ai_manage_explorer ai_manage_military: based on role, auto-settle, build

city, defend home, attack, fortify, run, escort, explore

Can ask for units (ai_wants_role_unit) and get closest possible match (due to tech limitations)

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The Unit AI - What It Does

Settlers - auto_settler_findwork Check terrain for value as city Rate usefulness of mining, roads, irrigation,

detox, etc.

Exploration Top priority = huts Then randomly explore

Caravan Find city building wonder not near completion,

send caravan

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Unit AI - Military

Calculate belligerence (offensive rating), vulnerability (defensive rating) of units

Military finds victim based on attack power vs. vulnerability Bodyguard - extra defensive unit finds vulnerable unit,

becomes its bodyguard; AI synchronizes movement Assigns roles to units (auto-settle, build city, defend home,

attack, fortify, run, escore, explore) find_something_to_kill - main controlling function for

coordinating attacks generate_warmap - assessment of distances, possibilities, &

costs of moving & attacking Choose most appealing targets, send forces to eradicate

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Military Advisor

Assess danger due to opposing forces Assess city defenses

If city in danger, may specify desire for coastal fortress, SAM, SDI

Based on this, may want prerequisite new technologies

Assess offensive capability and desires May wish to build new units, acquire new

technologies

Decide what to kill

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The City AI - structure

In aicity and advdomestic Main function in aicity is ai_manage_cities

ai_manage_city - arrange workers ai_manage_buildings - globally important bldgs (palace

& wonders) ai_city_choose_build - locally evaluate buildings (calls

domestic advisor)

Main function in advdomestic is ai_eval_buildings individually rates each type of building according to

food/prod/gold/bulb considers luxury result in terms of new available workers

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Money Management in the City AI

Money spent based on collaborative needs (“votes”)

Considers cities in order of greatest need Tries to buy something for every city Becomes frugal with remaining money if ever it

cannot afford something “important” (highly desired)

Some heuristics restrict buying Don’t spend too much at once (except for high want) Don’t build Wonder in threatened city

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Taxes, Luxury, and Sciency (oh my!)

Handled in ai_manage_taxes (in aihand) Luxuries considered first

Only if government is Republic Try to maximize population gain from “we love the

consul” Sacrifice if leaves insufficient tax rate to acquire needed

funds or if unable to reach optimal celebration point

Taxes next Gathers enough money to cover expenses plus

population dependent “spare change” requirements

Science gets leftovers

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Government changes

Simple module (partly because AI cheats!) Different possible goal governments

define track Change as soon as next government on

track becomes available No backtracking to earlier government Current AI hardwired to Republic track

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Useful code

AI code is hard to reuse AI uses many server structures unavailable to

client (e.g., warmap) Shared structures (such as tech_want) are

hard to disentangle from code

Smallest functions most useful City calculations in advdomestic such as

pollution caused/ameliorated by an improvement

Danger assessments?

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Bonus: FreeCiv AI Fun!

“hhjj” is a structure used to calculate celebration levels in cities at various luxury levels… what does hhjj stand for?

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Technology

tech_want structure maintains desire levels for various techs

Each component can up tech requirements Military wants better units/transports Missing wonder/governments cause want for enabling techs Global AI adds hardwired want for Republic track techs Cities want enabling techs for buildings Global AI fudges philosophy want upward (if not developed

yet)

Prerequisites of desired tech weighted Weights reduced according to time to reach tech