Factorio Under the Hood: Deterministic Sim, Transport Lines & UPS Tricks
Engine & Tech Stack
Engine / language: a custom C++ engine; Lua drives both the data/prototype stage and runtime mod scripting.
Map representation: a tile grid partitioned into 32×32 chunks; entities live on the grid and are tracked in per-system update lists.
Determinism: the simulation is fully deterministic, enabling lock-step multiplayer where only player inputs are sent over the network.
Core AI & Decision Making
Enemies (biters): pollution spreads per-chunk and raises aggression; enemies form unit groups that move and attack together.
Dispatch: attack targets and routes come from the pathfinder; groups gather, then path toward the nearest polluting structures.
State Management & Data Architecture
Model: custom C++ entities updated from active update lists; entities with nothing to do are removed from those lists ("sleeping") so idle factories cost almost nothing.
Transport lines: contiguous belt segments are merged into a single transport line with a compact item representation: items aren't simulated as individual objects, which is the core belt-throughput optimization.
Persistence: the entire deterministic world state is serialized to the save.
Tick Scheduling & Performance Tricks
Fixed 60 UPS: the simulation runs at a constant 60 updates/second, fully decoupled from render FPS.
Multithreading: later versions parallelize belt, fluid, electric-network and entity-preparation work across cores; the electric network and heat/fluid systems update on their own schedules.
Pathfinding & Spatial Systems
Algorithm:A* over the collision grid with an abstract, cached base-to-base pathfinder to keep long enemy paths cheap.
Spatial partitioning: chunk-based iteration and path caching bound the search space as the factory sprawls.
