game-engine-core

The deterministic heart of a real-time engine: a fixed-timestep simulation loop and a fixed-point math type — so physics and gameplay are stable and bit-for-bit reproducible across machines, frame rates, and replays.

Part of a low-level C++ portfolio focused on performance and determinism, alongside mcu-baremetal and microtensor.

Why determinism

Floating-point results can differ across compilers, CPUs, and optimisation levels — which breaks lockstep multiplayer, replays, and reproducible physics. This core solves the two foundations:

• Fixed (Q16.16) — a fixed-point number backed by a single int32. All arithmetic is integer, so it's bit-for-bit identical everywhere. 64-bit intermediates keep multiply/divide exact.
• FixedTimestep — the canonical "fix your timestep" accumulator: convert variable, jittery frame times into a whole number of equal simulation steps, expose an interpolation alpha() for smooth rendering, and cap work per frame to avoid the spiral of death.

Quick look

#include "game_engine_core/fixed.hpp"
#include "game_engine_core/timestep.hpp"

ge::FixedTimestep clock(1.0f / 60.0f);   // 60 Hz simulation
const ge::Fixed dt = ge::Fixed::fromFloat(clock.step());

// Per rendered frame:
int steps = clock.advance(frameSeconds); // however many fixed steps are due
for (int i = 0; i < steps; ++i) simulate(dt);
render(clock.alpha());                    // interpolate between sim states

See examples/bouncing_ball.cpp — a ball bouncing under gravity, fully in fixed-point, driven by deliberately irregular frame times.

Build & test

cmake -S . -B build -G Ninja
cmake --build build
ctest --test-dir build --output-on-failure
./build/bouncing_ball

Requires CMake ≥ 3.20 and a C++17 compiler. Tests are dependency-free.

Roadmap

• A cache-friendly entity/component (ECS) core with archetype storage.
• 2D/3D fixed-point vector & matrix math built on Fixed.
• A deterministic event/command buffer for lockstep networking and replays.
• A spatial hash / broadphase for collision.

License

MIT — see LICENSE.