PhySMC.jl - Physical Simulation Safe for Probabilistic Programming

NOTE!: this is a work in progress and the API is NOT stable

The purpose of PhySMC is to properly interface physics engines with probabilistic programming. Probabilistic programming, especially of the flavor found in Gen, composes stochastic computation with deterministic or pure functions (e.g., every time f is called with argument x you get the same result y). However, physics engines often rely on internal state (usually to cache computations), which can pose an issue for integrating them with Gen. For example, generating multiple traces using the same engine context (such as a particle filter) can lead to unintended side-effects when using Gen.update.

PhySMC provides an interface where calls to physics engines and retrieval/manipulation of the state behave in a pure way.

The interface is defined in https://github.com/CNCLgithub/PhySMC/blob/master/src/PhySMC.jl with the main function being step

"""
     step(sim::PhySim, st::PhyState)::SimState

Performs a stateless evolution of the simulation state.
"""
function step(sim::PhySim, st::PhyState) end

where sim::PhySim is some simulation backend and st::PhyState is some state for that backend.

The default implementation of step relies on two helper funtions that can be broadly useful in generative modeling:

• sync!
• forward_step

function step(sim::PhySim, st::PhyState)
    sync!(sim, st)
    new_st = forward_step(sim, st)
end

where these functions can be dispatched to particular simulator backends

"""
    sync!(sim::PhySim, st::PhyState)::Nothing

Synchronizes the context within `sim` using `st`.
"""
function sync! end

"""
    forward_step(sim::PhySim)::PhyState

Resolves physical interactions and obtains the next state representation.
"""
function forward_step end

Backends

Available

• PhyBullet : Backend for the bullet physics engine (via pybullet)

Planned

• Taichi : Backend for taichi physics

Contributions are always welcome! Documentation is a work in progress, please refer to docstrings in src/PhySMC.jl in the meantime.