Module potential 

Callback-based potential dispatch.

This module defines PotentialImpl, an opaque handle that wraps a C function pointer callback together with a void* user_data and an optional destructor. This is the core abstraction that lets existing C++ potentials (LJ, CuH2, or any future implementation) plug into the Rust infrastructure without the Rust side knowing the concrete type.

How it Works

1. The C++ side creates a potential object (e.g., LJPot).
2. A trampoline function with the PotentialCallback signature is registered, casting user_data back to the concrete type and calling forceImpl.
3. The Rust core dispatches through the function pointer, receiving results via the rgpot_force_out_t output struct.

Lifetime Contract

• The user_data pointer is borrowed by PotentialImpl. The caller must keep the underlying object alive for the lifetime of the handle.
• If a free_fn is provided, it is called on drop when user_data is non-null, transferring ownership to PotentialImpl.
• The handle is exposed to C as rgpot_potential_t — an opaque pointer managed via rgpot_potential_new / rgpot_potential_free.

Structs

PotentialImpl

Opaque potential handle wrapping a callback + user data.

Type Aliases

FreeFn

Destructor for the user_data pointer.

PotentialCallback

Function pointer type for a potential energy calculation.

rgpot_potential_t

Opaque handle exposed to C as rgpot_potential_t.