rssn 0.2.9

//! Handle-based FFI API for physics sim Navier-Stokes functions.

use crate::numerical::matrix::Matrix;
use crate::physics::physics_sim::navier_stokes_fluid;

/// Result handles for the Navier-Stokes simulation containing velocity and pressure fields.
///
/// This C-compatible struct encapsulates the output of an incompressible Navier-Stokes
/// solver, providing matrix handles to the computed velocity components and pressure.
#[repr(C)]
pub struct NavierStokesResultHandles {
    /// Pointer to a Matrix containing the horizontal velocity field u(x,y) in m/s.
    pub u: *mut Matrix<f64>,
    /// Pointer to a Matrix containing the vertical velocity field v(x,y) in m/s.
    pub v: *mut Matrix<f64>,
    /// Pointer to a Matrix containing the pressure field p(x,y) in Pa.
    pub p: *mut Matrix<f64>,
}

/// Runs the lid-driven cavity simulation and returns handles to the U, V, and P matrices.
#[unsafe(no_mangle)]
pub extern "C" fn rssn_physics_sim_navier_stokes_run_lid_driven_cavity(
    nx: usize,
    ny: usize,
    re: f64,
    dt: f64,
    n_iter: usize,
    lid_velocity: f64,
) -> NavierStokesResultHandles {
    let params = navier_stokes_fluid::NavierStokesParameters {
        nx,
        ny,
        re,
        dt,
        n_iter,
        lid_velocity,
    };

    match navier_stokes_fluid::run_lid_driven_cavity(&params) {
        | Ok((u, v, p)) => {
            let u_mat = Matrix::new(ny, nx, u.into_raw_vec_and_offset().0);

            let v_mat = Matrix::new(ny, nx, v.into_raw_vec_and_offset().0);

            let p_mat = Matrix::new(ny, nx, p.into_raw_vec_and_offset().0);

            NavierStokesResultHandles {
                u: Box::into_raw(Box::new(u_mat)),
                v: Box::into_raw(Box::new(v_mat)),
                p: Box::into_raw(Box::new(p_mat)),
            }
        },
        | Err(_) => {
            NavierStokesResultHandles {
                u: std::ptr::null_mut(),
                v: std::ptr::null_mut(),
                p: std::ptr::null_mut(),
            }
        },
    }
}

/// Frees the result handles.
///
/// # Safety
///
/// This function is unsafe because it dereferences raw pointers as part of the FFI boundary.
/// The caller must ensure:
/// 1. All pointer arguments are valid and point to initialized memory.
/// 2. The memory layout of passed structures matches the expected C-ABI layout.
/// 3. Any pointers returned by this function are managed according to the API's ownership rules.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn rssn_physics_sim_navier_stokes_free_results(
    handles: NavierStokesResultHandles
) {
    unsafe {
        if !handles.u.is_null() {
            let _ = Box::from_raw(handles.u);
        }

        if !handles.v.is_null() {
            let _ = Box::from_raw(handles.v);
        }

        if !handles.p.is_null() {
            let _ = Box::from_raw(handles.p);
        }
    }
}