Function fgrad_inplace 

pub fn fgrad_inplace(
    gx: &mut [f64],
    gy: &mut [f64],
    gz: &mut [f64],
    x: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
)

Forward difference gradient operator (in-place)

Computes forward differences along each axis with periodic boundary conditions. Writes results directly into pre-allocated output buffers.

Arguments

• gx, gy, gz - Output gradient components (must be pre-allocated to nxnynz)
• x - Input array (nx * ny * nz)
• nx, ny, nz - Array dimensions
• vsx, vsy, vsz - Voxel sizes