Function knn_within 

pub fn knn_within(
    positions: &[[f32; 3]],
    query: &AtomSelection,
    targets: &AtomSelection,
    k: usize,
    options: KnnOptions,
    backend: SpatialBackend,
    periodic: Option<&PeriodicBox>,
    context: &ExecutionContext,
) -> Result<KnnResult, SpatialError>

Finds the k nearest target atoms of every query atom.

Rows follow the query selection’s order. Within a row neighbours increase in distance, equal distances ordering by atom index, and unfilled slots hold NO_NEIGHBOR with an infinite squared distance. Periodic queries report exact minimum-image distances; an unbounded periodic search looks through two lattice shells of images, which covers every wrapped target of a cell that is not extremely skewed, and a max_distance widens it exactly.

BruteForce scans every target serially and exists as a reference; every other backend uses the parallel k-d tree, since a cell list has no radius-free nearest query. Distances are f32; a target whose f64 distance ties the k-th at the f32 rounding boundary may be ranked either way.

Errors

Returns an error for an invalid max_distance, out-of-range atom indices, memory admission failure or cancellation.