Function cross_sections_on_grid 

pub fn cross_sections_on_grid(
    data: &ResonanceData,
    energies: &[f64],
) -> Vec<CrossSections>

Compute cross-sections over a grid of energies.

Optimized batch evaluation: precomputes J-groups and per-resonance invariants (reduced width amplitudes, penetrability at E_r) once per resonance range, then evaluates each energy point using the cached data. This avoids redundant group_by_j + penetrability(l, rho_r) calls that the per-point API (cross_sections_at_energy) would repeat.

Issue #87: the precompute is hoisted above the energy loop so that precompute_jgroups_* runs O(ranges) times total, not O(ranges × energies).

Arguments

• data — Parsed resonance parameters from ENDF.
• energies — Slice of neutron energies in eV.

Returns

Vector of cross-sections, one per energy point.

Panics

Panics if any element of energies is non-finite or non-positive. Validating the entire grid up-front (O(n) branch, one pass) means a single bad energy fails fast with a clear message instead of being hidden inside the inner loop, matches the symmetric contract on cross_sections_at_energy, and protects direct Rust callers from the same release-mode silent-zero footgun that the SLBW / RML / URR leaf asserts guard against per-point.