Function select_kept 

pub fn select_kept<T: BackupLike>(backups: &[T], policy: &Retention) -> KeptSet

Select the GFS-kept set from backups under policy (ADR §4.4).

Returns a KeptSet partitioning every input id into keep/delete. Input order is irrelevant; keep is returned newest-first, delete newest-first too. Ties on end_time are broken by id for determinism.

use chrono::{DateTime, TimeZone, Utc};
use kopiur_api::{select_kept, BackupLike};
use kopiur_api::common::Retention;

// A trivial fake honoring BackupLike — no kube CRs needed for selection.
struct Snap { id: String, end: DateTime<Utc> }
impl BackupLike for Snap {
    fn end_time(&self) -> DateTime<Utc> { self.end }
    fn id(&self) -> &str { &self.id }
}
let day = |d: u32| Utc.with_ymd_and_hms(2026, 5, d, 2, 0, 0).single().unwrap();
let snaps = vec![
    Snap { id: "d24".into(), end: day(24) },
    Snap { id: "d23".into(), end: day(23) },
    Snap { id: "d22".into(), end: day(22) },
];

// keepDaily: 2 — keep the newest per day for the 2 newest days; prune the rest.
let policy: Retention =
    serde_json::from_value(serde_json::json!({ "keepDaily": 2 })).unwrap();
let kept = select_kept(&snaps, &policy);
assert_eq!(kept.keep, vec!["d24", "d23"]); // newest-first
assert_eq!(kept.delete, vec!["d22"]);