// lib/group.js — group saved assignments by subject for the sidebar tree and
// the Library page. Shared so both stay in sync.

export function groupBySubject(items) {
  const map = new Map();
  for (const a of items || []) {
    const key = (a.subject && a.subject.trim()) || a.gradeLevel || "Other";
    if (!map.has(key)) map.set(key, []);
    map.get(key).push(a);
  }
  const groups = [...map.entries()].map(([key, list]) => ({
    key,
    items: list.slice().sort((x, y) => String(y.updatedAt || "").localeCompare(String(x.updatedAt || ""))),
  }));
  // Stable, friendly order: alphabetical by subject, "Other" last.
  groups.sort((a, b) => {
    if (a.key === "Other") return 1;
    if (b.key === "Other") return -1;
    return a.key.localeCompare(b.key);
  });
  return groups;
}