// `sectionEnter`: the flag on the first clip of each section, which is what
// makes the legacy footer's marker slide from one node to the next
// (report-to-video/build-video.mjs reads it; README "The marker slides").
//
// report-to-video only ever READS it; the generators author it. This writes
// the rule down for umtool's one writer that re-orders a timeline
// (lib/report/manifest.mjs moveEntry), derived from what the builder reads and
// checked against every real manifest: in the one that carries the flag
// (quartering-employee-count, seven of nineteen clips) a clip carries it
// exactly when its `section` differs from the previous CLIP's -- the first
// clip counts, a card between two clips does not break a section, and a clip
// with no `section` never enters one. Re-applying it to every real manifest
// under ~/reports changes nothing (lib/report/sections.test.mjs has the shape).
//
// `section` itself is the author's: it says which chapter an entry belongs
// to, and a move does not change that. Only the flag follows the order.
//
// A manifest that carries no `sectionEnter` key at all (every deck-era cut:
// the deck draws no footer) is left exactly as it is: `applySectionEnter`
// changes nothing unless some entry already carries the key.

/**
 * The flag each entry should carry, by index: true on a clip whose `section`
 * is set and differs from the previous clip's; false everywhere else.
 *
 * @param {Array<Record<string, unknown>>} timeline
 * @returns {boolean[]}
 */
export function sectionEnterFlags(timeline) {
  let prev;
  return (timeline ?? []).map((e) => {
    if (e?.type !== "clip") return false;
    const s = e.section;
    const enters = s !== undefined && s !== null && s !== prev;
    prev = s;
    return enters;
  });
}

/** Does this timeline use the flag at all? */
export const usesSectionEnter = (timeline) => (timeline ?? []).some((e) => e && Object.hasOwn(e, "sectionEnter"));

/**
 * Recompute `sectionEnter` in place after a re-order. Only on a timeline that
 * already uses it; written as `true` or removed (an absent key reads as false,
 * and `"sectionEnter": false` is noise a human reads as a decision). Returns
 * the ids whose flag changed.
 *
 * @param {Array<Record<string, unknown>>} timeline
 * @returns {string[]}
 */
export function applySectionEnter(timeline) {
  if (!usesSectionEnter(timeline)) return [];
  const flags = sectionEnterFlags(timeline);
  const changed = [];
  timeline.forEach((e, i) => {
    const was = e.sectionEnter === true;
    if (flags[i] === was) return;
    if (flags[i]) e.sectionEnter = true;
    else delete e.sectionEnter;
    changed.push(String(e.id));
  });
  return changed;
}