/**
 * @file keepIfSame.ts
 * @description State-updater helper for polled data: keeps the previous value's
 * identity when the freshly fetched payload is value-identical, so a background
 * poll that found nothing new does not re-render the tree it feeds.
 *
 * Every poller in this app (`api.lanes.list`, `api.run.list`, `api.lanes.git`,
 * …) returns a brand-new object graph on each tick. Passing that straight into
 * `setState` re-renders every consumer several times a minute even when the
 * dashboard is completely idle — visible as the whole page "refreshing".
 *
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

/**
 * Wraps a freshly fetched value as a functional state update that returns the
 * previous state unchanged when the two are value-equal.
 *
 * Usage: `setLanes(keepIfSame(r.lanes))` instead of `setLanes(r.lanes)`.
 *
 * `prev` is typed wider than `T` so the same helper works for the `T | null`
 * states the pollers start out in; the cast is sound because it only happens on
 * the branch where `prev` serializes identically to `next`.
 *
 * @param next The value just fetched.
 * @returns An updater for `setState`, yielding `prev` when nothing changed.
 */
export function keepIfSame<T>(next: T): (prev: T | null | undefined) => T {
  // ponytail: JSON.stringify comparison — fine for the payloads this app polls
  // (lane lists, run lists, a git-facts object: tens of small records, key order
  // is stable because it comes from the same serializer every time). Swap for a
  // structural deep-equal if a poller ever returns something big or key-unstable.
  return (prev) => (JSON.stringify(prev) === JSON.stringify(next) ? (prev as T) : next);
}