// Infinite-scroll read-ahead policy for VirtualGrid, extracted as a pure function so the decision is
// unit-testable without a layout engine.
//
// The next page is pulled once the last rendered row comes within `lookahead` rows of the loaded end,
// where lookahead scales with the number of rows the viewport currently renders (so it adapts to
// window height / column count / row height) but is clamped to a floor and a ceiling:
//   - floor PREFETCH_ROWS: the finite-list / first-paint case where nothing is rendered yet.
//   - ceiling MAX_LOOKAHEAD_ROWS: a tall window renders many rows, and an uncapped lookahead would
//     eagerly prefetch several pages on every mount / filter change — wasted work on the app's
//     heaviest query (the feed). The cap bounds that while still buffering a couple of screens ahead.
export const PREFETCH_ROWS = 4;
export const PREFETCH_VIEWPORTS = 2;
export const MAX_LOOKAHEAD_ROWS = 40;

/** How many rows ahead of the last rendered row to keep buffered, for a viewport rendering
 *  `renderedRows` rows. Clamped to [PREFETCH_ROWS, MAX_LOOKAHEAD_ROWS]. */
export function readAheadLookahead(renderedRows: number): number {
  return Math.min(MAX_LOOKAHEAD_ROWS, Math.max(PREFETCH_ROWS, renderedRows * PREFETCH_VIEWPORTS));
}

/** Whether to fetch the next page: true once the last rendered row (`lastIndex`) is within the
 *  read-ahead window of the loaded end (`rowCount` total loaded rows). Because the caller re-runs
 *  this as each page settles, it chains — refilling the buffer as the user consumes it. */
export function shouldPrefetchNext(
  lastIndex: number,
  rowCount: number,
  renderedRows: number,
): boolean {
  return lastIndex >= rowCount - 1 - readAheadLookahead(renderedRows);
}