// Pure, DOM-free scroll-decision logic for MessageList. Unit-tested so the
// tricky math is verified without a browser (jsdom has no layout).

export interface ScrollMetrics {
  scrollTop: number;
  scrollHeight: number;
  clientHeight: number;
}

/** Distance from the bottom edge is within `threshold` px. */
export function isNearBottom(m: ScrollMetrics, threshold: number): boolean {
  return m.scrollHeight - (m.scrollTop + m.clientHeight) <= threshold;
}

/** Scroll offset is within `threshold` px of the top. */
export function isNearTop(m: ScrollMetrics, threshold: number): boolean {
  return m.scrollTop <= threshold;
}

export interface SavedPosition {
  topmostIndex: number;
  stickToBottom: boolean;
}

export interface Anchor {
  index: number;
  align: 'start' | 'end';
}

/**
 * Where a freshly-opened chat should start.
 * - default / "left at bottom" → last row, aligned to the viewport bottom.
 * - "left scrolled up" → the saved top-most row, aligned to the viewport top
 *   (clamped in case the cached row count shrank).
 */
export function resolveInitialAnchor(saved: SavedPosition | null, rowCount: number): Anchor {
  if (rowCount <= 0) return { index: 0, align: 'end' };
  if (saved && !saved.stickToBottom) {
    const index = Math.max(0, Math.min(saved.topmostIndex, rowCount - 1));
    return { index, align: 'start' };
  }
  return { index: rowCount - 1, align: 'end' };
}