/**
 * Index of the offset nearest to a scroll position.
 *
 * Used to derive the "current" slide from the gallery's real scroll offset
 * rather than from whichever item last crossed an IntersectionObserver
 * threshold — which, when several items are visible at once, is
 * direction-dependent and causes the focused index to drift.
 *
 * Ties resolve to the lower index. Returns 0 for an empty list.
 */
export const nearestIndex = (
  offsets: number[],
  scrollStart: number,
): number => {
  let bestIndex = 0
  let bestDistance = Number.POSITIVE_INFINITY
  for (let i = 0; i < offsets.length; i++) {
    const distance = Math.abs(offsets[i] - scrollStart)
    if (distance >= bestDistance) return bestIndex
    bestDistance = distance
    bestIndex = i
  }
  return bestIndex
}

/**
 * Whether a horizontal scroll container sits at its start / end edge, computed
 * from real scroll geometry.
 *
 * Index-based edge detection is wrong when several items are visible at once:
 * at the true end the left-most (focused) item is not the last item, so a
 * `focusedIndex === itemCount - 1` check never becomes true and a "next"
 * control never disables on its own.
 */
export const scrollEdges = (
  scrollLeft: number,
  scrollWidth: number,
  clientWidth: number,
  tolerance = 1,
): { atStart: boolean; atEnd: boolean } => {
  const max = scrollWidth - clientWidth
  return {
    atStart: scrollLeft <= tolerance,
    atEnd: scrollLeft >= max - tolerance,
  }
}