const SCROLL_BOUNDARY_THRESHOLD = 2

export type PageScrollBoundary = 'none' | 'top' | 'middle' | 'bottom'

export function getPageScrollBoundary(scrollTop: number): PageScrollBoundary {
  if (typeof window === 'undefined') {
    return 'none'
  }

  const scrollingElement = document.scrollingElement ?? document.documentElement
  const viewportHeight = window.visualViewport?.height ?? window.innerHeight
  const scrollHeight = Math.max(
    scrollingElement.scrollHeight,
    document.documentElement.scrollHeight,
    document.body?.scrollHeight ?? 0
  )
  const maxScrollTop = Math.max(0, scrollHeight - viewportHeight)

  if (maxScrollTop <= SCROLL_BOUNDARY_THRESHOLD) {
    return 'none'
  }

  if (scrollTop <= SCROLL_BOUNDARY_THRESHOLD) {
    return 'top'
  }

  if (maxScrollTop - scrollTop <= SCROLL_BOUNDARY_THRESHOLD) {
    return 'bottom'
  }

  return 'middle'
}