/**
 * useScrollSpy — reusable scroll-spy hook with URL hash sync.
 *
 * Observes a list of section elements by ID and returns the currently
 * visible one. Optionally syncs the URL hash and enables smooth scrolling.
 *
 * Analytics-agnostic: use `onSectionVisible` to fire tracking events.
 *
 * @example
 * ```tsx
 * const activeId = useScrollSpy(['hero', 'findings', 'table'], {
 *   onSectionVisible: (id) => trackView({ contentId: id, contentType: 'page' }),
 * })
 * ```
 */

import { useState, useEffect, useRef } from 'react'

export interface ScrollSpyOptions {
  /** Root margin for the IntersectionObserver. Default: '-80px 0px -50% 0px' */
  rootMargin?: string
  /** Enable smooth scrolling on the document element. Default: true */
  enableSmoothScroll?: boolean
  /** Sync the active section ID to the URL hash via replaceState. Default: true */
  updateUrlHash?: boolean
  /** Scroll to the URL hash fragment on mount. Default: true */
  scrollToHashOnMount?: boolean
  /** Called once per section when it first enters the viewport */
  onSectionVisible?: (id: string) => void
}

const DEFAULT_ROOT_MARGIN = '-80px 0px -50% 0px'

export function useScrollSpy(
  sectionIds: string[],
  options: ScrollSpyOptions = {},
): string {
  const {
    rootMargin = DEFAULT_ROOT_MARGIN,
    enableSmoothScroll = true,
    updateUrlHash = true,
    scrollToHashOnMount = true,
    onSectionVisible,
  } = options

  const [activeId, setActiveId] = useState('')
  const viewedRef = useRef(new Set<string>())
  const onVisibleRef = useRef(onSectionVisible)
  onVisibleRef.current = onSectionVisible

  // Enable smooth scrolling while mounted
  useEffect(() => {
    if (!enableSmoothScroll) return

    document.documentElement.style.scrollBehavior = 'smooth'
    return () => {
      document.documentElement.style.scrollBehavior = ''
    }
  }, [enableSmoothScroll])

  // Scroll to hash on mount
  useEffect(() => {
    if (!scrollToHashOnMount) return

    const hash = window.location.hash.slice(1)
    if (!hash) return

    requestAnimationFrame(() => {
      const el = document.getElementById(hash)
      if (el) {
        el.scrollIntoView({ behavior: 'smooth', block: 'start' })
        setActiveId(hash)
      }
    })
  }, [scrollToHashOnMount])

  // IntersectionObserver for scroll spy
  useEffect(() => {
    const observer = new IntersectionObserver(
      (entries) => {
        for (const entry of entries) {
          if (!entry.isIntersecting) continue

          const { id } = entry.target
          if (!id) continue

          setActiveId(id)

          if (updateUrlHash) {
            window.history.replaceState(null, '', `#${id}`)
          }

          if (!viewedRef.current.has(id)) {
            viewedRef.current.add(id)
            onVisibleRef.current?.(id)
          }
        }
      },
      { rootMargin, threshold: 0 },
    )

    for (const id of sectionIds) {
      const el = document.getElementById(id)
      if (el) observer.observe(el)
    }

    return () => observer.disconnect()
  }, [sectionIds, rootMargin, updateUrlHash])

  return activeId
}