/**
 * WCAG 1.4.10 Reflow (Level AA) — shared detection for sidebar flyout,
 * compact filter toolbar, disabled column stickies, and single-column nav scroll.
 *
 * Triggers when any of:
 * - viewport width ≤ 320 CSS px (1.4.10 test width),
 * - browser zoom ≥ ~200% (`visualViewport.scale`),
 * - short viewport height (≤ 640px or ≤ 420px).
 *
 * **Windows display scaling (125% / 150%) is not browser zoom.** The OS shrinks
 * the logical CSS viewport and raises DPR; `visualViewport.scale` stays ~1.
 * Do **not** invent an OS-DPI layout mode — rely on CSS px / rem, wrap identity
 * labels (`SHELL_IDENTITY_MENU_SURFACE_CLASS`), and this reflow signal for the
 * WCAG 200% / 320px path. Mid-scale QA: Windows 100/125/150% × browser 100/150/200%.
 */

import { rafThrottle } from "./raf-throttle"

/** WCAG 1.4.10 — content must reflow at 320 CSS px without horizontal scroll. */
export const REFLOW_MAX_WIDTH_PX = 320
export const REFLOW_MAX_WIDTH_MQ = `(max-width: ${REFLOW_MAX_WIDTH_PX}px)`
export const REFLOW_MAX_HEIGHT_MQ = "(max-height: 640px)"
export const COARSE_POINTER_MQ = "(pointer: coarse)"

/** Browser zoom ≥ ~200%, narrow 1.4.10 width, or a genuinely short viewport. */
export function computeReflowViewport(): boolean {
  if (typeof window === "undefined") return false
  const vv = window.visualViewport
  const scale = vv?.scale ?? 1
  const vvWidth = vv?.width
  const narrow =
    window.matchMedia(REFLOW_MAX_WIDTH_MQ).matches ||
    window.innerWidth <= REFLOW_MAX_WIDTH_PX ||
    (typeof vvWidth === "number" && vvWidth > 0 && vvWidth <= REFLOW_MAX_WIDTH_PX)
  const short = window.matchMedia(REFLOW_MAX_HEIGHT_MQ).matches
  const veryShort = window.innerHeight <= 420
  return narrow || scale >= 1.99 || short || veryShort
}

export function computeCoarsePointer(): boolean {
  if (typeof window === "undefined") return false
  return window.matchMedia(COARSE_POINTER_MQ).matches
}

type Listener = () => void

let reflowListeners: Set<Listener> | null = null
let reflowCached = false
let reflowScheduled: ReturnType<typeof rafThrottle> | null = null

function ensureReflowInitialized() {
  if (reflowListeners) return
  reflowListeners = new Set()
  if (typeof window === "undefined") return
  reflowCached = computeReflowViewport()
  reflowScheduled = rafThrottle(() => {
    const next = computeReflowViewport()
    if (next === reflowCached) return
    reflowCached = next
    reflowListeners?.forEach(cb => cb())
  })
  const vv = window.visualViewport
  vv?.addEventListener("resize", reflowScheduled, { passive: true })
  vv?.addEventListener("scroll", reflowScheduled, { passive: true })
  window.addEventListener("resize", reflowScheduled, { passive: true })
  window.matchMedia(REFLOW_MAX_HEIGHT_MQ).addEventListener("change", reflowScheduled)
  window.matchMedia(REFLOW_MAX_WIDTH_MQ).addEventListener("change", reflowScheduled)
}

export function subscribeReflowViewport(callback: Listener): () => void {
  ensureReflowInitialized()
  reflowListeners!.add(callback)
  return () => {
    reflowListeners!.delete(callback)
  }
}

export function getReflowViewportSnapshot(): boolean {
  ensureReflowInitialized()
  return reflowCached
}

export function getServerReflowViewportSnapshot(): boolean {
  return false
}

let coarseListeners: Set<Listener> | null = null
let coarseCached = false

function ensureCoarseInitialized() {
  if (coarseListeners) return
  coarseListeners = new Set()
  if (typeof window === "undefined") return
  coarseCached = computeCoarsePointer()
  const mql = window.matchMedia(COARSE_POINTER_MQ)
  mql.addEventListener("change", () => {
    const next = computeCoarsePointer()
    if (next === coarseCached) return
    coarseCached = next
    coarseListeners?.forEach(cb => cb())
  })
}

export function subscribeCoarsePointer(callback: Listener): () => void {
  ensureCoarseInitialized()
  coarseListeners!.add(callback)
  return () => {
    coarseListeners!.delete(callback)
  }
}

export function getCoarsePointerSnapshot(): boolean {
  ensureCoarseInitialized()
  return coarseCached
}

export function getServerCoarsePointerSnapshot(): boolean {
  return false
}