import { ref } from 'vue'

function createMomentumDrag({
  viewport,
  viewportHeight,
  contentHeight,
  setScrollTop,
  speed,
  reasonDragPointer,
  reasonDragTouch,
  reasonMomentum,
  allowTouch,
  hitTest,
}) {
  let dragging = false
  let startY = 0
  let startScroll = 0
  let velocity = 0
  let samples = [] // { timestamp, position }
  let momentumActive = false
  let momentumFrame = null
  let dragAccumY = 0 // used when pointer lock active
  let usingPointerLock = false
  const frictionPerMs = 0.0018
  const MIN_V = 0.03
  const VELOCITY_MS = 50

  function cancelMomentum() {
    if (!momentumActive) return
    momentumActive = false
    if (momentumFrame) cancelAnimationFrame(momentumFrame)
    momentumFrame = null
  }
  function startMomentum() {
    if (Math.abs(velocity) < MIN_V) return
    cancelMomentum()
    momentumActive = true
    let lastTs = performance.now()
    const step = () => {
      if (!momentumActive) return
      const now = performance.now()
      const dt = now - lastTs
      lastTs = now
      if (dt <= 0) {
        momentumFrame = requestAnimationFrame(step)
        return
      }
      const decay = Math.exp(-frictionPerMs * dt)
      velocity *= decay
      const delta = velocity * dt
      if (viewport.value) {
        let cur = viewport.value.scrollTop
        let target = cur + delta
        const maxScroll = Math.max(0, contentHeight.value - viewportHeight.value)
        if (target < 0) {
          target = 0
          velocity = 0
        } else if (target > maxScroll) {
          target = maxScroll
          velocity = 0
        }
        setScrollTop(target, reasonMomentum)
      }
      if (Math.abs(velocity) < MIN_V * 0.6) {
        momentumActive = false
        return
      }
      momentumFrame = requestAnimationFrame(step)
    }
    momentumFrame = requestAnimationFrame(step)
  }
  function applyDragByDelta(deltaY, reason) {
    const now = performance.now()
    while (samples[0]?.timestamp < now - VELOCITY_MS) samples.shift()
    samples.push({ timestamp: now, position: deltaY })
    const newScrollTop = startScroll - deltaY * speed
    const maxScroll = Math.max(0, contentHeight.value - viewportHeight.value)
    const clamped = Math.max(0, Math.min(newScrollTop, maxScroll))
    setScrollTop(clamped, reason)
  }
  function applyDragPosition(clientY, reason) {
    const deltaY = clientY - startY
    applyDragByDelta(deltaY, reason)
  }
  function endDrag() {
    if (!dragging) return
    dragging = false
    window.removeEventListener('pointermove', onPointerMove, true)
    window.removeEventListener('pointerup', endDrag, true)
    window.removeEventListener('pointercancel', endDrag, true)
    if (allowTouch) {
      window.removeEventListener('touchmove', onTouchMove)
      window.removeEventListener('touchend', endDrag)
      window.removeEventListener('touchcancel', endDrag)
    }
    document.removeEventListener('pointerlockchange', onPointerLockChange, true)
    if (usingPointerLock && document.pointerLockElement === viewport.value) {
      try {
        document.exitPointerLock()
      } catch {}
    }
    usingPointerLock = false
    velocity = 0
    if (samples.length) {
      const first = samples[0]
      const now = performance.now()
      const last = samples[samples.length - 1]
      const dy = last.position - first.position
      if (Math.abs(dy) > 5) velocity = (-dy * speed) / (now - first.timestamp)
    }
    samples = []
    startMomentum()
  }
  function onPointerMove(e) {
    if (!dragging || document.pointerLockElement !== viewport.value) return
    dragAccumY += e.movementY
    applyDragByDelta(dragAccumY, reasonDragPointer)
    e.preventDefault()
  }
  function onTouchMove(e) {
    if (!dragging) return
    if (e.touches.length !== 1) {
      endDrag()
      return
    }
    applyDragPosition(e.touches[0].clientY, reasonDragTouch)
    e.preventDefault()
  }
  function handlePointerDown(e) {
    if (e.button !== undefined && e.button !== 0) return
    if (hitTest && !hitTest(e)) return
    e.preventDefault()
    cancelMomentum()
    dragging = true
    startY = e.clientY
    startScroll = viewport.value?.scrollTop || 0
    velocity = 0
    dragAccumY = 0
    samples = [{ timestamp: performance.now(), position: e.clientY }]
    window.addEventListener('pointermove', onPointerMove, true)
    window.addEventListener('pointerup', endDrag, true)
    window.addEventListener('pointercancel', endDrag, true)
    document.addEventListener('pointerlockchange', onPointerLockChange, true)
    viewport.value.requestPointerLock({ unadjustedMovement: true })
  }
  function handleTouchStart(e) {
    if (!allowTouch) return
    if (e.touches.length !== 1) return
    if (hitTest && !hitTest(e.touches[0])) return
    cancelMomentum()
    dragging = true
    const t = e.touches[0]
    startY = t.clientY
    startScroll = viewport.value?.scrollTop || 0
    velocity = 0
    dragAccumY = 0
    samples = [{ timestamp: performance.now(), position: t.clientY }]
    window.addEventListener('touchmove', onTouchMove, { passive: false })
    window.addEventListener('touchend', endDrag, { passive: false })
    window.addEventListener('touchcancel', endDrag, { passive: false })
    e.preventDefault()
  }
  function onPointerLockChange() {
    const lockedEl = document.pointerLockElement
    if (dragging && lockedEl === viewport.value) {
      usingPointerLock = true
      return
    }
    if (dragging && usingPointerLock && lockedEl !== viewport.value) endDrag()
    if (!dragging) usingPointerLock = false
  }
  return { handlePointerDown, handleTouchStart, cancelMomentum }
}

export function createScrollManager({ viewport, scheduleRebuild }) {
  const scrollTop = ref(0)
  let lastProgrammatic = null
  let pendingTarget = null
  let pendingAttempts = 0
  let pendingLoopActive = false

  function setScrollTop(val, reason = 'programmatic') {
    let applied = val
    if (viewport.value) {
      const maxScroll = viewport.value.scrollHeight - viewport.value.clientHeight
      if (applied > maxScroll) {
        applied = maxScroll < 0 ? 0 : maxScroll
        pendingTarget = val
        pendingAttempts = 0
        startPendingLoop()
      }
      if (applied < 0) applied = 0
      viewport.value.scrollTop = applied
    }
    scrollTop.value = applied
    lastProgrammatic = applied
    scheduleRebuild(reason)
  }

  function onScroll() {
    if (!viewport.value) return
    const cur = viewport.value.scrollTop
    const maxScroll = viewport.value.scrollHeight - viewport.value.clientHeight
    let effective = cur
    if (cur < 0) effective = 0
    else if (cur > maxScroll) effective = maxScroll
    scrollTop.value = effective
    if (lastProgrammatic !== null && effective === lastProgrammatic) {
      lastProgrammatic = null
      return
    }
    if (pendingTarget !== null && Math.abs(effective - pendingTarget) > 4) {
      pendingTarget = null
    }
    scheduleRebuild('scroll')
  }

  function startPendingLoop() {
    if (pendingLoopActive || !viewport.value) return
    pendingLoopActive = true
    const loop = () => {
      if (pendingTarget == null || !viewport.value) {
        pendingLoopActive = false
        return
      }
      const maxScroll = viewport.value.scrollHeight - viewport.value.clientHeight
      if (pendingTarget <= maxScroll) {
        setScrollTop(pendingTarget, 'pending-fulfill')
        pendingTarget = null
        pendingLoopActive = false
        return
      }
      pendingAttempts++
      if (pendingAttempts > 120) {
        pendingTarget = null
        pendingLoopActive = false
        return
      }
      requestAnimationFrame(loop)
    }
    requestAnimationFrame(loop)
  }

  return { scrollTop, setScrollTop, onScroll }
}

export function createWeekColumnScrollManager({
  viewport,
  viewportHeight,
  contentHeight,
  setScrollTop,
}) {
  const isWeekColDragging = ref(false)
  function getWeekLabelRect() {
    const headerYear = document.querySelector('.calendar-header .year-label')
    if (headerYear) return headerYear.getBoundingClientRect()
    const weekLabel = viewport.value?.querySelector('.week-row .week-label')
    return weekLabel ? weekLabel.getBoundingClientRect() : null
  }
  const drag = createMomentumDrag({
    viewport,
    viewportHeight,
    contentHeight,
    setScrollTop,
    speed: 1,
    reasonDragPointer: 'week-col-drag',
    reasonDragTouch: 'week-col-drag',
    reasonMomentum: 'week-col-momentum',
    allowTouch: false,
    hitTest: (e) => {
      const rect = getWeekLabelRect()
      if (!rect) return false
      const x = e.clientX ?? e.pageX
      return x >= rect.left && x <= rect.right
    },
  })
  function handleWeekColMouseDown(e) {
    if (e.ctrlKey || e.metaKey || e.altKey || e.shiftKey) return
    isWeekColDragging.value = true
    drag.handlePointerDown(e)
    const end = () => {
      isWeekColDragging.value = false
      window.removeEventListener('pointerup', end, true)
      window.removeEventListener('pointercancel', end, true)
    }
    window.addEventListener('pointerup', end, true)
    window.addEventListener('pointercancel', end, true)
  }
  function handlePointerLockChange() {
    if (document.pointerLockElement !== viewport.value) {
      isWeekColDragging.value = false
    }
  }
  return { isWeekColDragging, handleWeekColMouseDown, handlePointerLockChange }
}

export function createMonthScrollManager({
  viewport,
  viewportHeight,
  contentHeight,
  setScrollTop,
}) {
  const drag = createMomentumDrag({
    viewport,
    viewportHeight,
    contentHeight,
    setScrollTop,
    speed: 10,
    reasonDragPointer: 'month-scroll-drag',
    reasonDragTouch: 'month-scroll-touch',
    reasonMomentum: 'month-scroll-momentum',
    allowTouch: true,
    hitTest: null,
  })
  function handleMonthScrollPointerDown(e) {
    drag.handlePointerDown(e)
  }
  function handleMonthScrollTouchStart(e) {
    drag.handleTouchStart(e)
  }
  function handleMonthScrollWheel(e) {
    drag.cancelMomentum()
    const currentScroll = viewport.value?.scrollTop || 0
    const newScrollTop = currentScroll + e.deltaY * 10
    const maxScroll = Math.max(0, contentHeight.value - viewportHeight.value)
    const clamped = Math.max(0, Math.min(newScrollTop, maxScroll))
    setScrollTop(clamped, 'month-scroll-wheel')
    e.preventDefault()
  }
  return { handleMonthScrollPointerDown, handleMonthScrollTouchStart, handleMonthScrollWheel }
}