core/packages/runtime-core/src/renderer.ts

import {
  Text,
  Fragment,
  Comment,
  Portal,
  cloneIfMounted,
  normalizeVNode,
  VNode,
  VNodeArrayChildren,
  createVNode,
  isSameVNodeType
} from './vnode'
import {
  ComponentInternalInstance,
  createComponentInstance,
  Component,
  Data,
  setupComponent
} from './component'
import {
  renderComponentRoot,
  shouldUpdateComponent,
  updateHOCHostEl
} from './componentRenderUtils'
import {
  isString,
  EMPTY_OBJ,
  EMPTY_ARR,
  isReservedProp,
  isFunction,
  PatchFlags
} from '@vue/shared'
import { queueJob, queuePostFlushCb, flushPostFlushCbs } from './scheduler'
import {
  effect,
  stop,
  ReactiveEffectOptions,
  isRef,
  Ref,
  toRaw,
  DebuggerEvent
} from '@vue/reactivity'
import { resolveProps } from './componentProps'
import { resolveSlots } from './componentSlots'
import { ShapeFlags } from './shapeFlags'
import { pushWarningContext, popWarningContext, warn } from './warning'
import { invokeDirectiveHook } from './directives'
import { ComponentPublicInstance } from './componentProxy'
import { createAppAPI, CreateAppFunction } from './apiCreateApp'
import {
  SuspenseBoundary,
  queueEffectWithSuspense,
  SuspenseImpl
} from './components/Suspense'
import { ErrorCodes, callWithErrorHandling } from './errorHandling'
import { KeepAliveSink, isKeepAlive } from './components/KeepAlive'
import { registerHMR, unregisterHMR } from './hmr'

const __HMR__ = __BUNDLER__ && __DEV__

export interface RendererOptions<HostNode = any, HostElement = any> {
  patchProp(
    el: HostElement,
    key: string,
    value: any,
    oldValue: any,
    isSVG?: boolean,
    prevChildren?: VNode<HostNode, HostElement>[],
    parentComponent?: ComponentInternalInstance | null,
    parentSuspense?: SuspenseBoundary<HostNode, HostElement> | null,
    unmountChildren?: (
      children: VNode<HostNode, HostElement>[],
      parentComponent: ComponentInternalInstance | null,
      parentSuspense: SuspenseBoundary<HostNode, HostElement> | null
    ) => void
  ): void
  insert(el: HostNode, parent: HostElement, anchor?: HostNode | null): void
  remove(el: HostNode): void
  createElement(type: string, isSVG?: boolean): HostElement
  createText(text: string): HostNode
  createComment(text: string): HostNode
  setText(node: HostNode, text: string): void
  setElementText(node: HostElement, text: string): void
  parentNode(node: HostNode): HostElement | null
  nextSibling(node: HostNode): HostNode | null
  querySelector(selector: string): HostElement | null
  setScopeId(el: HostNode, id: string): void
}

export type RootRenderFunction<HostNode, HostElement> = (
  vnode: VNode<HostNode, HostElement> | null,
  dom: HostElement
) => void

// An object exposing the internals of a renderer, passed to tree-shakeable
// features so that they can be decoupled from this file.
export interface RendererInternals<HostNode = any, HostElement = any> {
  patch: (
    n1: VNode<HostNode, HostElement> | null, // null means this is a mount
    n2: VNode<HostNode, HostElement>,
    container: HostElement,
    anchor?: HostNode | null,
    parentComponent?: ComponentInternalInstance | null,
    parentSuspense?: SuspenseBoundary<HostNode, HostElement> | null,
    isSVG?: boolean,
    optimized?: boolean
  ) => void
  unmount: (
    vnode: VNode<HostNode, HostElement>,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: SuspenseBoundary<HostNode, HostElement> | null,
    doRemove?: boolean
  ) => void
  move: (
    vnode: VNode<HostNode, HostElement>,
    container: HostElement,
    anchor: HostNode | null,
    type: MoveType,
    parentSuspense?: SuspenseBoundary<HostNode, HostElement> | null
  ) => void
  next: (vnode: VNode<HostNode, HostElement>) => HostNode | null
  options: RendererOptions<HostNode, HostElement>
}

export const enum MoveType {
  ENTER,
  LEAVE,
  REORDER
}

const prodEffectOptions = {
  scheduler: queueJob
}

function createDevEffectOptions(
  instance: ComponentInternalInstance
): ReactiveEffectOptions {
  return {
    scheduler: queueJob,
    onTrack: instance.rtc ? e => invokeHooks(instance.rtc!, e) : void 0,
    onTrigger: instance.rtg ? e => invokeHooks(instance.rtg!, e) : void 0
  }
}

export function invokeHooks(hooks: Function[], arg?: DebuggerEvent) {
  for (let i = 0; i < hooks.length; i++) {
    hooks[i](arg)
  }
}

export const queuePostRenderEffect = __FEATURE_SUSPENSE__
  ? queueEffectWithSuspense
  : queuePostFlushCb

/**
 * The createRenderer function accepts two generic arguments:
 * HostNode and HostElement, corresponding to Node and Element types in the
 * host environment. For example, for runtime-dom, HostNode would be the DOM
 * `Node` interface and HostElement would be the DOM `Element` interface.
 *
 * Custom renderers can pass in the platform specific types like this:
 *
 * ``` js
 * const { render, createApp } = createRenderer<Node, Element>({
 *   patchProp,
 *   ...nodeOps
 * })
 * ```
 */
export function createRenderer<
  HostNode extends object = any,
  HostElement extends HostNode = any
>(
  options: RendererOptions<HostNode, HostElement>
): {
  render: RootRenderFunction<HostNode, HostElement>
  createApp: CreateAppFunction<HostElement>
} {
  type HostVNode = VNode<HostNode, HostElement>
  type HostVNodeChildren = VNodeArrayChildren<HostNode, HostElement>
  type HostSuspenseBoundary = SuspenseBoundary<HostNode, HostElement>

  const {
    insert: hostInsert,
    remove: hostRemove,
    patchProp: hostPatchProp,
    createElement: hostCreateElement,
    createText: hostCreateText,
    createComment: hostCreateComment,
    setText: hostSetText,
    setElementText: hostSetElementText,
    parentNode: hostParentNode,
    nextSibling: hostNextSibling,
    querySelector: hostQuerySelector,
    setScopeId: hostSetScopeId
  } = options

  const internals: RendererInternals<HostNode, HostElement> = {
    patch,
    unmount,
    move,
    next: getNextHostNode,
    options
  }

  function patch(
    n1: HostVNode | null, // null means this is a mount
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null = null,
    parentComponent: ComponentInternalInstance | null = null,
    parentSuspense: HostSuspenseBoundary | null = null,
    isSVG: boolean = false,
    optimized: boolean = false
  ) {
    // patching & not same type, unmount old tree
    if (n1 != null && !isSameVNodeType(n1, n2)) {
      anchor = getNextHostNode(n1)
      unmount(n1, parentComponent, parentSuspense, true)
      n1 = null
    }

    const { type, shapeFlag } = n2
    switch (type) {
      case Text:
        processText(n1, n2, container, anchor)
        break
      case Comment:
        processCommentNode(n1, n2, container, anchor)
        break
      case Fragment:
        processFragment(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
        break
      case Portal:
        processPortal(
          n1,
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
        break
      default:
        if (shapeFlag & ShapeFlags.ELEMENT) {
          processElement(
            n1,
            n2,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized
          )
        } else if (shapeFlag & ShapeFlags.COMPONENT) {
          processComponent(
            n1,
            n2,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized
          )
        } else if (__FEATURE_SUSPENSE__ && shapeFlag & ShapeFlags.SUSPENSE) {
          ;(type as typeof SuspenseImpl).process(
            n1,
            n2,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized,
            internals
          )
        } else if (__DEV__) {
          warn('Invalid HostVNode type:', type, `(${typeof type})`)
        }
    }
  }

  function processText(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null
  ) {
    if (n1 == null) {
      hostInsert(
        (n2.el = hostCreateText(n2.children as string)),
        container,
        anchor
      )
    } else {
      const el = (n2.el = n1.el) as HostNode
      if (n2.children !== n1.children) {
        hostSetText(el, n2.children as string)
      }
    }
  }

  function processCommentNode(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null
  ) {
    if (n1 == null) {
      hostInsert(
        (n2.el = hostCreateComment((n2.children as string) || '')),
        container,
        anchor
      )
    } else {
      // there's no support for dynamic comments
      n2.el = n1.el
    }
  }

  function processElement(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    isSVG = isSVG || (n2.type as string) === 'svg'
    if (n1 == null) {
      mountElement(
        n2,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        optimized
      )
    } else {
      patchElement(n1, n2, parentComponent, parentSuspense, isSVG, optimized)
    }
    if (n2.ref !== null && parentComponent !== null) {
      setRef(n2.ref, n1 && n1.ref, parentComponent, n2.el)
    }
  }

  function mountElement(
    vnode: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    const el = (vnode.el = hostCreateElement(vnode.type as string, isSVG))
    const { type, props, shapeFlag, transition, scopeId } = vnode

    // props
    if (props != null) {
      for (const key in props) {
        if (isReservedProp(key)) continue
        hostPatchProp(el, key, props[key], null, isSVG)
      }
      if (props.onVnodeBeforeMount != null) {
        invokeDirectiveHook(props.onVnodeBeforeMount, parentComponent, vnode)
      }
    }

    // scopeId
    if (__BUNDLER__) {
      if (scopeId !== null) {
        hostSetScopeId(el, scopeId)
      }
      const treeOwnerId = parentComponent && parentComponent.type.__scopeId
      // vnode's own scopeId and the current patched component's scopeId is
      // different - this is a slot content node.
      if (treeOwnerId != null && treeOwnerId !== scopeId) {
        hostSetScopeId(el, treeOwnerId + '-s')
      }
    }

    // children
    if (shapeFlag & ShapeFlags.TEXT_CHILDREN) {
      hostSetElementText(el, vnode.children as string)
    } else if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
      mountChildren(
        vnode.children as HostVNodeChildren,
        el,
        null,
        parentComponent,
        parentSuspense,
        isSVG && type !== 'foreignObject',
        optimized || vnode.dynamicChildren !== null
      )
    }
    if (transition != null && !transition.persisted) {
      transition.beforeEnter(el)
    }
    hostInsert(el, container, anchor)
    const vnodeMountedHook = props && props.onVnodeMounted
    if (
      vnodeMountedHook != null ||
      (transition != null && !transition.persisted)
    ) {
      queuePostRenderEffect(() => {
        vnodeMountedHook &&
          invokeDirectiveHook(vnodeMountedHook, parentComponent, vnode)
        transition && !transition.persisted && transition.enter(el)
      }, parentSuspense)
    }
  }

  function mountChildren(
    children: HostVNodeChildren,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean,
    start: number = 0
  ) {
    for (let i = start; i < children.length; i++) {
      const child = (children[i] = optimized
        ? cloneIfMounted(children[i] as HostVNode)
        : normalizeVNode(children[i]))
      patch(
        null,
        child,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        optimized
      )
    }
  }

  function patchElement(
    n1: HostVNode,
    n2: HostVNode,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    const el = (n2.el = n1.el) as HostElement
    let { patchFlag, dynamicChildren } = n2
    const oldProps = (n1 && n1.props) || EMPTY_OBJ
    const newProps = n2.props || EMPTY_OBJ

    if (newProps.onVnodeBeforeUpdate != null) {
      invokeDirectiveHook(newProps.onVnodeBeforeUpdate, parentComponent, n2, n1)
    }

    if (__HMR__ && parentComponent && parentComponent.renderUpdated) {
      // HMR updated, force full diff
      patchFlag = 0
      optimized = false
      dynamicChildren = null
    }

    if (patchFlag > 0) {
      // the presence of a patchFlag means this element's render code was
      // generated by the compiler and can take the fast path.
      // in this path old node and new node are guaranteed to have the same shape
      // (i.e. at the exact same position in the source template)

      if (patchFlag & PatchFlags.FULL_PROPS) {
        // element props contain dynamic keys, full diff needed
        patchProps(
          el,
          n2,
          oldProps,
          newProps,
          parentComponent,
          parentSuspense,
          isSVG
        )
      } else {
        // class
        // this flag is matched when the element has dynamic class bindings.
        if (patchFlag & PatchFlags.CLASS) {
          if (oldProps.class !== newProps.class) {
            hostPatchProp(el, 'class', newProps.class, null, isSVG)
          }
        }

        // style
        // this flag is matched when the element has dynamic style bindings
        if (patchFlag & PatchFlags.STYLE) {
          hostPatchProp(el, 'style', newProps.style, oldProps.style, isSVG)
        }

        // props
        // This flag is matched when the element has dynamic prop/attr bindings
        // other than class and style. The keys of dynamic prop/attrs are saved for
        // faster iteration.
        // Note dynamic keys like :[foo]="bar" will cause this optimization to
        // bail out and go through a full diff because we need to unset the old key
        if (patchFlag & PatchFlags.PROPS) {
          // if the flag is present then dynamicProps must be non-null
          const propsToUpdate = n2.dynamicProps!
          for (let i = 0; i < propsToUpdate.length; i++) {
            const key = propsToUpdate[i]
            const prev = oldProps[key]
            const next = newProps[key]
            if (prev !== next) {
              hostPatchProp(
                el,
                key,
                next,
                prev,
                isSVG,
                n1.children as HostVNode[],
                parentComponent,
                parentSuspense,
                unmountChildren
              )
            }
          }
        }
      }

      // text
      // This flag is matched when the element has only dynamic text children.
      // this flag is terminal (i.e. skips children diffing).
      if (patchFlag & PatchFlags.TEXT) {
        if (n1.children !== n2.children) {
          hostSetElementText(el, n2.children as string)
        }
        return // terminal
      }
    } else if (!optimized && dynamicChildren == null) {
      // unoptimized, full diff
      patchProps(
        el,
        n2,
        oldProps,
        newProps,
        parentComponent,
        parentSuspense,
        isSVG
      )
    }

    const areChildrenSVG = isSVG && n2.type !== 'foreignObject'
    if (dynamicChildren != null) {
      patchBlockChildren(
        n1.dynamicChildren!,
        dynamicChildren,
        el,
        parentComponent,
        parentSuspense,
        areChildrenSVG
      )
    } else if (!optimized) {
      // full diff
      patchChildren(
        n1,
        n2,
        el,
        null,
        parentComponent,
        parentSuspense,
        areChildrenSVG
      )
    }

    if (newProps.onVnodeUpdated != null) {
      queuePostRenderEffect(() => {
        invokeDirectiveHook(newProps.onVnodeUpdated, parentComponent, n2, n1)
      }, parentSuspense)
    }
  }

  // The fast path for blocks.
  function patchBlockChildren(
    oldChildren: HostVNode[],
    newChildren: HostVNode[],
    fallbackContainer: HostElement,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean
  ) {
    for (let i = 0; i < newChildren.length; i++) {
      const oldVNode = oldChildren[i]
      const newVNode = newChildren[i]
      // Determine the container (parent element) for the patch.
      const container =
        // - In the case of a Fragment, we need to provide the actual parent
        // of the Fragment itself so it can move its children.
        oldVNode.type === Fragment ||
        // - In the case of different nodes, there is going to be a replacement
        // which also requires the correct parent container
        !isSameVNodeType(oldVNode, newVNode) ||
        // - In the case of a component, it could contain anything.
        oldVNode.shapeFlag & ShapeFlags.COMPONENT
          ? hostParentNode(oldVNode.el!)!
          : // In other cases, the parent container is not actually used so we
            // just pass the block element here to avoid a DOM parentNode call.
            fallbackContainer
      patch(
        oldVNode,
        newVNode,
        container,
        null,
        parentComponent,
        parentSuspense,
        isSVG,
        true
      )
    }
  }

  function patchProps(
    el: HostElement,
    vnode: HostVNode,
    oldProps: Data,
    newProps: Data,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean
  ) {
    if (oldProps !== newProps) {
      for (const key in newProps) {
        if (isReservedProp(key)) continue
        const next = newProps[key]
        const prev = oldProps[key]
        if (next !== prev) {
          hostPatchProp(
            el,
            key,
            next,
            prev,
            isSVG,
            vnode.children as HostVNode[],
            parentComponent,
            parentSuspense,
            unmountChildren
          )
        }
      }
      if (oldProps !== EMPTY_OBJ) {
        for (const key in oldProps) {
          if (!isReservedProp(key) && !(key in newProps)) {
            hostPatchProp(
              el,
              key,
              null,
              null,
              isSVG,
              vnode.children as HostVNode[],
              parentComponent,
              parentSuspense,
              unmountChildren
            )
          }
        }
      }
    }
  }

  let devFragmentID = 0

  function processFragment(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    const showID = __DEV__ && !__TEST__
    const fragmentStartAnchor = (n2.el = n1
      ? n1.el
      : hostCreateComment(showID ? `fragment-${devFragmentID}-start` : ''))!
    const fragmentEndAnchor = (n2.anchor = n1
      ? n1.anchor
      : hostCreateComment(showID ? `fragment-${devFragmentID}-end` : ''))!

    let { patchFlag, dynamicChildren } = n2
    if (patchFlag > 0) {
      optimized = true
    }

    if (__HMR__ && parentComponent && parentComponent.renderUpdated) {
      // HMR updated, force full diff
      patchFlag = 0
      optimized = false
      dynamicChildren = null
    }

    if (n1 == null) {
      if (showID) {
        devFragmentID++
      }
      hostInsert(fragmentStartAnchor, container, anchor)
      hostInsert(fragmentEndAnchor, container, anchor)
      // a fragment can only have array children
      // since they are either generated by the compiler, or implicitly created
      // from arrays.
      mountChildren(
        n2.children as HostVNodeChildren,
        container,
        fragmentEndAnchor,
        parentComponent,
        parentSuspense,
        isSVG,
        optimized
      )
    } else {
      if (patchFlag & PatchFlags.STABLE_FRAGMENT && dynamicChildren != null) {
        // a stable fragment (template root or <template v-for>) doesn't need to
        // patch children order, but it may contain dynamicChildren.
        patchBlockChildren(
          n1.dynamicChildren!,
          dynamicChildren,
          container,
          parentComponent,
          parentSuspense,
          isSVG
        )
      } else {
        // keyed / unkeyed, or manual fragments.
        // for keyed & unkeyed, since they are compiler generated from v-for,
        // each child is guaranteed to be a block so the fragment will never
        // have dynamicChildren.
        patchChildren(
          n1,
          n2,
          container,
          fragmentEndAnchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
      }
    }
  }

  function processPortal(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    const targetSelector = n2.props && n2.props.target
    const { patchFlag, shapeFlag, children } = n2
    if (n1 == null) {
      const target = (n2.target = isString(targetSelector)
        ? hostQuerySelector(targetSelector)
        : targetSelector)
      if (target != null) {
        if (shapeFlag & ShapeFlags.TEXT_CHILDREN) {
          hostSetElementText(target, children as string)
        } else if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
          mountChildren(
            children as HostVNodeChildren,
            target,
            null,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized
          )
        }
      } else if (__DEV__) {
        warn('Invalid Portal target on mount:', target, `(${typeof target})`)
      }
    } else {
      // update content
      const target = (n2.target = n1.target)!
      if (patchFlag === PatchFlags.TEXT) {
        hostSetElementText(target, children as string)
      } else if (n2.dynamicChildren) {
        // fast path when the portal happens to be a block root
        patchBlockChildren(
          n1.dynamicChildren!,
          n2.dynamicChildren,
          container,
          parentComponent,
          parentSuspense,
          isSVG
        )
      } else if (!optimized) {
        patchChildren(
          n1,
          n2,
          target,
          null,
          parentComponent,
          parentSuspense,
          isSVG
        )
      }
      // target changed
      if (targetSelector !== (n1.props && n1.props.target)) {
        const nextTarget = (n2.target = isString(targetSelector)
          ? hostQuerySelector(targetSelector)
          : targetSelector)
        if (nextTarget != null) {
          // move content
          if (shapeFlag & ShapeFlags.TEXT_CHILDREN) {
            hostSetElementText(target, '')
            hostSetElementText(nextTarget, children as string)
          } else if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
            for (let i = 0; i < (children as HostVNode[]).length; i++) {
              move(
                (children as HostVNode[])[i],
                nextTarget,
                null,
                MoveType.REORDER
              )
            }
          }
        } else if (__DEV__) {
          warn('Invalid Portal target on update:', target, `(${typeof target})`)
        }
      }
    }
    // insert an empty node as the placeholder for the portal
    processCommentNode(n1, n2, container, anchor)
  }

  function processComponent(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    if (n1 == null) {
      if (n2.shapeFlag & ShapeFlags.COMPONENT_KEPT_ALIVE) {
        ;(parentComponent!.sink as KeepAliveSink).activate(
          n2,
          container,
          anchor
        )
      } else {
        mountComponent(
          n2,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG
        )
      }
    } else {
      const instance = (n2.component = n1.component)!

      if (shouldUpdateComponent(n1, n2, parentComponent, optimized)) {
        if (
          __FEATURE_SUSPENSE__ &&
          instance.asyncDep &&
          !instance.asyncResolved
        ) {
          // async & still pending - just update props and slots
          // since the component's reactive effect for render isn't set-up yet
          if (__DEV__) {
            pushWarningContext(n2)
          }
          updateComponentPreRender(instance, n2)
          if (__DEV__) {
            popWarningContext()
          }
          return
        } else {
          // normal update
          instance.next = n2
          // instance.update is the reactive effect runner.
          instance.update()
        }
      } else {
        // no update needed. just copy over properties
        n2.component = n1.component
        n2.el = n1.el
      }
    }
    if (n2.ref !== null && parentComponent !== null) {
      if (__DEV__ && !(n2.shapeFlag & ShapeFlags.STATEFUL_COMPONENT)) {
        pushWarningContext(n2)
        warn(
          `Functional components do not support "ref" because they do not ` +
            `have instances.`
        )
        popWarningContext()
      }
      setRef(n2.ref, n1 && n1.ref, parentComponent, n2.component!.proxy)
    }
  }

  function mountComponent(
    initialVNode: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean
  ) {
    const instance: ComponentInternalInstance = (initialVNode.component = createComponentInstance(
      initialVNode,
      parentComponent
    ))

    if (__HMR__ && instance.type.__hmrId != null) {
      registerHMR(instance)
    }

    if (__DEV__) {
      pushWarningContext(initialVNode)
    }

    // inject renderer internals for keepAlive
    if (isKeepAlive(initialVNode)) {
      const sink = instance.sink as KeepAliveSink
      sink.renderer = internals
      sink.parentSuspense = parentSuspense
    }

    // resolve props and slots for setup context
    setupComponent(instance, parentSuspense)

    // setup() is async. This component relies on async logic to be resolved
    // before proceeding
    if (__FEATURE_SUSPENSE__ && instance.asyncDep) {
      if (!parentSuspense) {
        if (__DEV__) warn('async setup() is used without a suspense boundary!')
        return
      }

      parentSuspense.registerDep(instance, setupRenderEffect)

      // give it a placeholder
      const placeholder = (instance.subTree = createVNode(Comment))
      processCommentNode(null, placeholder, container, anchor)
      initialVNode.el = placeholder.el
      return
    }

    setupRenderEffect(
      instance,
      parentSuspense,
      initialVNode,
      container,
      anchor,
      isSVG
    )

    if (__DEV__) {
      popWarningContext()
    }
  }

  function setupRenderEffect(
    instance: ComponentInternalInstance,
    parentSuspense: HostSuspenseBoundary | null,
    initialVNode: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    isSVG: boolean
  ) {
    // create reactive effect for rendering
    instance.update = effect(function componentEffect() {
      if (!instance.isMounted) {
        const subTree = (instance.subTree = renderComponentRoot(instance))
        // beforeMount hook
        if (instance.bm !== null) {
          invokeHooks(instance.bm)
        }
        patch(null, subTree, container, anchor, instance, parentSuspense, isSVG)
        initialVNode.el = subTree.el
        // mounted hook
        if (instance.m !== null) {
          queuePostRenderEffect(instance.m, parentSuspense)
        }
        // activated hook for keep-alive roots.
        if (
          instance.a !== null &&
          instance.vnode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE
        ) {
          queuePostRenderEffect(instance.a, parentSuspense)
        }
        instance.isMounted = true
      } else {
        // updateComponent
        // This is triggered by mutation of component's own state (next: null)
        // OR parent calling processComponent (next: HostVNode)
        const { next } = instance

        if (__DEV__) {
          pushWarningContext(next || instance.vnode)
        }

        if (next !== null) {
          updateComponentPreRender(instance, next)
        }
        const nextTree = renderComponentRoot(instance)
        const prevTree = instance.subTree
        instance.subTree = nextTree
        // beforeUpdate hook
        if (instance.bu !== null) {
          invokeHooks(instance.bu)
        }
        // reset refs
        // only needed if previous patch had refs
        if (instance.refs !== EMPTY_OBJ) {
          instance.refs = {}
        }
        patch(
          prevTree,
          nextTree,
          // parent may have changed if it's in a portal
          hostParentNode(prevTree.el as HostNode) as HostElement,
          // anchor may have changed if it's in a fragment
          getNextHostNode(prevTree),
          instance,
          parentSuspense,
          isSVG
        )
        instance.vnode.el = nextTree.el
        if (next === null) {
          // self-triggered update. In case of HOC, update parent component
          // vnode el. HOC is indicated by parent instance's subTree pointing
          // to child component's vnode
          updateHOCHostEl(instance, nextTree.el)
        }
        // updated hook
        if (instance.u !== null) {
          queuePostRenderEffect(instance.u, parentSuspense)
        }

        if (__DEV__) {
          popWarningContext()
        }
      }
    }, __DEV__ ? createDevEffectOptions(instance) : prodEffectOptions)
  }

  function updateComponentPreRender(
    instance: ComponentInternalInstance,
    nextVNode: HostVNode
  ) {
    nextVNode.component = instance
    instance.vnode = nextVNode
    instance.next = null
    resolveProps(instance, nextVNode.props, (nextVNode.type as Component).props)
    resolveSlots(instance, nextVNode.children)
  }

  function patchChildren(
    n1: HostVNode | null,
    n2: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean = false
  ) {
    const c1 = n1 && n1.children
    const prevShapeFlag = n1 ? n1.shapeFlag : 0
    const c2 = n2.children

    const { patchFlag, shapeFlag } = n2
    if (patchFlag === PatchFlags.BAIL) {
      optimized = false
    }
    // fast path
    if (patchFlag > 0) {
      if (patchFlag & PatchFlags.KEYED_FRAGMENT) {
        // this could be either fully-keyed or mixed (some keyed some not)
        // presence of patchFlag means children are guaranteed to be arrays
        patchKeyedChildren(
          c1 as HostVNode[],
          c2 as HostVNodeChildren,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
        return
      } else if (patchFlag & PatchFlags.UNKEYED_FRAGMENT) {
        // unkeyed
        patchUnkeyedChildren(
          c1 as HostVNode[],
          c2 as HostVNodeChildren,
          container,
          anchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
        return
      }
    }

    // children has 3 possibilities: text, array or no children.
    if (shapeFlag & ShapeFlags.TEXT_CHILDREN) {
      // text children fast path
      if (prevShapeFlag & ShapeFlags.ARRAY_CHILDREN) {
        unmountChildren(c1 as HostVNode[], parentComponent, parentSuspense)
      }
      if (c2 !== c1) {
        hostSetElementText(container, c2 as string)
      }
    } else {
      if (prevShapeFlag & ShapeFlags.ARRAY_CHILDREN) {
        // prev children was array
        if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
          // two arrays, cannot assume anything, do full diff
          patchKeyedChildren(
            c1 as HostVNode[],
            c2 as HostVNodeChildren,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized
          )
        } else {
          // no new children, just unmount old
          unmountChildren(
            c1 as HostVNode[],
            parentComponent,
            parentSuspense,
            true
          )
        }
      } else {
        // prev children was text OR null
        // new children is array OR null
        if (prevShapeFlag & ShapeFlags.TEXT_CHILDREN) {
          hostSetElementText(container, '')
        }
        // mount new if array
        if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
          mountChildren(
            c2 as HostVNodeChildren,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized
          )
        }
      }
    }
  }

  function patchUnkeyedChildren(
    c1: HostVNode[],
    c2: HostVNodeChildren,
    container: HostElement,
    anchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    c1 = c1 || EMPTY_ARR
    c2 = c2 || EMPTY_ARR
    const oldLength = c1.length
    const newLength = c2.length
    const commonLength = Math.min(oldLength, newLength)
    let i
    for (i = 0; i < commonLength; i++) {
      const nextChild = (c2[i] = optimized
        ? cloneIfMounted(c2[i] as HostVNode)
        : normalizeVNode(c2[i]))
      patch(
        c1[i],
        nextChild,
        container,
        null,
        parentComponent,
        parentSuspense,
        isSVG,
        optimized
      )
    }
    if (oldLength > newLength) {
      // remove old
      unmountChildren(c1, parentComponent, parentSuspense, true, commonLength)
    } else {
      // mount new
      mountChildren(
        c2,
        container,
        anchor,
        parentComponent,
        parentSuspense,
        isSVG,
        optimized,
        commonLength
      )
    }
  }

  // can be all-keyed or mixed
  function patchKeyedChildren(
    c1: HostVNode[],
    c2: HostVNodeChildren,
    container: HostElement,
    parentAnchor: HostNode | null,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    isSVG: boolean,
    optimized: boolean
  ) {
    let i = 0
    const l2 = c2.length
    let e1 = c1.length - 1 // prev ending index
    let e2 = l2 - 1 // next ending index

    // 1. sync from start
    // (a b) c
    // (a b) d e
    while (i <= e1 && i <= e2) {
      const n1 = c1[i]
      const n2 = (c2[i] = optimized
        ? cloneIfMounted(c2[i] as HostVNode)
        : normalizeVNode(c2[i]))
      if (isSameVNodeType(n1, n2)) {
        patch(
          n1,
          n2,
          container,
          parentAnchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
      } else {
        break
      }
      i++
    }

    // 2. sync from end
    // a (b c)
    // d e (b c)
    while (i <= e1 && i <= e2) {
      const n1 = c1[e1]
      const n2 = (c2[e2] = optimized
        ? cloneIfMounted(c2[e2] as HostVNode)
        : normalizeVNode(c2[e2]))
      if (isSameVNodeType(n1, n2)) {
        patch(
          n1,
          n2,
          container,
          parentAnchor,
          parentComponent,
          parentSuspense,
          isSVG,
          optimized
        )
      } else {
        break
      }
      e1--
      e2--
    }

    // 3. common sequence + mount
    // (a b)
    // (a b) c
    // i = 2, e1 = 1, e2 = 2
    // (a b)
    // c (a b)
    // i = 0, e1 = -1, e2 = 0
    if (i > e1) {
      if (i <= e2) {
        const nextPos = e2 + 1
        const anchor =
          nextPos < l2 ? (c2[nextPos] as HostVNode).el : parentAnchor
        while (i <= e2) {
          patch(
            null,
            (c2[i] = optimized
              ? cloneIfMounted(c2[i] as HostVNode)
              : normalizeVNode(c2[i])),
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG
          )
          i++
        }
      }
    }

    // 4. common sequence + unmount
    // (a b) c
    // (a b)
    // i = 2, e1 = 2, e2 = 1
    // a (b c)
    // (b c)
    // i = 0, e1 = 0, e2 = -1
    else if (i > e2) {
      while (i <= e1) {
        unmount(c1[i], parentComponent, parentSuspense, true)
        i++
      }
    }

    // 5. unknown sequence
    // [i ... e1 + 1]: a b [c d e] f g
    // [i ... e2 + 1]: a b [e d c h] f g
    // i = 2, e1 = 4, e2 = 5
    else {
      const s1 = i // prev starting index
      const s2 = i // next starting index

      // 5.1 build key:index map for newChildren
      const keyToNewIndexMap: Map<string | number, number> = new Map()
      for (i = s2; i <= e2; i++) {
        const nextChild = (c2[i] = optimized
          ? cloneIfMounted(c2[i] as HostVNode)
          : normalizeVNode(c2[i]))
        if (nextChild.key != null) {
          if (__DEV__ && keyToNewIndexMap.has(nextChild.key)) {
            warn(
              `Duplicate keys found during update:`,
              JSON.stringify(nextChild.key),
              `Make sure keys are unique.`
            )
          }
          keyToNewIndexMap.set(nextChild.key, i)
        }
      }

      // 5.2 loop through old children left to be patched and try to patch
      // matching nodes & remove nodes that are no longer present
      let j
      let patched = 0
      const toBePatched = e2 - s2 + 1
      let moved = false
      // used to track whether any node has moved
      let maxNewIndexSoFar = 0
      // works as Map<newIndex, oldIndex>
      // Note that oldIndex is offset by +1
      // and oldIndex = 0 is a special value indicating the new node has
      // no corresponding old node.
      // used for determining longest stable subsequence
      const newIndexToOldIndexMap = new Array(toBePatched)
      for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0

      for (i = s1; i <= e1; i++) {
        const prevChild = c1[i]
        if (patched >= toBePatched) {
          // all new children have been patched so this can only be a removal
          unmount(prevChild, parentComponent, parentSuspense, true)
          continue
        }
        let newIndex
        if (prevChild.key != null) {
          newIndex = keyToNewIndexMap.get(prevChild.key)
        } else {
          // key-less node, try to locate a key-less node of the same type
          for (j = s2; j <= e2; j++) {
            if (
              newIndexToOldIndexMap[j - s2] === 0 &&
              isSameVNodeType(prevChild, c2[j] as HostVNode)
            ) {
              newIndex = j
              break
            }
          }
        }
        if (newIndex === undefined) {
          unmount(prevChild, parentComponent, parentSuspense, true)
        } else {
          newIndexToOldIndexMap[newIndex - s2] = i + 1
          if (newIndex >= maxNewIndexSoFar) {
            maxNewIndexSoFar = newIndex
          } else {
            moved = true
          }
          patch(
            prevChild,
            c2[newIndex] as HostVNode,
            container,
            null,
            parentComponent,
            parentSuspense,
            isSVG,
            optimized
          )
          patched++
        }
      }

      // 5.3 move and mount
      // generate longest stable subsequence only when nodes have moved
      const increasingNewIndexSequence = moved
        ? getSequence(newIndexToOldIndexMap)
        : EMPTY_ARR
      j = increasingNewIndexSequence.length - 1
      // looping backwards so that we can use last patched node as anchor
      for (i = toBePatched - 1; i >= 0; i--) {
        const nextIndex = s2 + i
        const nextChild = c2[nextIndex] as HostVNode
        const anchor =
          nextIndex + 1 < l2
            ? (c2[nextIndex + 1] as HostVNode).el
            : parentAnchor
        if (newIndexToOldIndexMap[i] === 0) {
          // mount new
          patch(
            null,
            nextChild,
            container,
            anchor,
            parentComponent,
            parentSuspense,
            isSVG
          )
        } else if (moved) {
          // move if:
          // There is no stable subsequence (e.g. a reverse)
          // OR current node is not among the stable sequence
          if (j < 0 || i !== increasingNewIndexSequence[j]) {
            move(nextChild, container, anchor, MoveType.REORDER)
          } else {
            j--
          }
        }
      }
    }
  }

  function move(
    vnode: HostVNode,
    container: HostElement,
    anchor: HostNode | null,
    type: MoveType,
    parentSuspense: HostSuspenseBoundary | null = null
  ) {
    if (vnode.shapeFlag & ShapeFlags.COMPONENT) {
      move(vnode.component!.subTree, container, anchor, type)
      return
    }
    if (__FEATURE_SUSPENSE__ && vnode.shapeFlag & ShapeFlags.SUSPENSE) {
      vnode.suspense!.move(container, anchor, type)
      return
    }
    if (vnode.type === Fragment) {
      hostInsert(vnode.el!, container, anchor)
      const children = vnode.children as HostVNode[]
      for (let i = 0; i < children.length; i++) {
        move(children[i], container, anchor, type)
      }
      hostInsert(vnode.anchor!, container, anchor)
    } else {
      // Plain element
      const { el, transition, shapeFlag } = vnode
      const needTransition =
        type !== MoveType.REORDER &&
        shapeFlag & ShapeFlags.ELEMENT &&
        transition != null
      if (needTransition) {
        if (type === MoveType.ENTER) {
          transition!.beforeEnter(el!)
          hostInsert(el!, container, anchor)
          queuePostRenderEffect(() => transition!.enter(el!), parentSuspense)
        } else {
          const { leave, delayLeave, afterLeave } = transition!
          const remove = () => hostInsert(el!, container, anchor)
          const performLeave = () => {
            leave(el!, () => {
              remove()
              afterLeave && afterLeave()
            })
          }
          if (delayLeave) {
            delayLeave(el!, remove, performLeave)
          } else {
            performLeave()
          }
        }
      } else {
        hostInsert(el!, container, anchor)
      }
    }
  }

  function unmount(
    vnode: HostVNode,
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    doRemove?: boolean
  ) {
    const { props, ref, children, dynamicChildren, shapeFlag } = vnode

    // unset ref
    if (ref !== null && parentComponent !== null) {
      setRef(ref, null, parentComponent, null)
    }

    if (shapeFlag & ShapeFlags.COMPONENT) {
      if (shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE) {
        ;(parentComponent!.sink as KeepAliveSink).deactivate(vnode)
      } else {
        unmountComponent(vnode.component!, parentSuspense, doRemove)
      }
      return
    }

    if (__FEATURE_SUSPENSE__ && shapeFlag & ShapeFlags.SUSPENSE) {
      vnode.suspense!.unmount(parentSuspense, doRemove)
      return
    }

    if (props != null && props.onVnodeBeforeUnmount != null) {
      invokeDirectiveHook(props.onVnodeBeforeUnmount, parentComponent, vnode)
    }

    if (dynamicChildren != null) {
      // fast path for block nodes: only need to unmount dynamic children.
      unmountChildren(dynamicChildren, parentComponent, parentSuspense)
    } else if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
      unmountChildren(children as HostVNode[], parentComponent, parentSuspense)
    }

    if (doRemove) {
      remove(vnode)
    }

    if (props != null && props.onVnodeUnmounted != null) {
      queuePostRenderEffect(() => {
        invokeDirectiveHook(props.onVnodeUnmounted!, parentComponent, vnode)
      }, parentSuspense)
    }
  }

  function remove(vnode: HostVNode) {
    const { type, el, anchor, transition } = vnode
    if (type === Fragment) {
      removeFragment(el!, anchor!)
      return
    }

    const performRemove = () => {
      hostRemove(el!)
      if (
        transition != null &&
        !transition.persisted &&
        transition.afterLeave
      ) {
        transition.afterLeave()
      }
    }

    if (
      vnode.shapeFlag & ShapeFlags.ELEMENT &&
      transition != null &&
      !transition.persisted
    ) {
      const { leave, delayLeave } = transition
      const performLeave = () => leave(el!, performRemove)
      if (delayLeave) {
        delayLeave(vnode.el!, performRemove, performLeave)
      } else {
        performLeave()
      }
    } else {
      performRemove()
    }
  }

  function removeFragment(cur: HostNode, end: HostNode) {
    // For fragments, directly remove all contained DOM nodes.
    // (fragment child nodes cannot have transition)
    let next
    while (cur !== end) {
      next = hostNextSibling(cur)!
      hostRemove(cur)
      cur = next
    }
    hostRemove(end)
  }

  function unmountComponent(
    instance: ComponentInternalInstance,
    parentSuspense: HostSuspenseBoundary | null,
    doRemove?: boolean
  ) {
    if (__HMR__ && instance.type.__hmrId != null) {
      unregisterHMR(instance)
    }

    const { bum, effects, update, subTree, um, da, isDeactivated } = instance
    // beforeUnmount hook
    if (bum !== null) {
      invokeHooks(bum)
    }
    if (effects !== null) {
      for (let i = 0; i < effects.length; i++) {
        stop(effects[i])
      }
    }
    // update may be null if a component is unmounted before its async
    // setup has resolved.
    if (update !== null) {
      stop(update)
      unmount(subTree, instance, parentSuspense, doRemove)
    }
    // unmounted hook
    if (um !== null) {
      queuePostRenderEffect(um, parentSuspense)
    }
    // deactivated hook
    if (
      da !== null &&
      !isDeactivated &&
      instance.vnode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE
    ) {
      queuePostRenderEffect(da, parentSuspense)
    }
    queuePostFlushCb(() => {
      instance.isUnmounted = true
    })

    // A component with async dep inside a pending suspense is unmounted before
    // its async dep resolves. This should remove the dep from the suspense, and
    // cause the suspense to resolve immediately if that was the last dep.
    if (
      __FEATURE_SUSPENSE__ &&
      parentSuspense !== null &&
      !parentSuspense.isResolved &&
      !parentSuspense.isUnmounted &&
      instance.asyncDep !== null &&
      !instance.asyncResolved
    ) {
      parentSuspense.deps--
      if (parentSuspense.deps === 0) {
        parentSuspense.resolve()
      }
    }
  }

  function unmountChildren(
    children: HostVNode[],
    parentComponent: ComponentInternalInstance | null,
    parentSuspense: HostSuspenseBoundary | null,
    doRemove?: boolean,
    start: number = 0
  ) {
    for (let i = start; i < children.length; i++) {
      unmount(children[i], parentComponent, parentSuspense, doRemove)
    }
  }

  function getNextHostNode(vnode: HostVNode): HostNode | null {
    if (vnode.shapeFlag & ShapeFlags.COMPONENT) {
      return getNextHostNode(vnode.component!.subTree)
    }
    if (__FEATURE_SUSPENSE__ && vnode.shapeFlag & ShapeFlags.SUSPENSE) {
      return vnode.suspense!.next()
    }
    return hostNextSibling((vnode.anchor || vnode.el)!)
  }

  function setRef(
    ref: string | Function | Ref,
    oldRef: string | Function | Ref | null,
    parent: ComponentInternalInstance,
    value: HostNode | ComponentPublicInstance | null
  ) {
    const refs = parent.refs === EMPTY_OBJ ? (parent.refs = {}) : parent.refs
    const renderContext = toRaw(parent.renderContext)

    // unset old ref
    if (oldRef !== null && oldRef !== ref) {
      if (isString(oldRef)) {
        refs[oldRef] = null
        const oldSetupRef = renderContext[oldRef]
        if (isRef(oldSetupRef)) {
          oldSetupRef.value = null
        }
      } else if (isRef(oldRef)) {
        oldRef.value = null
      }
    }

    if (isString(ref)) {
      const setupRef = renderContext[ref]
      if (isRef(setupRef)) {
        setupRef.value = value
      }
      refs[ref] = value
    } else if (isRef(ref)) {
      ref.value = value
    } else if (isFunction(ref)) {
      callWithErrorHandling(ref, parent, ErrorCodes.FUNCTION_REF, [value, refs])
    } else if (__DEV__) {
      warn('Invalid template ref type:', value, `(${typeof value})`)
    }
  }

  type HostRootElement = HostElement & { _vnode: HostVNode | null }

  const render: RootRenderFunction<HostNode, HostElement> = (
    vnode,
    container: HostRootElement
  ) => {
    if (vnode == null) {
      if (container._vnode) {
        unmount(container._vnode, null, null, true)
      }
    } else {
      patch(container._vnode || null, vnode, container)
    }
    flushPostFlushCbs()
    container._vnode = vnode
  }

  return {
    render,
    createApp: createAppAPI(render)
  }
}

// https://en.wikipedia.org/wiki/Longest_increasing_subsequence
function getSequence(arr: number[]): number[] {
  const p = arr.slice()
  const result = [0]
  let i, j, u, v, c
  const len = arr.length
  for (i = 0; i < len; i++) {
    const arrI = arr[i]
    if (arrI !== 0) {
      j = result[result.length - 1]
      if (arr[j] < arrI) {
        p[i] = j
        result.push(i)
        continue
      }
      u = 0
      v = result.length - 1
      while (u < v) {
        c = ((u + v) / 2) | 0
        if (arr[result[c]] < arrI) {
          u = c + 1
        } else {
          v = c
        }
      }
      if (arrI < arr[result[u]]) {
        if (u > 0) {
          p[i] = result[u - 1]
        }
        result[u] = i
      }
    }
  }
  u = result.length
  v = result[u - 1]
  while (u-- > 0) {
    result[u] = v
    v = p[v]
  }
  return result
}