lrucache.js

module.exports = LRUCache

// This will be a proper iterable 'Map' in engines that support it,
// or a fakey-fake PseudoMap in older versions.
var Map = require('pseudomap')
var util = require('util')

// A linked list to keep track of recently-used-ness
var Yallist = require('yallist')

// use symbols if possible, otherwise just _props
var hasSymbol = typeof Symbol === 'function'
var makeSymbol
if (hasSymbol) {
  makeSymbol = function (key) {
    return Symbol(key)
  }
} else {
  makeSymbol = function (key) {
    return '_' + key
  }
}

var MAX = makeSymbol('max')
var LENGTH = makeSymbol('length')
var LENGTH_CALCULATOR = makeSymbol('lengthCalculator')
var ALLOW_STALE = makeSymbol('allowStale')
var MAX_AGE = makeSymbol('maxAge')
var DISPOSE = makeSymbol('dispose')
var NO_DISPOSE_ON_SET = makeSymbol('noDisposeOnSet')
var LRU_LIST = makeSymbol('lruList')
var CACHE = makeSymbol('cache')

function naiveLength () { return 1 }

// lruList is a yallist where the head is the youngest
// item, and the tail is the oldest.  the list contains the Hit
// objects as the entries.
// Each Hit object has a reference to its Yallist.Node.  This
// never changes.
//
// cache is a Map (or PseudoMap) that matches the keys to
// the Yallist.Node object.
function LRUCache (options) {
  if (!(this instanceof LRUCache)) {
    return new LRUCache(options)
  }

  if (typeof options === 'number') {
    options = { max: options }
  }

  if (!options) {
    options = {}
  }

  var max = this[MAX] = options.max
  // Kind of weird to have a default max of Infinity, but oh well.
  if (!max ||
      !(typeof max === 'number') ||
      max <= 0) {
    this[MAX] = Infinity
  }

  var lc = options.length || naiveLength
  if (typeof lc !== 'function') {
    lc = naiveLength
  }
  this[LENGTH_CALCULATOR] = lc

  this[ALLOW_STALE] = options.stale || false
  this[MAX_AGE] = options.maxAge || 0
  this[DISPOSE] = options.dispose
  this[NO_DISPOSE_ON_SET] = options.noDisposeOnSet || false
  this.reset()
}

// resize the cache when the max changes.
Object.defineProperty(LRUCache.prototype, 'max', {
  set: function (mL) {
    if (!mL || !(typeof mL === 'number') || mL <= 0) {
      mL = Infinity
    }
    this[MAX] = mL
    trim(this)
  },
  get: function () {
    return this[MAX]
  },
  enumerable: true
})

Object.defineProperty(LRUCache.prototype, 'allowStale', {
  set: function (allowStale) {
    this[ALLOW_STALE] = !!allowStale
  },
  get: function () {
    return this[ALLOW_STALE]
  },
  enumerable: true
})

Object.defineProperty(LRUCache.prototype, 'maxAge', {
  set: function (mA) {
    if (!mA || !(typeof mA === 'number') || mA < 0) {
      mA = 0
    }
    this[MAX_AGE] = mA
    trim(this)
  },
  get: function () {
    return this[MAX_AGE]
  },
  enumerable: true
})

// resize the cache when the lengthCalculator changes.
Object.defineProperty(LRUCache.prototype, 'lengthCalculator', {
  set: function (lC) {
    if (typeof lC !== 'function') {
      lC = naiveLength
    }
    if (lC !== this[LENGTH_CALCULATOR]) {
      this[LENGTH_CALCULATOR] = lC
      this[LENGTH] = 0
      this[LRU_LIST].forEach(function (hit) {
        hit.length = this[LENGTH_CALCULATOR](hit.value, hit.key)
        this[LENGTH] += hit.length
      }, this)
    }
    trim(this)
  },
  get: function () { return this[LENGTH_CALCULATOR] },
  enumerable: true
})

Object.defineProperty(LRUCache.prototype, 'length', {
  get: function () { return this[LENGTH] },
  enumerable: true
})

Object.defineProperty(LRUCache.prototype, 'itemCount', {
  get: function () { return this[LRU_LIST].length },
  enumerable: true
})

LRUCache.prototype.rforEach = function (fn, thisp) {
  thisp = thisp || this
  for (var walker = this[LRU_LIST].tail; walker !== null;) {
    var prev = walker.prev
    forEachStep(this, fn, walker, thisp)
    walker = prev
  }
}

function forEachStep (self, fn, node, thisp) {
  var hit = node.value
  if (isStale(self, hit)) {
    del(self, node)
    if (!self[ALLOW_STALE]) {
      hit = undefined
    }
  }
  if (hit) {
    fn.call(thisp, hit.value, hit.key, self)
  }
}

LRUCache.prototype.forEach = function (fn, thisp) {
  thisp = thisp || this
  for (var walker = this[LRU_LIST].head; walker !== null;) {
    var next = walker.next
    forEachStep(this, fn, walker, thisp)
    walker = next
  }
}

LRUCache.prototype.keys = function () {
  return this[LRU_LIST].toArray().map(function (k) {
    return k.key
  }, this)
}

LRUCache.prototype.values = function () {
  return this[LRU_LIST].toArray().map(function (k) {
    return k.value
  }, this)
}

LRUCache.prototype.reset = function () {
  if (this[DISPOSE] &&
      this[LRU_LIST] &&
      this[LRU_LIST].length) {
    this[LRU_LIST].forEach(function (hit) {
      this[DISPOSE](hit.key, hit.value)
    }, this)
  }

  this[CACHE] = new Map() // hash of items by key
  this[LRU_LIST] = new Yallist() // list of items in order of use recency
  this[LENGTH] = 0 // length of items in the list
}

LRUCache.prototype.dump = function () {
  return this[LRU_LIST].map(function (hit) {
    if (!isStale(this, hit)) {
      return {
        k: hit.key,
        v: hit.value,
        e: hit.now + (hit.maxAge || 0)
      }
    }
  }, this).toArray().filter(function (h) {
    return h
  })
}

LRUCache.prototype.dumpLru = function () {
  return this[LRU_LIST]
}

LRUCache.prototype.inspect = function (n, opts) {
  var str = 'LRUCache {'
  var extras = false

  var as = this[ALLOW_STALE]
  if (as) {
    str += '\n  allowStale: true'
    extras = true
  }

  var max = this[MAX]
  if (max && max !== Infinity) {
    if (extras) {
      str += ','
    }
    str += '\n  max: ' + util.inspect(max, opts)
    extras = true
  }

  var maxAge = this[MAX_AGE]
  if (maxAge) {
    if (extras) {
      str += ','
    }
    str += '\n  maxAge: ' + util.inspect(maxAge, opts)
    extras = true
  }

  var lc = this[LENGTH_CALCULATOR]
  if (lc && lc !== naiveLength) {
    if (extras) {
      str += ','
    }
    str += '\n  length: ' + util.inspect(this[LENGTH], opts)
    extras = true
  }

  var didFirst = false
  this[LRU_LIST].forEach(function (item) {
    if (didFirst) {
      str += ',\n  '
    } else {
      if (extras) {
        str += ',\n'
      }
      didFirst = true
      str += '\n  '
    }
    var key = util.inspect(item.key).split('\n').join('\n  ')
    var val = { value: item.value }
    if (item.maxAge !== maxAge) {
      val.maxAge = item.maxAge
    }
    if (lc !== naiveLength) {
      val.length = item.length
    }
    if (isStale(this, item)) {
      val.stale = true
    }

    val = util.inspect(val, opts).split('\n').join('\n  ')
    str += key + ' => ' + val
  })

  if (didFirst || extras) {
    str += '\n'
  }
  str += '}'

  return str
}

LRUCache.prototype.set = function (key, value, maxAge) {
  maxAge = maxAge || this[MAX_AGE]

  var now = maxAge ? Date.now() : 0
  var len = this[LENGTH_CALCULATOR](value, key)

  if (this[CACHE].has(key)) {
    if (len > this[MAX]) {
      del(this, this[CACHE].get(key))
      return false
    }

    var node = this[CACHE].get(key)
    var item = node.value

    // dispose of the old one before overwriting
    // split out into 2 ifs for better coverage tracking
    if (this[DISPOSE]) {
      if (!this[NO_DISPOSE_ON_SET]) {
        this[DISPOSE](key, item.value)
      }
    }

    item.now = now
    item.maxAge = maxAge
    item.value = value
    this[LENGTH] += len - item.length
    item.length = len
    this.get(key)
    trim(this)
    return true
  }

  var hit = new Entry(key, value, len, now, maxAge)

  // oversized objects fall out of cache automatically.
  if (hit.length > this[MAX]) {
    if (this[DISPOSE]) {
      this[DISPOSE](key, value)
    }
    return false
  }

  this[LENGTH] += hit.length
  this[LRU_LIST].unshift(hit)
  this[CACHE].set(key, this[LRU_LIST].head)
  trim(this)
  return true
}

LRUCache.prototype.has = function (key) {
  if (!this[CACHE].has(key)) return false
  var hit = this[CACHE].get(key).value
  if (isStale(this, hit)) {
    return false
  }
  return true
}

LRUCache.prototype.get = function (key) {
  return get(this, key, true)
}

LRUCache.prototype.peek = function (key) {
  return get(this, key, false)
}

LRUCache.prototype.pop = function () {
  var node = this[LRU_LIST].tail
  if (!node) return null
  del(this, node)
  return node.value
}

LRUCache.prototype.del = function (key) {
  del(this, this[CACHE].get(key))
}

LRUCache.prototype.load = function (arr) {
  // reset the cache
  this.reset()

  var now = Date.now()
  // A previous serialized cache has the most recent items first
  for (var l = arr.length - 1; l >= 0; l--) {
    var hit = arr[l]
    var expiresAt = hit.e || 0
    if (expiresAt === 0) {
      // the item was created without expiration in a non aged cache
      this.set(hit.k, hit.v)
    } else {
      var maxAge = expiresAt - now
      // dont add already expired items
      if (maxAge > 0) {
        this.set(hit.k, hit.v, maxAge)
      }
    }
  }
}

LRUCache.prototype.prune = function () {
  var self = this
  this[CACHE].forEach(function (value, key) {
    get(self, key, false)
  })
}

function get (self, key, doUse) {
  var node = self[CACHE].get(key)
  if (node) {
    var hit = node.value
    if (isStale(self, hit)) {
      del(self, node)
      if (!self[ALLOW_STALE]) hit = undefined
    } else {
      if (doUse) {
        self[LRU_LIST].unshiftNode(node)
      }
    }
    if (hit) hit = hit.value
  }
  return hit
}

function isStale (self, hit) {
  if (!hit || (!hit.maxAge && !self[MAX_AGE])) {
    return false
  }
  var stale = false
  var diff = Date.now() - hit.now
  if (hit.maxAge) {
    stale = diff > hit.maxAge
  } else {
    stale = self[MAX_AGE] && (diff > self[MAX_AGE])
  }
  return stale
}

function trim (self) {
  if (self[LENGTH] > self[MAX]) {
    for (var walker = self[LRU_LIST].tail;
         self[LENGTH] > self[MAX] && walker !== null;) {
      // We know that we're about to delete this one, and also
      // what the next least recently used key will be, so just
      // go ahead and set it now.
      var prev = walker.prev
      del(self, walker)
      walker = prev
    }
  }
}

function del (self, node) {
  if (node) {
    var hit = node.value
    if (self[DISPOSE]) {
      self[DISPOSE](hit.key, hit.value)
    }
    self[LENGTH] -= hit.length
    self[CACHE].delete(hit.key)
    self[LRU_LIST].removeNode(node)
  }
}

// classy, since V8 prefers predictable objects.
function Entry (key, value, length, now, maxAge) {
  this.key = key
  this.value = value
  this.length = length
  this.now = now
  this.maxAge = maxAge || 0
}

react.js

import type {ReactProviderType, ReactContext} from 'shared/ReactTypes';
import type {Fiber} from 'react-reconciler/src/ReactFiber';
import type {FiberRoot} from './ReactFiberRoot';
import type {ExpirationTime} from './ReactFiberExpirationTime';
import checkPropTypes from 'prop-types/checkPropTypes';

import {
  IndeterminateComponent,
  FunctionalComponent,
  FunctionalComponentLazy,
  ClassComponent,
  ClassComponentLazy,
  HostRoot,
  HostComponent,
  HostText,
  HostPortal,
  ForwardRef,
  ForwardRefLazy,
  Fragment,
  Mode,
  ContextProvider,
  ContextConsumer,
  Profiler,
  PlaceholderComponent,
} from 'shared/ReactWorkTags';
import {
  NoEffect,
  PerformedWork,
  Placement,
  ContentReset,
  DidCapture,
  Update,
  Ref,
} from 'shared/ReactSideEffectTags';
import ReactSharedInternals from 'shared/ReactSharedInternals';
import {
  enableGetDerivedStateFromCatch,
  enableSuspense,
  debugRenderPhaseSideEffects,
  debugRenderPhaseSideEffectsForStrictMode,
  enableProfilerTimer,
} from 'shared/ReactFeatureFlags';
import invariant from 'shared/invariant';
import getComponentName from 'shared/getComponentName';
import ReactStrictModeWarnings from './ReactStrictModeWarnings';
import warning from 'shared/warning';
import warningWithoutStack from 'shared/warningWithoutStack';
import * as ReactCurrentFiber from './ReactCurrentFiber';
import {cancelWorkTimer} from './ReactDebugFiberPerf';

import {applyDerivedStateFromProps} from './ReactFiberClassComponent';
import {
  mountChildFibers,
  reconcileChildFibers,
  cloneChildFibers,
} from './ReactChildFiber';
import {processUpdateQueue} from './ReactUpdateQueue';
import {NoWork, Never} from './ReactFiberExpirationTime';
import {AsyncMode, StrictMode} from './ReactTypeOfMode';
import {
  shouldSetTextContent,
  shouldDeprioritizeSubtree,
} from './ReactFiberHostConfig';
import {pushHostContext, pushHostContainer} from './ReactFiberHostContext';
import {
  pushProvider,
  propagateContextChange,
  readContext,
  prepareToReadContext,
  calculateChangedBits,
} from './ReactFiberNewContext';
import {stopProfilerTimerIfRunning} from './ReactProfilerTimer';
import {
  getMaskedContext,
  getUnmaskedContext,
  hasContextChanged as hasLegacyContextChanged,
  pushContextProvider as pushLegacyContextProvider,
  isContextProvider as isLegacyContextProvider,
  pushTopLevelContextObject,
  invalidateContextProvider,
} from './ReactFiberContext';
import {
  enterHydrationState,
  resetHydrationState,
  tryToClaimNextHydratableInstance,
} from './ReactFiberHydrationContext';
import {
  adoptClassInstance,
  constructClassInstance,
  mountClassInstance,
  resumeMountClassInstance,
  updateClassInstance,
} from './ReactFiberClassComponent';
import {readLazyComponentType} from './ReactFiberLazyComponent';
import {getResultFromResolvedThenable} from 'shared/ReactLazyComponent';
import {resolveLazyComponentTag} from './ReactFiber';

const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;

let didWarnAboutBadClass;
let didWarnAboutGetDerivedStateOnFunctionalComponent;
let didWarnAboutStatelessRefs;

if (__DEV__) {
  didWarnAboutBadClass = {};
  didWarnAboutGetDerivedStateOnFunctionalComponent = {};
  didWarnAboutStatelessRefs = {};
}

export function reconcileChildren(
  current: Fiber | null,
  workInProgress: Fiber,
  nextChildren: any,
  renderExpirationTime: ExpirationTime,
) {
  if (current === null) {
    workInProgress.child = mountChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderExpirationTime,
    );
  } else {
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      current.child,
      nextChildren,
      renderExpirationTime,
    );
  }
}

function updateForwardRef(
  current: Fiber | null,
  workInProgress: Fiber,
  type: any,
  nextProps: any,
  renderExpirationTime: ExpirationTime,
) {
  const render = type.render;
  const ref = workInProgress.ref;
  if (hasLegacyContextChanged()) {
  } else if (workInProgress.memoizedProps === nextProps) {
    const currentRef = current !== null ? current.ref : null;
    if (ref === currentRef) {
      return bailoutOnAlreadyFinishedWork(
        current,
        workInProgress,
        renderExpirationTime,
      );
    }
  }

  let nextChildren;
  if (__DEV__) {
    ReactCurrentOwner.current = workInProgress;
    ReactCurrentFiber.setCurrentPhase('render');
    nextChildren = render(nextProps, ref);
    ReactCurrentFiber.setCurrentPhase(null);
  } else {
    nextChildren = render(nextProps, ref);
  }

  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  memoizeProps(workInProgress, nextProps);
  return workInProgress.child;
}

function updateFragment(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  const nextChildren = workInProgress.pendingProps;
  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  memoizeProps(workInProgress, nextChildren);
  return workInProgress.child;
}

function updateMode(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  const nextChildren = workInProgress.pendingProps.children;
  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  memoizeProps(workInProgress, nextChildren);
  return workInProgress.child;
}

function updateProfiler(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  if (enableProfilerTimer) {
    workInProgress.effectTag |= Update;
  }
  const nextProps = workInProgress.pendingProps;
  const nextChildren = nextProps.children;
  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  memoizeProps(workInProgress, nextProps);
  return workInProgress.child;
}

function markRef(current: Fiber | null, workInProgress: Fiber) {
  const ref = workInProgress.ref;
  if (
    (current === null && ref !== null) ||
    (current !== null && current.ref !== ref)
  ) {
    workInProgress.effectTag |= Ref;
  }
}

function updateFunctionalComponent(
  current,
  workInProgress,
  Component,
  nextProps: any,
  renderExpirationTime,
) {
  const unmaskedContext = getUnmaskedContext(workInProgress, Component, true);
  const context = getMaskedContext(workInProgress, unmaskedContext);

  let nextChildren;
  prepareToReadContext(workInProgress, renderExpirationTime);
  if (__DEV__) {
    ReactCurrentOwner.current = workInProgress;
    ReactCurrentFiber.setCurrentPhase('render');
    nextChildren = Component(nextProps, context);
    ReactCurrentFiber.setCurrentPhase(null);
  } else {
    nextChildren = Component(nextProps, context);
  }

  workInProgress.effectTag |= PerformedWork;
  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  memoizeProps(workInProgress, nextProps);
  return workInProgress.child;
}

function updateClassComponent(
  current: Fiber | null,
  workInProgress: Fiber,
  Component: any,
  nextProps,
  renderExpirationTime: ExpirationTime,
) {
  let hasContext;
  if (isLegacyContextProvider(Component)) {
    hasContext = true;
    pushLegacyContextProvider(workInProgress);
  } else {
    hasContext = false;
  }
  prepareToReadContext(workInProgress, renderExpirationTime);

  let shouldUpdate;
  if (current === null) {
    if (workInProgress.stateNode === null) {
      constructClassInstance(
        workInProgress,
        Component,
        nextProps,
        renderExpirationTime,
      );
      mountClassInstance(
        workInProgress,
        Component,
        nextProps,
        renderExpirationTime,
      );
      shouldUpdate = true;
    } else {
      shouldUpdate = resumeMountClassInstance(
        workInProgress,
        Component,
        nextProps,
        renderExpirationTime,
      );
    }
  } else {
    shouldUpdate = updateClassInstance(
      current,
      workInProgress,
      Component,
      nextProps,
      renderExpirationTime,
    );
  }
  return finishClassComponent(
    current,
    workInProgress,
    Component,
    shouldUpdate,
    hasContext,
    renderExpirationTime,
  );
}

function finishClassComponent(
  current: Fiber | null,
  workInProgress: Fiber,
  Component: any,
  shouldUpdate: boolean,
  hasContext: boolean,
  renderExpirationTime: ExpirationTime,
) {
  // Refs should update even if shouldComponentUpdate returns false
  markRef(current, workInProgress);

  const didCaptureError = (workInProgress.effectTag & DidCapture) !== NoEffect;

  if (!shouldUpdate && !didCaptureError) {
    // Context providers should defer to sCU for rendering
    if (hasContext) {
      invalidateContextProvider(workInProgress, Component, false);
    }

    return bailoutOnAlreadyFinishedWork(
      current,
      workInProgress,
      renderExpirationTime,
    );
  }

  const instance = workInProgress.stateNode;

  // Rerender
  ReactCurrentOwner.current = workInProgress;
  let nextChildren;
  if (
    didCaptureError &&
    (!enableGetDerivedStateFromCatch ||
      typeof Component.getDerivedStateFromCatch !== 'function')
  ) {
    // If we captured an error, but getDerivedStateFrom catch is not defined,
    // unmount all the children. componentDidCatch will schedule an update to
    // re-render a fallback. This is temporary until we migrate everyone to
    // the new API.
    // TODO: Warn in a future release.
    nextChildren = null;

    if (enableProfilerTimer) {
      stopProfilerTimerIfRunning(workInProgress);
    }
  } else {
    if (__DEV__) {
      ReactCurrentFiber.setCurrentPhase('render');
      nextChildren = instance.render();
      if (
        debugRenderPhaseSideEffects ||
        (debugRenderPhaseSideEffectsForStrictMode &&
          workInProgress.mode & StrictMode)
      ) {
        instance.render();
      }
      ReactCurrentFiber.setCurrentPhase(null);
    } else {
      nextChildren = instance.render();
    }
  }

  // React DevTools reads this flag.
  workInProgress.effectTag |= PerformedWork;
  if (current !== null && didCaptureError) {
    // If we're recovering from an error, reconcile twice: first to delete
    // all the existing children.
    reconcileChildren(current, workInProgress, null, renderExpirationTime);
    workInProgress.child = null;
    // Now we can continue reconciling like normal. This has the effect of
    // remounting all children regardless of whether their their
    // identity matches.
  }
  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  // Memoize props and state using the values we just used to render.
  // TODO: Restructure so we never read values from the instance.
  memoizeState(workInProgress, instance.state);
  memoizeProps(workInProgress, instance.props);

  // The context might have changed so we need to recalculate it.
  if (hasContext) {
    invalidateContextProvider(workInProgress, Component, true);
  }

  return workInProgress.child;
}

function pushHostRootContext(workInProgress) {
  const root = (workInProgress.stateNode: FiberRoot);
  if (root.pendingContext) {
    pushTopLevelContextObject(
      workInProgress,
      root.pendingContext,
      root.pendingContext !== root.context,
    );
  } else if (root.context) {
    // Should always be set
    pushTopLevelContextObject(workInProgress, root.context, false);
  }
  pushHostContainer(workInProgress, root.containerInfo);
}

function updateHostRoot(current, workInProgress, renderExpirationTime) {
  pushHostRootContext(workInProgress);
  const updateQueue = workInProgress.updateQueue;
  invariant(
    updateQueue !== null,
    'If the root does not have an updateQueue, we should have already ' +
      'bailed out. This error is likely caused by a bug in React. Please ' +
      'file an issue.',
  );
  const nextProps = workInProgress.pendingProps;
  const prevState = workInProgress.memoizedState;
  const prevChildren = prevState !== null ? prevState.element : null;
  processUpdateQueue(
    workInProgress,
    updateQueue,
    nextProps,
    null,
    renderExpirationTime,
  );
  const nextState = workInProgress.memoizedState;
  // Caution: React DevTools currently depends on this property
  // being called "element".
  const nextChildren = nextState.element;
  if (nextChildren === prevChildren) {
    // If the state is the same as before, that's a bailout because we had
    // no work that expires at this time.
    resetHydrationState();
    return bailoutOnAlreadyFinishedWork(
      current,
      workInProgress,
      renderExpirationTime,
    );
  }
  const root: FiberRoot = workInProgress.stateNode;
  if (
    (current === null || current.child === null) &&
    root.hydrate &&
    enterHydrationState(workInProgress)
  ) {
    // If we don't have any current children this might be the first pass.
    // We always try to hydrate. If this isn't a hydration pass there won't
    // be any children to hydrate which is effectively the same thing as
    // not hydrating.

    // This is a bit of a hack. We track the host root as a placement to
    // know that we're currently in a mounting state. That way isMounted
    // works as expected. We must reset this before committing.
    // TODO: Delete this when we delete isMounted and findDOMNode.
    workInProgress.effectTag |= Placement;

    // Ensure that children mount into this root without tracking
    // side-effects. This ensures that we don't store Placement effects on
    // nodes that will be hydrated.
    workInProgress.child = mountChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderExpirationTime,
    );
  } else {
    // Otherwise reset hydration state in case we aborted and resumed another
    // root.
    reconcileChildren(
      current,
      workInProgress,
      nextChildren,
      renderExpirationTime,
    );
    resetHydrationState();
  }
  return workInProgress.child;
}

function updateHostComponent(current, workInProgress, renderExpirationTime) {
  pushHostContext(workInProgress);

  if (current === null) {
    tryToClaimNextHydratableInstance(workInProgress);
  }

  const type = workInProgress.type;
  const nextProps = workInProgress.pendingProps;
  const prevProps = current !== null ? current.memoizedProps : null;

  let nextChildren = nextProps.children;
  const isDirectTextChild = shouldSetTextContent(type, nextProps);

  if (isDirectTextChild) {
    // We special case a direct text child of a host node. This is a common
    // case. We won't handle it as a reified child. We will instead handle
    // this in the host environment that also have access to this prop. That
    // avoids allocating another HostText fiber and traversing it.
    nextChildren = null;
  } else if (prevProps !== null && shouldSetTextContent(type, prevProps)) {
    // If we're switching from a direct text child to a normal child, or to
    // empty, we need to schedule the text content to be reset.
    workInProgress.effectTag |= ContentReset;
  }

  markRef(current, workInProgress);

  // Check the host config to see if the children are offscreen/hidden.
  if (
    renderExpirationTime !== Never &&
    workInProgress.mode & AsyncMode &&
    shouldDeprioritizeSubtree(type, nextProps)
  ) {
    // Schedule this fiber to re-render at offscreen priority. Then bailout.
    workInProgress.expirationTime = Never;
    workInProgress.memoizedProps = nextProps;
    return null;
  }

  reconcileChildren(
    current,
    workInProgress,
    nextChildren,
    renderExpirationTime,
  );
  memoizeProps(workInProgress, nextProps);
  return workInProgress.child;
}

function updateHostText(current, workInProgress) {
  if (current === null) {
    tryToClaimNextHydratableInstance(workInProgress);
  }
  const nextProps = workInProgress.pendingProps;
  memoizeProps(workInProgress, nextProps);
  // Nothing to do here. This is terminal. We'll do the completion step
  // immediately after.
  return null;
}

function resolveDefaultProps(Component, baseProps) {
  if (Component && Component.defaultProps) {
    // Resolve default props. Taken from ReactElement
    const props = Object.assign({}, baseProps);
    const defaultProps = Component.defaultProps;
    for (let propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
    return props;
  }
  return baseProps;
}

function mountIndeterminateComponent(
  current,
  workInProgress,
  Component,
  renderExpirationTime,
) {
  invariant(
    current === null,
    'An indeterminate component should never have mounted. This error is ' +
      'likely caused by a bug in React. Please file an issue.',
  );

  const props = workInProgress.pendingProps;
  if (
    typeof Component === 'object' &&
    Component !== null &&
    typeof Component.then === 'function'
  ) {
    Component = readLazyComponentType(Component);
    const resolvedTag = (workInProgress.tag = resolveLazyComponentTag(
      workInProgress,
      Component,
    ));
    const resolvedProps = resolveDefaultProps(Component, props);
    switch (resolvedTag) {
      case FunctionalComponentLazy: {
        return updateFunctionalComponent(
          current,
          workInProgress,
          Component,
          resolvedProps,
          renderExpirationTime,
        );
      }
      case ClassComponentLazy: {
        return updateClassComponent(
          current,
          workInProgress,
          Component,
          resolvedProps,
          renderExpirationTime,
        );
      }
      case ForwardRefLazy: {
        return updateForwardRef(
          current,
          workInProgress,
          Component,
          resolvedProps,
          renderExpirationTime,
        );
      }
      default: {
        // This message intentionally doesn't metion ForwardRef because the
        // fact that it's a separate type of work is an implementation detail.
        invariant(
          false,
          'Element type is invalid. Received a promise that resolves to: %s. ' +
            'Promise elements must resolve to a class or function.',
          Component,
        );
      }
    }
  }

  const unmaskedContext = getUnmaskedContext(workInProgress, Component, false);
  const context = getMaskedContext(workInProgress, unmaskedContext);

  prepareToReadContext(workInProgress, renderExpirationTime);

  let value;

  if (__DEV__) {
    if (
      Component.prototype &&
      typeof Component.prototype.render === 'function'
    ) {
      const componentName = getComponentName(Component) || 'Unknown';

      if (!didWarnAboutBadClass[componentName]) {
        warningWithoutStack(
          false,
          "The <%s /> component appears to have a render method, but doesn't extend React.Component. " +
            'This is likely to cause errors. Change %s to extend React.Component instead.',
          componentName,
          componentName,
        );
        didWarnAboutBadClass[componentName] = true;
      }
    }

    if (workInProgress.mode & StrictMode) {
      ReactStrictModeWarnings.recordLegacyContextWarning(workInProgress, null);
    }

    ReactCurrentOwner.current = workInProgress;
    value = Component(props, context);
  } else {
    value = Component(props, context);
  }
  // React DevTools reads this flag.
  workInProgress.effectTag |= PerformedWork;

  if (
    typeof value === 'object' &&
    value !== null &&
    typeof value.render === 'function' &&
    value.$$typeof === undefined
  ) {
    // Proceed under the assumption that this is a class instance
    workInProgress.tag = ClassComponent;

    // Push context providers early to prevent context stack mismatches.
    // During mounting we don't know the child context yet as the instance doesn't exist.
    // We will invalidate the child context in finishClassComponent() right after rendering.
    let hasContext = false;
    if (isLegacyContextProvider(Component)) {
      hasContext = true;
      pushLegacyContextProvider(workInProgress);
    } else {
      hasContext = false;
    }

    workInProgress.memoizedState =
      value.state !== null && value.state !== undefined ? value.state : null;

    const getDerivedStateFromProps = Component.getDerivedStateFromProps;
    if (typeof getDerivedStateFromProps === 'function') {
      applyDerivedStateFromProps(
        workInProgress,
        Component,
        getDerivedStateFromProps,
        props,
      );
    }

    adoptClassInstance(workInProgress, value);
    mountClassInstance(workInProgress, Component, props, renderExpirationTime);
    return finishClassComponent(
      current,
      workInProgress,
      Component,
      true,
      hasContext,
      renderExpirationTime,
    );
  } else {
    workInProgress.tag = FunctionalComponent;
    if (__DEV__) {
      if (Component) {
        warningWithoutStack(
          !Component.childContextTypes,
          '%s(...): childContextTypes cannot be defined on a functional component.',
          Component.displayName || Component.name || 'Component',
        );
      }
      if (workInProgress.ref !== null) {
        let info = '';
        const ownerName = ReactCurrentFiber.getCurrentFiberOwnerNameInDevOrNull();
        if (ownerName) {
          info += '\n\nCheck the render method of `' + ownerName + '`.';
        }

        let warningKey = ownerName || workInProgress._debugID || '';
        const debugSource = workInProgress._debugSource;
        if (debugSource) {
          warningKey = debugSource.fileName + ':' + debugSource.lineNumber;
        }
        if (!didWarnAboutStatelessRefs[warningKey]) {
          didWarnAboutStatelessRefs[warningKey] = true;
          warning(
            false,
            'Stateless function components cannot be given refs. ' +
              'Attempts to access this ref will fail.%s',
            info,
          );
        }
      }

      if (typeof Component.getDerivedStateFromProps === 'function') {
        const componentName = getComponentName(Component) || 'Unknown';

        if (!didWarnAboutGetDerivedStateOnFunctionalComponent[componentName]) {
          warningWithoutStack(
            false,
            '%s: Stateless functional components do not support getDerivedStateFromProps.',
            componentName,
          );
          didWarnAboutGetDerivedStateOnFunctionalComponent[
            componentName
          ] = true;
        }
      }
    }
    reconcileChildren(current, workInProgress, value, renderExpirationTime);
    memoizeProps(workInProgress, props);
    return workInProgress.child;
  }
}

function updatePlaceholderComponent(
  current,
  workInProgress,
  renderExpirationTime,
) {
  if (enableSuspense) {
    const nextProps = workInProgress.pendingProps;
    const alreadyCaptured =
      (workInProgress.effectTag & DidCapture) === NoEffect;

    let nextDidTimeout;
    if (current !== null && workInProgress.updateQueue !== null) {
      workInProgress.updateQueue = null;
      nextDidTimeout = true;
      reconcileChildren(current, workInProgress, null, renderExpirationTime);
      current.child = null;
    } else {
      nextDidTimeout = !alreadyCaptured;
    }

    if ((workInProgress.mode & StrictMode) !== NoEffect) {
      if (nextDidTimeout) {
        workInProgress.effectTag |= Update;
      } else {
        workInProgress.stateNode = null;
      }
    }
    const children = nextProps.children;
    let nextChildren;
    if (typeof children === 'function') {
      nextChildren = children(nextDidTimeout);
    } else {
      nextChildren = nextDidTimeout ? nextProps.fallback : children;
    }

    workInProgress.memoizedProps = nextProps;
    workInProgress.memoizedState = nextDidTimeout;
    reconcileChildren(
      current,
      workInProgress,
      nextChildren,
      renderExpirationTime,
    );
    return workInProgress.child;
  } else {
    return null;
  }
}

function updatePortalComponent(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  pushHostContainer(workInProgress, workInProgress.stateNode.containerInfo);
  const nextChildren = workInProgress.pendingProps;
  if (current === null) {
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderExpirationTime,
    );
    memoizeProps(workInProgress, nextChildren);
  } else {
    reconcileChildren(
      current,
      workInProgress,
      nextChildren,
      renderExpirationTime,
    );
    memoizeProps(workInProgress, nextChildren);
  }
  return workInProgress.child;
}

function updateContextProvider(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  const providerType: ReactProviderType<any> = workInProgress.type;
  const context: ReactContext<any> = providerType._context;

  const newProps = workInProgress.pendingProps;
  const oldProps = workInProgress.memoizedProps;

  const newValue = newProps.value;
  workInProgress.memoizedProps = newProps;

  if (__DEV__) {
    const providerPropTypes = workInProgress.type.propTypes;

    if (providerPropTypes) {
      checkPropTypes(
        providerPropTypes,
        newProps,
        'prop',
        'Context.Provider',
        ReactCurrentFiber.getCurrentFiberStackInDev,
      );
    }
  }

  pushProvider(workInProgress, newValue);

  if (oldProps !== null) {
    const oldValue = oldProps.value;
    const changedBits = calculateChangedBits(context, newValue, oldValue);
    if (changedBits === 0) {
      if (
        oldProps.children === newProps.children &&
        !hasLegacyContextChanged()
      ) {
        return bailoutOnAlreadyFinishedWork(
          current,
          workInProgress,
          renderExpirationTime,
        );
      }
    } else {
      // The context value changed. Search for matching consumers and schedule
      // them to update.
      propagateContextChange(
        workInProgress,
        context,
        changedBits,
        renderExpirationTime,
      );
    }
  }

  const newChildren = newProps.children;
  reconcileChildren(current, workInProgress, newChildren, renderExpirationTime);
  return workInProgress.child;
}

function updateContextConsumer(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  const context: ReactContext<any> = workInProgress.type;
  const newProps = workInProgress.pendingProps;
  const render = newProps.children;

  if (__DEV__) {
    warningWithoutStack(
      typeof render === 'function',
      'A context consumer was rendered with multiple children, or a child ' +
        "that isn't a function. A context consumer expects a single child " +
        'that is a function. If you did pass a function, make sure there ' +
        'is no trailing or leading whitespace around it.',
    );
  }

  prepareToReadContext(workInProgress, renderExpirationTime);
  const newValue = readContext(context, newProps.unstable_observedBits);
  let newChildren;
  if (__DEV__) {
    ReactCurrentOwner.current = workInProgress;
    ReactCurrentFiber.setCurrentPhase('render');
    newChildren = render(newValue);
    ReactCurrentFiber.setCurrentPhase(null);
  } else {
    newChildren = render(newValue);
  }

  workInProgress.effectTag |= PerformedWork;
  reconcileChildren(current, workInProgress, newChildren, renderExpirationTime);
  workInProgress.memoizedProps = newProps;
  return workInProgress.child;
}


function bailoutOnAlreadyFinishedWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
): Fiber | null {
  cancelWorkTimer(workInProgress);

  if (current !== null) {

    workInProgress.firstContextDependency = current.firstContextDependency;
  }

  if (enableProfilerTimer) {
    stopProfilerTimerIfRunning(workInProgress);
  }


  const childExpirationTime = workInProgress.childExpirationTime;
  if (
    childExpirationTime === NoWork ||
    childExpirationTime > renderExpirationTime
  ) {
    return null;
  } else {
    cloneChildFibers(current, workInProgress);
    return workInProgress.child;
  }
}


function memoizeProps(workInProgress: Fiber, nextProps: any) {
  workInProgress.memoizedProps = nextProps;
}

function memoizeState(workInProgress: Fiber, nextState: any) {
  workInProgress.memoizedState = nextState;
}

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
): Fiber | null {
  const updateExpirationTime = workInProgress.expirationTime;
  if (
    !hasLegacyContextChanged() &&
    (updateExpirationTime === NoWork ||
      updateExpirationTime > renderExpirationTime)
  ) {
    // This fiber does not have any pending work. Bailout without entering
    // the begin phase. There's still some bookkeeping we that needs to be done
    // in this optimized path, mostly pushing stuff onto the stack.
    switch (workInProgress.tag) {
      case HostRoot:
        pushHostRootContext(workInProgress);
        resetHydrationState();
        break;
      case HostComponent:
        pushHostContext(workInProgress);
        break;
      case ClassComponent: {
        const Component = workInProgress.type;
        if (isLegacyContextProvider(Component)) {
          pushLegacyContextProvider(workInProgress);
        }
        break;
      }
      case ClassComponentLazy: {
        const thenable = workInProgress.type;
        const Component = getResultFromResolvedThenable(thenable);
        if (isLegacyContextProvider(Component)) {
          pushLegacyContextProvider(workInProgress);
        }
        break;
      }
      case HostPortal:
        pushHostContainer(
          workInProgress,
          workInProgress.stateNode.containerInfo,
        );
        break;
      case ContextProvider: {
        const newValue = workInProgress.memoizedProps.value;
        pushProvider(workInProgress, newValue);
        break;
      }
      case Profiler:
        if (enableProfilerTimer) {
          workInProgress.effectTag |= Update;
        }
        break;
    }
    return bailoutOnAlreadyFinishedWork(
      current,
      workInProgress,
      renderExpirationTime,
    );
  }

  workInProgress.expirationTime = NoWork;

  switch (workInProgress.tag) {
    case IndeterminateComponent: {
      const Component = workInProgress.type;
      return mountIndeterminateComponent(
        current,
        workInProgress,
        Component,
        renderExpirationTime,
      );
    }
    case FunctionalComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      return updateFunctionalComponent(
        current,
        workInProgress,
        Component,
        unresolvedProps,
        renderExpirationTime,
      );
    }
    case FunctionalComponentLazy: {
      const thenable = workInProgress.type;
      const Component = getResultFromResolvedThenable(thenable);
      const unresolvedProps = workInProgress.pendingProps;
      const child = updateFunctionalComponent(
        current,
        workInProgress,
        Component,
        resolveDefaultProps(Component, unresolvedProps),
        renderExpirationTime,
      );
      workInProgress.memoizedProps = unresolvedProps;
      return child;
    }
    case ClassComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      return updateClassComponent(
        current,
        workInProgress,
        Component,
        unresolvedProps,
        renderExpirationTime,
      );
    }
    case ClassComponentLazy: {
      const thenable = workInProgress.type;
      const Component = getResultFromResolvedThenable(thenable);
      const unresolvedProps = workInProgress.pendingProps;
      const child = updateClassComponent(
        current,
        workInProgress,
        Component,
        resolveDefaultProps(Component, unresolvedProps),
        renderExpirationTime,
      );
      workInProgress.memoizedProps = unresolvedProps;
      return child;
    }
    case HostRoot:
      return updateHostRoot(current, workInProgress, renderExpirationTime);
    case HostComponent:
      return updateHostComponent(current, workInProgress, renderExpirationTime);
    case HostText:
      return updateHostText(current, workInProgress);
    case PlaceholderComponent:
      return updatePlaceholderComponent(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case HostPortal:
      return updatePortalComponent(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case ForwardRef: {
      const type = workInProgress.type;
      return updateForwardRef(
        current,
        workInProgress,
        type,
        workInProgress.pendingProps,
        renderExpirationTime,
      );
    }
    case ForwardRefLazy:
      const thenable = workInProgress.type;
      const Component = getResultFromResolvedThenable(thenable);
      const unresolvedProps = workInProgress.pendingProps;
      const child = updateForwardRef(
        current,
        workInProgress,
        Component,
        resolveDefaultProps(Component, unresolvedProps),
        renderExpirationTime,
      );
      workInProgress.memoizedProps = unresolvedProps;
      return child;
    case Fragment:
      return updateFragment(current, workInProgress, renderExpirationTime);
    case Mode:
      return updateMode(current, workInProgress, renderExpirationTime);
    case Profiler:
      return updateProfiler(current, workInProgress, renderExpirationTime);
    case ContextProvider:
      return updateContextProvider(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case ContextConsumer:
      return updateContextConsumer(
        current,
        workInProgress,
        renderExpirationTime,
      );
    default:
      invariant(
        false,
        'Unknown unit of work tag. This error is likely caused by a bug in ' +
          'React. Please file an issue.',
      );
  }
}

export {beginWork};