function reconcileChildrenArray(returnFiber, currentFirstChild, newChildren, lanes) {
      // This algorithm can't optimize by searching from both ends since we
      // don't have backpointers on fibers. I'm trying to see how far we can get
      // with that model. If it ends up not being worth the tradeoffs, we can
      // add it later.
      // Even with a two ended optimization, we'd want to optimize for the case
      // where there are few changes and brute force the comparison instead of
      // going for the Map. It'd like to explore hitting that path first in
      // forward-only mode and only go for the Map once we notice that we need
      // lots of look ahead. This doesn't handle reversal as well as two ended
      // search but that's unusual. Besides, for the two ended optimization to
      // work on Iterables, we'd need to copy the whole set.
      // In this first iteration, we'll just live with hitting the bad case
      // (adding everything to a Map) in for every insert/move.
      // If you change this code, also update reconcileChildrenIterator() which
      // uses the same algorithm.
      {
        // First, validate keys.
        var knownKeys = null;

        for (var i = 0; i < newChildren.length; i++) {
          var child = newChildren[i];
          knownKeys = warnOnInvalidKey(child, knownKeys, returnFiber);
        }
      }

      var resultingFirstChild = null;
      var previousNewFiber = null;
      var oldFiber = currentFirstChild;
      var lastPlacedIndex = 0;
      var newIdx = 0;
      var nextOldFiber = null;

      for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
        if (oldFiber.index > newIdx) {
          nextOldFiber = oldFiber;
          oldFiber = null;
        } else {
          nextOldFiber = oldFiber.sibling;
        }

        var newFiber = updateSlot(returnFiber, oldFiber, newChildren[newIdx], lanes);

        if (newFiber === null) {
          // TODO: This breaks on empty slots like null children. That's
          // unfortunate because it triggers the slow path all the time. We need
          // a better way to communicate whether this was a miss or null,
          // boolean, undefined, etc.
          if (oldFiber === null) {
            oldFiber = nextOldFiber;
          }

          break;
        }

        if (shouldTrackSideEffects) {
          if (oldFiber && newFiber.alternate === null) {
            // We matched the slot, but we didn't reuse the existing fiber, so we
            // need to delete the existing child.
            deleteChild(returnFiber, oldFiber);
          }
        }

        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);

        if (previousNewFiber === null) {
          // TODO: Move out of the loop. This only happens for the first run.
          resultingFirstChild = newFiber;
        } else {
          // TODO: Defer siblings if we're not at the right index for this slot.
          // I.e. if we had null values before, then we want to defer this
          // for each null value. However, we also don't want to call updateSlot
          // with the previous one.
          previousNewFiber.sibling = newFiber;
        }

        previousNewFiber = newFiber;
        oldFiber = nextOldFiber;
      }

      if (newIdx === newChildren.length) {
        // We've reached the end of the new children. We can delete the rest.
        deleteRemainingChildren(returnFiber, oldFiber);

        if (getIsHydrating()) {
          var numberOfForks = newIdx;
          pushTreeFork(returnFiber, numberOfForks);
        }

        return resultingFirstChild;
      }

      if (oldFiber === null) {
        // If we don't have any more existing children we can choose a fast path
        // since the rest will all be insertions.
        for (; newIdx < newChildren.length; newIdx++) {
          var _newFiber = createChild(returnFiber, newChildren[newIdx], lanes);

          if (_newFiber === null) {
            continue;
          }

          lastPlacedIndex = placeChild(_newFiber, lastPlacedIndex, newIdx);

          if (previousNewFiber === null) {
            // TODO: Move out of the loop. This only happens for the first run.
            resultingFirstChild = _newFiber;
          } else {
            previousNewFiber.sibling = _newFiber;
          }

          previousNewFiber = _newFiber;
        }

        if (getIsHydrating()) {
          var _numberOfForks = newIdx;
          pushTreeFork(returnFiber, _numberOfForks);
        }

        return resultingFirstChild;
      } // Add all children to a key map for quick lookups.


      var existingChildren = mapRemainingChildren(returnFiber, oldFiber); // Keep scanning and use the map to restore deleted items as moves.

      for (; newIdx < newChildren.length; newIdx++) {
        var _newFiber2 = updateFromMap(existingChildren, returnFiber, newIdx, newChildren[newIdx], lanes);

        if (_newFiber2 !== null) {
          if (shouldTrackSideEffects) {
            if (_newFiber2.alternate !== null) {
              // The new fiber is a work in progress, but if there exists a
              // current, that means that we reused the fiber. We need to delete
              // it from the child list so that we don't add it to the deletion
              // list.
              existingChildren.delete(_newFiber2.key === null ? newIdx : _newFiber2.key);
            }
          }

          lastPlacedIndex = placeChild(_newFiber2, lastPlacedIndex, newIdx);

          if (previousNewFiber === null) {
            resultingFirstChild = _newFiber2;
          } else {
            previousNewFiber.sibling = _newFiber2;
          }

          previousNewFiber = _newFiber2;
        }
      }

      if (shouldTrackSideEffects) {
        // Any existing children that weren't consumed above were deleted. We need
        // to add them to the deletion list.
        existingChildren.forEach(function (child) {
          return deleteChild(returnFiber, child);
        });
      }

      if (getIsHydrating()) {
        var _numberOfForks2 = newIdx;
        pushTreeFork(returnFiber, _numberOfForks2);
      }

      return resultingFirstChild;
    }
Warns if there is a duplicate or missing key