/// <summary>
/// Finds the leaf quadrant which contains <paramref name="location"/> by
/// traversing downward starting at <paramref name="initialQuadrant"/> using
/// upgradeable locks.
/// </summary>
/// <returns>The leaf quadrant containing the location. The current thread
/// will hold an upgradable lock on this quadrant.</returns>
/// <param name="initialQuadrant">
/// The quadrant from which to start downward traversal. This must contain
/// <paramref name="location"/> and the current thread should hold an
/// upgradeable lock on it. This lock will be released unless this quadrant
/// happens to be the return value.
/// </param>
/// <param name="location">The location for which to find a leaf quadrant.</param>
Quadrant FindAndLockTargetLeafQuadrant(Quadrant initialQuadrant, GeoCoordinates location)
{
Debug.Assert(initialQuadrant.Contains(location));
Debug.Assert(initialQuadrant.Lock.ThreadOwnsUpgradeableLock);
Quadrant targetQuadrant = initialQuadrant;
while (!targetQuadrant.IsLeaf)
{
Quadrant next = targetQuadrant.GetChildContaining(location);
next.Lock.EnterUpgradeableLock();
targetQuadrant.Lock.ExitUpgradeableLock();
targetQuadrant = next;
}
Debug.Assert(targetQuadrant.Contains(location));
return(targetQuadrant);
}
/// <summary>
/// Moves the element to a new position. This will return false and do
/// nothing if another thread is currently adjusting this element.
/// </summary>
/// <returns><c>true</c>, if element was moved, <c>false</c> otherwise.</returns>
/// <param name="elementRef">Element.</param>
/// <param name="newCoordinates">New coordinates.</param>
public bool MoveElement(IElement elementRef, GeoCoordinates newCoordinates)
{
Element element = (Element)elementRef;
if (element.IsMarkedForRemoval)
{
return(false);
}
if (!element.TrySetBusy())
{
return(false);
}
// This check has to happen after TrySetBusy() to avoid a race
// condition with RemoveElement(). After TrySetBusy() succeeds,
// no other move or remove operation can run on this element,
// so we are guaranteed not to move the element after it has been
// removed. If the element will be removed, we just abort the move.
if (element.IsMarkedForRemoval)
{
element.SetNotBusy();
return(false);
}
while (true)
{
Quadrant originalQuadrant = GetAndLockQuadrantForElement(element, QuadrantLock.LockType.Shared);
// Catch tricky issues early.
Debug.Assert(originalQuadrant.Lock.ThreadOwnsSharedLock);
Debug.Assert(!originalQuadrant.IsDisconnected);
Debug.Assert(originalQuadrant.IsLeaf);
if (originalQuadrant.Contains(newCoordinates))
{
// Case (1): new position is inside the old quadrant. Just change x and y.
element.Coordinates = newCoordinates;
originalQuadrant.Lock.ExitSharedLock();
break;
}
// Case (2): new position is inside a new quadrant. Need to move
// to a new quadrant.
// The root node should contain all elements, so the above if statement
// should prevent this assert.
Debug.Assert(originalQuadrant != rootNode);
// We have to lock the parent to avoid deadlocking with a join
// operation if the new quadrant is also a child of the parent.
if (!SwitchSharedLockToExclusiveLockAndLockParent(originalQuadrant))
{
// If in between releasing the shared lock and acquiring the
// exclusive lock the quadrant became disconnected (due to
// a join on its parent) or became subdivided, we have to
// restart the move.
continue;
}
Debug.Assert(originalQuadrant.Lock.ThreadOwnsExlusiveLock);
Debug.Assert(originalQuadrant.Parent.Lock.ThreadOwnsSharedLock);
// If originalQuadrant was not subdivided or joined (or was
// subdivided and joined an equal number of times), then element
// could not have been moved because we set it to busy.
Debug.Assert(originalQuadrant.Data.Contains(element));
// Traverse upward to search for the nearest ancestor that contains
// the new position. No locking is necessary because ancestors
// cannot be subdivided since they're not leaves, and they cannot
// be joined because we hold a lock on a child node.
Quadrant closestContainingAncestor = originalQuadrant.Parent;
while (!closestContainingAncestor.Contains(newCoordinates))
{
closestContainingAncestor = closestContainingAncestor.Parent;
// This only happens if rootNode doesn't contain the new
// position, but that cannot happen.
Debug.Assert(closestContainingAncestor != null);
}
// Traverse downward to target quadrant. Use locking like in an insert operation.
// It cannot be a leaf node because it is an ancestor node.
Debug.Assert(!closestContainingAncestor.IsLeaf);
// Find the leaf quadrant that contains the new coordinates.
Quadrant targetQuadrant = closestContainingAncestor.GetChildContaining(newCoordinates);
targetQuadrant.Lock.EnterUpgradeableLock();
targetQuadrant = FindAndLockTargetLeafQuadrant(targetQuadrant, newCoordinates);
targetQuadrant.Lock.EnterExclusiveLock();
// Change the element's coordinates and move it to the new quadrant.
element.Coordinates = newCoordinates;
originalQuadrant.Data.Remove(element);
targetQuadrant.Data.Add(element);
elementsToNodes[element] = targetQuadrant;
// If the target quadrant now contains too much data, mark it as a candidate
// for subdivision.
if (targetQuadrant.Data.Count > maxLeafCapacity && targetQuadrant.SubdivisionLevel < maxSubdivisionLevel)
{
candidatesForSubdivide.TryAdd(targetQuadrant);
}
// If the original quadrant is now nearly empty, mark its parent
// as a candidate for a join operation.
if (originalQuadrant.Parent != null && originalQuadrant.Data.Count < minLeafCapacity)
{
candidatesForJoin.TryAdd(originalQuadrant.Parent);
}
targetQuadrant.Lock.ExitExclusiveLock();
targetQuadrant.Lock.ExitUpgradeableLock();
// Release the locks on the original quadrant and its parent,
// which were acquired earlier.
originalQuadrant.Lock.ExitExclusiveLock();
originalQuadrant.Parent.Lock.ExitSharedLock();
break;
}
element.SetNotBusy();
return(true);
}
