/// <summary>
/// Makes node within the tree the root child, reorganizing the nodes and child arrays.
///
/// Critically, we do this operation in situ to avoid having to transiently allocate
/// extremely large memory objects.
///
/// The operation consists of 3 stages:
/// - traverse the subtree breadth-first starting at the new root,
/// building a bitmap of which nodes are children.
///
/// - traverse the node array sequentially, processing each node that is a member of this bitmap by
/// moving it into the new position in the store (including modifying associated child and parent references)
/// Also we build a table of all the new children that need to be moved.
///
/// - using the child table built above, shift all children down. Because nodes may have children written
/// out of order, we don't know for sure there is enough space available. Therefore we sort the table
/// based on their new position in the table and then shift them down, insuring we don't overwrite
/// children yet to be shifted.
///
/// Additionally we may have to recreate the transposition roots dictionary because
/// the set of roots is smaller (the retined subtree only) and the node indices will change.
/// </summary>
/// <param name="store"></param>
/// <param name="newRootChild"></param>
/// <param name="newPriorMoves"></param>
/// <param name="transpositionRoots"></param>
public static void MakeChildNewRoot(MCTSNodeStore store, float policySoftmax,
ref MCTSNodeStruct newRootChild,
PositionWithHistory newPriorMoves,
PositionEvalCache cacheNonRetainedNodes,
TranspositionRootsDict transpositionRoots)
{
#if DEBUG
store.Validate();
#endif
COUNT++;
// Nothing to do if the requested node is already currently the root
if (newRootChild.Index == store.RootNode.Index)
{
// Nothing changing in the tree, just flush the cache references
store.ClearAllCacheIndices();
}
else
{
DoMakeChildNewRoot(store, policySoftmax, ref newRootChild, newPriorMoves, cacheNonRetainedNodes, transpositionRoots);
}
#if DEBUG
store.Validate();
#endif
}
static void DoMakeChildNewRoot(MCTSNodeStore store, float policySoftmax, ref MCTSNodeStruct newRootChild,
PositionWithHistory newPriorMoves,
PositionEvalCache cacheNonRetainedNodes,
TranspositionRootsDict transpositionRoots)
{
ChildStartIndexToNodeIndex[] childrenToNodes;
uint numNodesUsed;
uint numChildrenUsed;
BitArray includedNodes;
int newRootChildIndex = newRootChild.Index.Index;
int newIndexOfNewParent = -1;
int nextAvailableNodeIndex = 1;
// Traverse this subtree, building a bit array of visited nodes
includedNodes = MCTSNodeStructUtils.BitArrayNodesInSubtree(store, ref newRootChild, out numNodesUsed);
//using (new TimingBlock("Build position cache "))
if (cacheNonRetainedNodes != null)
{
long estNumNodes = store.RootNode.N - numNodesUsed;
cacheNonRetainedNodes.InitializeWithSize((int)estNumNodes);
ExtractPositionCacheNonRetainedNodes(store, policySoftmax, includedNodes, in newRootChild, cacheNonRetainedNodes);
}
// We will constract a table indicating the starting index and length of
// children associated with the nodes we are extracting
childrenToNodes = GC.AllocateUninitializedArray <ChildStartIndexToNodeIndex>((int)numNodesUsed);
void RewriteNodes()
{
// TODO: Consider that the above is possibly all we need to do in some case
// Suppose the subtree is very large relative to the whole
// This approach would be much faster, and orphan an only small part of the storage
// Now scan all above nodes.
// If they don't belong, ignore.
// If they do belong, swap them down to the next available lower location
// Note that this can't be parallelized, since we have to do it strictly in order of node index
int numRewrittenNodesDone = 0;
for (int i = 2; i < store.Nodes.nextFreeIndex; i++)
{
if (includedNodes.Get(i))
{
ref MCTSNodeStruct thisNode = ref store.Nodes.nodes[i];
// Reset any cache entry
thisNode.CacheIndex = 0;
// Not possible to support transposition linked nodes,
// since the root may be in a part of the tree that is not retained
// and possibly already overwritten.
// We expect them to have already been materialized by the time we reach this point.
Debug.Assert(!thisNode.IsTranspositionLinked);
Debug.Assert(thisNode.NumNodesTranspositionExtracted == 0);
// Remember this location if this is the new parent
if (i == newRootChildIndex)
{
newIndexOfNewParent = nextAvailableNodeIndex;
}
// Move the actual node
MoveNodePosition(store, new MCTSNodeStructIndex(i), new MCTSNodeStructIndex(nextAvailableNodeIndex));
// Reset all transposition information
thisNode.NextTranspositionLinked = 0;
childrenToNodes[numRewrittenNodesDone] = new ChildStartIndexToNodeIndex(thisNode.childStartBlockIndex, nextAvailableNodeIndex, thisNode.NumPolicyMoves);
// Re-insert this into the transpositionRoots (with the updated node index)
if (transpositionRoots != null)
{
transpositionRoots.TryAdd(thisNode.ZobristHash, nextAvailableNodeIndex);
}
Debug.Assert(thisNode.NumNodesTranspositionExtracted == 0);
numRewrittenNodesDone++;
nextAvailableNodeIndex++;
}
}
}
