/// <summary>
/// See https://www.youtube.com/watch?v=sD1IoalFomA
/// The graph has to be rooted. Since the algorithm is intended to work for trees I made an adjustment
/// such that I process branching nodes only once.
/// Otherwise we would process the same sub graphs again and again.
/// </summary>
public GraphNode FindCommonAncestor(GraphNode node1, GraphNode node2)
{
if (_preprocessData is null)
{
_preprocessData = PreprocessLeastCommonAncestor();
}
var index1 = _preprocessData.GraphNodeToIndex[node1];
var index2 = _preprocessData.GraphNodeToIndex[node2];
var from = Math.Min(index1, index2);
var to = Math.Max(index1, index2);
var lcaIndex = -1;
var minDepth = int.MaxValue;
for (var i = from; i <= to; i++)
{
if (_preprocessData.Depth[i] < minDepth)
{
minDepth = _preprocessData.Depth[i];
lcaIndex = i;
}
}
return(_preprocessData.EulerPath[lcaIndex]);
}
public void UpdateGraph(string hash, IEnumerable <string> allParents)
{
lock (_lockObj)
{
_preprocessData = null;
// GraphNode for the given hash.
var node = GetOrAddNode(hash);
// Update parents and child relationships
foreach (var parentHash in allParents)
{
node.Parents.Add(GetOrAddNode(parentHash));
var parent = GetOrAddNode(parentHash);
parent.Children.Add(GetOrAddNode(hash));
}
}
}
private LeaseCommonAncestorPreprocessData PreprocessLeastCommonAncestor()
{
void TraverseDepthFirst(LeaseCommonAncestorPreprocessData preprocessData, GraphNode node, int currentDepth = 0)
{
if (preprocessData.AlreadyProcessed.Add(node) is false)
{
return;
}
preprocessData.Record(node, currentDepth);
foreach (var child in node.Children)
{
TraverseDepthFirst(preprocessData, child, currentDepth + 1);
preprocessData.Record(node, currentDepth);
}
}
var data = new LeaseCommonAncestorPreprocessData();
var root = AllNodes.Single(node => !node.Parents.Any());
TraverseDepthFirst(data, root);
return(data);
}
