/// <summary>
/// Uses Prim's algorithm to build an MST spanning the mstNodes.
/// O(|mstNodes|^2) runtime.
/// </summary>
/// <param name="startIndex">The node index to start from.</param>
public void Span(int startIndex)
{
// All nodes that are not yet included.
var toAdd = new List <int>(_mstNodes.Count);
// If the index node is already included.
var inMst = new bool[_distances.CacheSize];
// The spanning edges.
var mstEdges = new List <DirectedGraphEdge>(_mstNodes.Count);
using (var adjacentEdgeQueue = new LinkedListPriorityQueue <DirectedGraphEdge>(100, _mstNodes.Count * _mstNodes.Count))
{
foreach (var t in _mstNodes)
{
if (t != startIndex)
{
toAdd.Add(t);
adjacentEdgeQueue.Enqueue(new DirectedGraphEdge(startIndex, t),
_distances[startIndex, t]);
}
}
inMst[startIndex] = true;
while (toAdd.Count > 0 && !adjacentEdgeQueue.IsEmpty)
{
int newIn;
DirectedGraphEdge shortestEdge;
// Dequeue and ignore edges that are already inside the MST.
// Add the first one that is not.
do
{
shortestEdge = adjacentEdgeQueue.Dequeue();
newIn = shortestEdge.Outside;
} while (inMst[newIn]);
mstEdges.Add(shortestEdge);
inMst[newIn] = true;
// Find all newly adjacent edges and enqueue them.
for (var i = 0; i < toAdd.Count; i++)
{
var otherNode = toAdd[i];
if (otherNode == newIn)
{
toAdd.RemoveAt(i--);
}
else
{
adjacentEdgeQueue.Enqueue(new DirectedGraphEdge(newIn, otherNode),
_distances[newIn, otherNode]);
}
}
}
}
SpanningEdges = mstEdges;
}
private void InitializeDataStructures(
int startIndex, int[] notIncluded, ref int notIncludedCount,
LinkedListPriorityQueue <DirectedGraphEdge> adjacentEdgeQueue)
{
for (var i = 0; i < _mstNodes.Count; i++)
{
var t = _mstNodes[i];
if (t != startIndex)
{
notIncluded[notIncludedCount] = t;
notIncludedCount++;
var distance = _distances[startIndex, t];
adjacentEdgeQueue.Enqueue(new DirectedGraphEdge(startIndex, t), distance);
}
}
}
public void Test()
{
int[] queueTestOrder = { 10, 3, 11, 6, -3, 17, 13, -6, 2, 8, -2, -8 };
var queue = new LinkedListPriorityQueue <TestNode>(30, queueTestOrder.Length);
foreach (int t in queueTestOrder)
{
if (t > 0)
{
queue.Enqueue(new TestNode(t), (uint)t);
}
if (t < 0)
{
Assert.IsTrue(queue.Dequeue().Priority == -t);
}
}
}
/// <summary>
/// Uses Prim's algorithm to build an MST spanning the mstNodes.
/// O(|mstNodes|^2) runtime.
/// </summary>
/// <param name="startIndex">The node index to start from.</param>
public IReadOnlyList <DirectedGraphEdge> Span(int startIndex)
{
var notIncluded = ArrayPool <int> .Shared.Rent(_mstNodes.Count);
var notIncludedCount = 0;
var isIncluded = ArrayPool <bool> .Shared.Rent(_distances.CacheSize);
Array.Clear(isIncluded, 0, isIncluded.Length);
_spanningEdges?.Dispose();
_spanningEdges = new PooledList <DirectedGraphEdge>(_mstNodes.Count);
using (var adjacentEdgeQueue = new LinkedListPriorityQueue <DirectedGraphEdge>(100, _mstNodes.Count * _mstNodes.Count))
{
InitializeDataStructures(startIndex, notIncluded, ref notIncludedCount, adjacentEdgeQueue);
isIncluded[startIndex] = true;
while (notIncludedCount > 0 && !adjacentEdgeQueue.IsEmpty)
{
int newIn;
DirectedGraphEdge shortestEdge;
// Dequeue and ignore edges that are already inside the MST.
// Add the first one that is not.
do
{
shortestEdge = adjacentEdgeQueue.Dequeue();
newIn = shortestEdge.Outside;
} while (isIncluded[newIn]);
_spanningEdges.Add(shortestEdge);
isIncluded[newIn] = true;
// Find all newly adjacent edges and enqueue them.
IncludeNode(notIncluded, ref notIncludedCount, adjacentEdgeQueue, newIn);
}
}
ArrayPool <bool> .Shared.Return(isIncluded);
ArrayPool <int> .Shared.Return(notIncluded);
return(_spanningEdges);
}
private void IncludeNode(
int[] notIncluded, ref int notIncludedCount,
LinkedListPriorityQueue <DirectedGraphEdge> adjacentEdgeQueue,
int node)
{
for (var i = 0; i < notIncludedCount; i++)
{
var otherNode = notIncluded[i];
if (otherNode == node)
{
RemoveAt(notIncluded, ref notIncludedCount, i);
i--;
}
else
{
var distance = _distances[node, otherNode];
adjacentEdgeQueue.Enqueue(new DirectedGraphEdge(node, otherNode), distance);
}
}
}
