/// <summary>
/// Creates the root node for a new <see cref="KdTree{T}"/> given
/// a set of data points and their respective stored values.
/// </summary>
///
/// <param name="points">The data points to be inserted in the tree.</param>
/// <param name="values">The values associated with each point.</param>
/// <param name="leaves">Return the number of leaves in the root subtree.</param>
/// <param name="inPlace">Whether the given <paramref name="points"/> vector
/// can be ordered in place. Passing true will change the original order of
/// the vector. If set to false, all operations will be performed on an extra
/// copy of the vector.</param>
///
/// <returns>The root node for a new <see cref="KdTree{T}"/>
/// contained the given <paramref name="points"/>.</returns>
///
protected static KDTreeNode <T> CreateRoot(double[][] points, T[] values, bool inPlace, out int leaves)
{
// Initial argument checks for creating the tree
if (points == null)
{
throw new ArgumentNullException("points");
}
if (values != null && points.Length != values.Length)
{
throw new Exception("values");
}
if (!inPlace)
{
points = (double[][])points.Clone();
if (values != null)
{
values = (T[])values.Clone();
}
}
leaves = 0;
int dimensions = points[0].Length;
// Create a comparer to compare individual array
// elements at specified positions when sorting
ElementComparer comparer = new ElementComparer();
// Call the recursive algorithm to operate on the whole array (from 0 to points.Length)
KDTreeNode <T> root = create(points, values, 0, dimensions, 0, points.Length, comparer, ref leaves);
// Create and return the newly formed tree
return(root);
}
private static KDTreeNode <T> create(double[][] points, T[] values,
int depth, int k, int start, int length, ElementComparer comparer, ref int leaves)
{
if (length <= 0)
{
return(null);
}
// We will be doing sorting in place
int axis = comparer.Index = depth % k;
Array.Sort(points, values, start, length, comparer);
// Middle of the input section
int half = start + length / 2;
// Start and end of the left branch
int leftStart = start;
int leftLength = half - start;
// Start and end of the right branch
int rightStart = half + 1;
int rightLength = length - length / 2 - 1;
// The median will be located halfway in the sorted array
var median = points[half];
var value = values != null ? values[half] : default(T);
depth++;
// Continue with the recursion, passing the appropriate left and right array sections
KDTreeNode <T> left = create(points, values, depth, k, leftStart, leftLength, comparer, ref leaves);
KDTreeNode <T> right = create(points, values, depth, k, rightStart, rightLength, comparer, ref leaves);
if (left == null && right == null)
{
leaves++;
}
// Backtrack and create
return(new KDTreeNode <T>()
{
Axis = axis,
Position = median,
Value = value,
Left = left,
Right = right,
});
}
