posts: kd-tree: add construction

content/posts/2024-08-10-kd-tree/index.md

@@ -25,3 +25,102 @@ possible acceleration structures for [ray-casting] operations.
 [ray-casting]: https://en.wikipedia.org/wiki/Ray_casting
 
 <!--more-->
+
+## Implementation
+
+As usual, this will be in Python, though its lack of proper discriminated enums
+makes it more verbose than would otherwise be necessary.
+
+### Pre-requisites
+
+Let's first define what kind of space our _k-d Tree_ is dealing with. In this
+instance $k = 3$ just like in the normal world.
+
+```python
+class Point(NamedTuple):
+    x: float
+    y: float
+    z: float
+
+class Axis(IntEnum):
+    X = 0
+    Y = 1
+    Z = 2
+
+    def next(self) -> Axis:
+        # Each level of the tree is split along a different axis
+        return Axis((self + 1) % 3)
+```
+
+### Representation
+
+The tree is represented by `KdTree`, each of its leaf nodes is a `KdLeafNode`
+and its inner nodes are `KdSplitNode`s.
+
+For each point in space, the tree can also keep track of an associated value,
+similar to a dictionary or other mapping data structure. Hence we will make our
+`KdTree` generic to this mapped type `T`.
+
+#### Leaf node
+
+A leaf node contains a number of points that were added to the tree. For each
+point, we also track their mapped value, hence the `dict[Point, T]`.
+
+```python
+class KdLeafNode[T]:
+    points: dict[Point, T]
+
+    def __init__(self):
+        self.points = {}
+
+    # Convenience constructor used when splitting a node
+    @classmethod
+    def from_items(cls, items: Iterable[tuple[Point, T]]) -> KdLeafNode[T]:
+        res = cls()
+        res.points.update(items)
+        return res
+```
+
+#### Split node
+
+An inner node must partition the space into two sub-spaces along a given axis
+and mid-point (thus defining a plane). All points that are "to the left" of the
+plane will be kept in one child, while all the points "to the right" will be in
+the other. Similar to a [_Binary Search Tree_][bst]'s inner nodes.
+
+[bst]: https://en.wikipedia.org/wiki/Binary_search_tree
+
+```python
+class KdSplitNode[T]:
+    axis: Axis
+    mid: float
+    children: tuple[KdTreeNode[T], KdTreeNode[T]]
+
+    # Convenience function to index into the child which contains `point`
+    def _index(self, point: Point) -> int:
+        return 0 if point[self.axis] <= self.mid else 1
+```
+
+#### Tree
+
+The tree itself is merely a wrapper around its inner nodes.
+
+Once annoying issue about writing this in Python is the lack of proper
+discriminated enum types. So we need to create a wrapper type for the nodes
+(`KdNode`) to allow for splitting when updating the tree.
+
+```python
+class KdNode[T]:
+    # Wrapper around leaf/inner nodes, the poor man's discriminated enum
+    inner: KdLeafNode[T] | KdSplitNode[T]
+
+    def __init__(self):
+        self.inner = KdLeafNode()
+
+class KdTree[T]:
+    _root: KdNode[T]
+
+    def __init__(self):
+        # Tree starts out empty
+        self._root = KdNode()
+```