Add intersection tree plotting capability

source_code/intersection_trees/intersection_tree.py

@@ -254,3 +254,69 @@ def populate_db(db: Node | None, intervals: typing.Sequence[Interval]) -> Node:
     for interval in intervals[start_idx:]:
         db.insert(interval)
     return db
+
+
+def plot_intersection_tree(tree: Node) -> None:
+    """Visualize the intersection tree using :mod:`matplotlib`.
+
+    Each node in the tree is drawn as a horizontal line spanning the interval
+    ``[start, end]``.  The root of the tree is shown at the bottom of the
+    figure, with each subsequent level plotted above it.  The start and end
+    values of the interval are annotated next to their respective end points
+    together with the ``max_end`` value for that node.  Lines are also drawn
+    from the midpoint of each interval to the midpoints of its children to
+    illustrate the tree structure.
+
+    Parameters
+    ----------
+    tree:
+        Root node of the intersection tree to plot.
+    """
+
+    import matplotlib.pyplot as plt
+
+    # Collect all nodes along with their depth in the tree.
+    nodes: list[tuple[Node, int]] = []
+
+    def _traverse(node: Node | None, depth: int) -> None:
+        if node is None:
+            return
+        nodes.append((node, depth))
+        _traverse(node._left, depth + 1)
+        _traverse(node._right, depth + 1)
+
+    _traverse(tree, 0)
+
+    if not nodes:
+        return
+
+    fig, ax = plt.subplots()
+    max_depth = max(depth for _, depth in nodes)
+
+    # Draw intervals and record midpoints for connecting lines.
+    midpoints: dict[Node, tuple[float, int]] = {}
+    for node, depth in nodes:
+        start, end, max_end = node._start, node._end, node._max_end
+        y = depth
+        ax.hlines(y, start, end, colors="tab:blue")
+        ax.plot([start, end], [y, y], "o", color="tab:blue", markersize=3)
+        ax.text(start, y + 0.1, f"{start}", ha="center", va="bottom", fontsize=8)
+        ax.text(end, y + 0.1, f"{end}", ha="center", va="bottom", fontsize=8)
+        ax.text(end, y - 0.1, f"max={max_end}", ha="left", va="top", fontsize=8)
+        midpoints[node] = ((start + end) / 2, y)
+
+    # Connect each node's midpoint to its children's midpoints.
+    for node, depth in nodes:
+        parent_mid, parent_y = midpoints[node]
+        if node._left is not None:
+            child_mid, child_y = midpoints[node._left]
+            ax.plot([parent_mid, child_mid], [parent_y, child_y], color="tab:gray")
+        if node._right is not None:
+            child_mid, child_y = midpoints[node._right]
+            ax.plot([parent_mid, child_mid], [parent_y, child_y], color="tab:gray")
+
+    ax.set_xlabel("value")
+    ax.set_ylabel("depth")
+    ax.set_ylim(-1, max_depth + 1)
+    ax.set_title("Intersection tree")
+    plt.show()