RFR: 8314512: IGV: clean up hierarchical layout code [v2]

[Roberto Castañeda Lozano]

On Thu, 28 Nov 2024 08:33:23 GMT, Tobias Holenstein <tholenstein at openjdk.org> wrote:

>> This refactoring enhances layer and node management by encapsulating computations within LayoutGraph, LayoutLayer, and LayoutNode, improving modularity and delegation of responsibilities for handling dummy nodes and self-edges to dedicated utility classes. Code duplication was reduced by consolidating common logic, such as edge reversal, into reusable methods, and optimizing processes like updating node positions and crossings. Additionally, method names were updated to better reflect their functionality,
>> 
>> ### LayoutGraph
>> The LayoutGraph class is responsible for organizing and arranging a graph's nodes and edges for visual display. It takes a collection of nodes (Vertex) and connections between them (Link) and structures them into layers, creating a hierarchical layout. The class handles complexities like edges that span multiple layers by inserting temporary "dummy" nodes to maintain a clear hierarchy. This organization helps ensure that when the graph is displayed, it is easy to understand and visually coherent, making the relationships between nodes clear and straightforward.
>> 
>> ### LayoutLayer
>> The LayoutLayer class represents a single horizontal layer in a hierarchical graph layout. It holds a list of nodes (LayoutNode) that are all on the same vertical level. This class provides simple methods to manage these nodes: you can add nodes to the layer, calculate the maximum height needed to fit all nodes, center the nodes vertically within the layer, and set their horizontal positions with proper spacing. In essence, LayoutLayer helps organize nodes neatly in a graph, making it easier to display the graph clearly and understand the relationships between nodes.
>> 
>> ### LayoutNode
>> The LayoutNode class represents a node in a hierarchical graph layout. It can be either an actual node from the original graph or a temporary "dummy" node added during the layout process to handle complex edge connections. This class stores important layout information like the node's position (x and y coordinates), size (width and height), layer level, and connections to other nodes through incoming and outgoing edges. It provides methods to calculate optimal positions, manage margins, and handle reversed edges, all aimed at arranging the nodes neatly in layers to create a clear and visually organized graph display.
>> 
>> ### LayoutEdge
>> The LayoutEdge class represents a connection between two nodes (LayoutNode) in a hierarchical graph layout. It stores information ...
>
> Tobias Holenstein has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fixed graph objects equality

src/utils/IdealGraphVisualizer/HierarchicalLayout/src/main/java/com/sun/hotspot/igv/hierarchicallayout/HierarchicalLayoutManager.java line 75:

> 73: 
> 74:         static public void apply(LayoutGraph graph) {
> 75:             removeSelfEdges(graph);

The proposed code removes self-edges unconditionally. This is OK for the sea-of-nodes layout, but for the CFG layout we do need to draw self-edges (think about single basic block loops). Here is an (artificially edited) example of how the new algorithm misses drawing a self-edge for B7 (left is current IGV, right is IGV with your proposed changes):

![Screenshot from 2024-11-28 10-09-09](https://github.com/user-attachments/assets/e91b55af-0fbf-4f28-b7e6-558dfcada42a)

Here is the artificially edited graph file that illustrates the issue: [self-edges.zip](https://github.com/user-attachments/files/17945729/self-edges.zip)

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PR Review Comment: https://git.openjdk.org/jdk/pull/22402#discussion_r1861795723