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Nested Subgraph Layout

Overview

This document describes the implementation of nested subgraph support in Ferrite's flowchart renderer. Mermaid supports subgraphs that can contain other subgraphs, creating a hierarchical structure.

Key Changes (Task 56)

1. Nested Margin Support

Added nested_subgraph_margin configuration (10.0 pixels by default) to create proper visual separation between parent and child subgraph boundaries.

When computing subgraph bounds in compute_subgraph_layouts():

  • Child subgraph bounds include extra margin around them
  • Parent subgraphs with nested children use increased effective padding
  • This prevents parent and child borders from appearing too close together

2. Accurate Depth Calculation

The compute_subgraph_depths() function computes actual nesting depth:

  • Depth 0 = top-level subgraph (no parent)
  • Depth 1 = child of top-level
  • Depth 2 = grandchild, etc.

This enables proper alternating fill colors for visual distinction between nesting levels.

3. Subgraph Direction Overrides

Infrastructure added to support per-subgraph direction overrides:

  • FlowGraph.subgraph_directions stores direction overrides from parsing
  • SubgraphLayoutEngine uses effective direction for each subgraph
  • Example: A TD flowchart can have a subgraph with direction LR

File Structure

src/markdown/mermaid/flowchart.rs
├── FlowLayoutConfig
│   └── nested_subgraph_margin: f32  // Extra margin between nested boundaries
├── FlowGraph
│   └── subgraph_directions: HashMap<String, FlowDirection>  // Direction overrides
├── SubgraphLayoutEngine
│   ├── layout_subgraph() - Uses effective direction per subgraph
│   ├── layout_simple_subgraph() - Accepts direction parameter
│   └── layout_hierarchical_subgraph() - Accepts direction parameter
├── compute_subgraph_layouts() - Adds nested margins to bounds
└── compute_subgraph_depths() - Calculates actual nesting depth

Configuration Values

Parameter Value Description
subgraph_padding 15.0 Standard padding around subgraph content
subgraph_title_height 24.0 Height reserved for title
nested_subgraph_margin 10.0 Extra margin between nested boundaries

Test Cases

See test_md/test_flowcharts.md for nested subgraph test cases:

  1. True Nested Subgraphs - Parent-child relationship
  2. Deeply Nested Subgraphs - 3 levels deep
  3. Nested with Direction Override - Child uses different direction

Known Limitations

  1. Internal Layouts Not Applied: The SubgraphLayoutEngine computes internal layouts for subgraphs, but these aren't currently applied to actual node positions. Nodes are positioned by the global Sugiyama algorithm.

  2. Direction Override Visual Effect: While direction overrides are parsed and stored, the visual effect is limited because internal layouts aren't applied to final positions.

Future Improvements

  • Apply internal subgraph layouts to node positions
  • Support treating subgraphs as "super-nodes" in the global layout
  • Improve edge routing through multiple nested levels

Related Documentation

  • flowchart-subgraph-title.md - Title width expansion (Task 55)
  • subgraph-internal-layout.md - Internal positioning