# Node Collection Feature - Implementation Complete

## Summary

Added comprehensive node collection and traversal utilities to `ascii-dag` for gathering all reachable nodes in a graph while automatically handling cycles.

## Changes Made

### New Module: `src/layout/generic/traversal.rs`

**Functions:**
- `collect_all_nodes_fn()` - BFS traversal collecting all reachable nodes
- `collect_all_nodes_dfs_fn()` - DFS variant for deep graphs
- `NodeCollectable` trait - Convenient traversal methods

**Key Features:**
- Automatic cycle detection and handling via visited tracking
- Works with any data structure via closures (generic algorithms)
- Both BFS and DFS variants available
- Multiple starting points supported
- Zero-copy where possible

### Integration

**Updated Files:**
- `src/layout/generic.rs` - Added `pub mod traversal;` and documentation
- `CHANGELOG.md` - Added unreleased section documenting new feature
- `README.md` - Added usage example in generic algorithms section

**New Example:**
- `examples/node_collection.rs` - Comprehensive demonstrations including:
  - Simple tree traversal
  - Diamond dependency (node visited once despite multiple paths)
  - Cyclic graph handling
  - Multiple starting points
  - BFS vs DFS order comparison
  - Real-world use case: PII redaction in error diagnostics

## Test Results

```
running 53 tests
...
test layout::generic::traversal::tests::test_collect_diamond ... ok
test layout::generic::traversal::tests::test_collect_multiple_starts ... ok
test layout::generic::traversal::tests::test_collect_simple_tree ... ok
test layout::generic::traversal::tests::test_collect_with_cycle ... ok
test layout::generic::traversal::tests::test_dfs_order ... ok
...

test result: ok. 53 passed; 0 failed; 0 ignored; 0 measured
```

All existing tests continue to pass. 5 new tests added for traversal functionality.

## Use Cases for Quackpatch

This enables correct filter implementation in quackpatch:

```rust
// Collect all diagnostics in error chain (including nested caused_by and related)
let all_diagnostic_ids = collect_all_nodes_fn(&[root_id], |&id| {
    let diag = &diagnostics[id];
    let mut children = Vec::new();
    
    // Add caused_by chain
    if let Some(cause_id) = diag.caused_by {
        children.push(cause_id);
    }
    
    // Add related diagnostics
    children.extend(&diag.related);
    
    children
});

// Now PII redaction filter can process ALL diagnostics, not just root
for id in all_diagnostic_ids {
    redact_pii(&mut diagnostics[id]);
}
```

## Next Steps for Quackpatch

1. Update `Filter` trait to use summary-based filtering:
   ```rust
   fn filter(&self, summary: &mut DiagnosticSummary) -> FilterResult
   ```

2. Use `collect_all_nodes_fn()` in filter implementations to traverse diagnostic graphs

3. Add helper methods to `DiagnosticSummary`:
   ```rust
   impl DiagnosticSummary {
       pub fn all_diagnostic_ids(&self) -> Vec<usize> {
           collect_all_nodes_fn(&self.roots, |id| self.get_children(id))
       }
   }
   ```

## API Example

```rust
use ascii_dag::layout::generic::traversal::collect_all_nodes_fn;

// Simple usage
let all_nodes = collect_all_nodes_fn(&start_nodes, |node| {
    get_children(node)
});

// Trait-based usage
use ascii_dag::layout::generic::traversal::NodeCollectable;

impl NodeCollectable for MyGraph {
    type Id = usize;
    
    fn get_all_ids(&self) -> Vec<usize> { self.nodes.clone() }
    fn get_children(&self, id: &usize) -> Vec<usize> { 
        self.edges[id].clone() 
    }
}

// Now you can call:
let all = graph.collect_all_nodes(&[start]);
```

## Documentation

Full documentation with examples in:
- Doc comments in `src/layout/generic/traversal.rs`
- `examples/node_collection.rs` - 6 comprehensive examples
- `README.md` - Quick start in generic algorithms section
- `CHANGELOG.md` - Feature documentation for next release

## Performance

- **Time Complexity**: O(V + E) where V = nodes, E = edges
- **Space Complexity**: O(V) for visited set and result vector
- **Memory Efficient**: DFS variant available for deep graphs
- **Cycle Safe**: Visited tracking prevents infinite loops
