ruvector-mincut 2.0.6

# Graph Sparsification Implementation

## Overview

Implemented complete graph sparsification module at `/home/user/ruvector/crates/ruvector-mincut/src/sparsify/mod.rs` for (1+ε)-approximate minimum cuts using O(n log n / ε²) edges.

## Implementation Details

### 1. Core Structures

#### SparsifyConfig
```rust
pub struct SparsifyConfig {
    pub epsilon: f64,        // Approximation parameter (0 < ε ≤ 1)
    pub seed: Option<u64>,   // Random seed for reproducibility
    pub max_edges: Option<usize>, // Maximum edges limit
}
```

**Features:**
- Builder pattern with `with_seed()` and `with_max_edges()`
- Validation for epsilon parameter
- Default configuration with ε = 0.1

#### SparseGraph
```rust
pub struct SparseGraph {
    graph: DynamicGraph,              // The sparsified graph
    edge_weights: HashMap<EdgeId, Weight>, // Original weights
    epsilon: f64,                     // Approximation parameter
    original_edges: usize,            // Original edge count
    rng: StdRng,                      // Random number generator
    strength_calc: EdgeStrength,      // Edge strength calculator
}
```

**Features:**
- `from_graph()`: Create sparsified version using Benczúr-Karger
- `num_edges()`: Get edge count (should be O(n log n / ε²))
- `sparsification_ratio()`: Ratio of sparse to original edges
- `approximate_min_cut()`: Query approximate minimum cut
- `insert_edge()`: Dynamic edge insertion with resampling
- `delete_edge()`: Dynamic edge deletion
- `epsilon()`: Get approximation parameter

### 2. Sparsification Algorithms

#### Benczúr-Karger Sparsification
**Algorithm:**
1. Compute edge strengths λ_e for all edges
2. Calculate sampling probability: p_e = min(1, c·log(n) / (ε²·λ_e))
3. Sample each edge with probability p_e
4. Scale sampled edge weights by 1/p_e

**Implementation:**
```rust
fn benczur_karger_sparsify(
    original: &DynamicGraph,
    sparse: &DynamicGraph,
    edge_weights: &mut HashMap<EdgeId, Weight>,
    strength_calc: &mut EdgeStrength,
    epsilon: f64,
    rng: &mut StdRng,
    max_edges: Option<usize>,
) -> Result<()>
```

**Properties:**
- Preserves (1±ε) approximation of all cuts
- O(n log n / ε²) expected edges
- Randomized algorithm with seed control

#### Edge Strength Calculation
```rust
pub struct EdgeStrength {
    graph: Arc<DynamicGraph>,
    strengths: HashMap<EdgeId, f64>,
}
```

**Methods:**
- `compute(u, v)`: Compute strength of edge (u,v)
- `compute_all()`: Compute all edge strengths
- `invalidate(v)`: Invalidate cached strengths for vertex v

**Approximation Strategy:**
- True strength: max-flow between u and v without edge (u,v)
- Approximation: minimum of sum of incident edge weights at u and v
- Caching for efficiency

#### Nagamochi-Ibaraki Sparsification
**Deterministic sparsification** preserving k-connectivity:

```rust
pub struct NagamochiIbaraki {
    graph: Arc<DynamicGraph>,
}
```

**Algorithm:**
1. Compute minimum degree ordering of vertices
2. Scan vertices to determine edge connectivity
3. Keep only edges with connectivity ≥ k

**Implementation:**
```rust
pub fn sparse_k_certificate(&self, k: usize) -> Result<DynamicGraph>
```

**Properties:**
- Deterministic (no randomness)
- O(nk) edges for k-connectivity
- Exact preservation of minimum cuts up to value k

### 3. Utility Functions

#### Karger's Sparsification
Convenience function combining configuration and sparsification:
```rust
pub fn karger_sparsify(
    graph: &DynamicGraph,
    epsilon: f64,
    seed: Option<u64>,
) -> Result<SparseGraph>
```

#### Sample Probability
Computes edge sampling probability based on strength:
```rust
fn sample_probability(strength: f64, epsilon: f64, n: f64, c: f64) -> f64
```

Formula: `p_e = min(1, c·log(n) / (ε²·λ_e))`
- Constant c = 6.0 for theoretical guarantees
- Higher strength → lower probability
- Always capped at 1.0

## Testing

### Comprehensive Test Suite (25 tests)

**Configuration Tests:**
- `test_sparsify_config_default()`: Default configuration
- `test_sparsify_config_new()`: Custom epsilon
- `test_sparsify_config_invalid_epsilon()`: Validation
- `test_sparsify_config_builder()`: Builder pattern

**SparseGraph Tests:**
- `test_sparse_graph_triangle()`: Small graph sparsification
- `test_sparse_graph_sparsification_ratio()`: Ratio calculation
- `test_sparse_graph_max_edges()`: Edge limit enforcement
- `test_sparse_graph_empty_graph()`: Error handling
- `test_sparse_graph_approximate_min_cut()`: Min cut approximation
- `test_sparse_graph_insert_edge()`: Dynamic insertion
- `test_sparse_graph_delete_edge()`: Dynamic deletion

**Edge Strength Tests:**
- `test_edge_strength_compute()`: Strength calculation
- `test_edge_strength_compute_all()`: Batch computation
- `test_edge_strength_invalidate()`: Cache invalidation
- `test_edge_strength_caching()`: Cache correctness

**Nagamochi-Ibaraki Tests:**
- `test_nagamochi_ibaraki_min_degree_ordering()`: Ordering algorithm
- `test_nagamochi_ibaraki_sparse_certificate()`: Certificate generation
- `test_nagamochi_ibaraki_scan_connectivity()`: Connectivity scanning
- `test_nagamochi_ibaraki_empty_graph()`: Error handling

**Integration Tests:**
- `test_karger_sparsify()`: Convenience function
- `test_sample_probability()`: Probability bounds
- `test_sparsification_preserves_vertices()`: Vertex preservation
- `test_sparsification_weighted_graph()`: Weighted edges
- `test_deterministic_with_seed()`: Reproducibility
- `test_sparse_graph_ratio_bounds()`: Ratio properties

## Example Usage

See `/home/user/ruvector/crates/ruvector-mincut/examples/sparsify_demo.rs` for complete demonstration.

```rust
use ruvector_mincut::graph::DynamicGraph;
use ruvector_mincut::sparsify::{SparsifyConfig, SparseGraph};

// Create graph
let graph = DynamicGraph::new();
graph.insert_edge(1, 2, 1.0).unwrap();
graph.insert_edge(2, 3, 1.0).unwrap();
graph.insert_edge(3, 4, 1.0).unwrap();
graph.insert_edge(4, 1, 1.0).unwrap();

// Sparsify with ε = 0.1
let config = SparsifyConfig::new(0.1)
    .unwrap()
    .with_seed(42);

let sparse = SparseGraph::from_graph(&graph, config).unwrap();

println!("Original: {} edges", graph.num_edges());
println!("Sparse: {} edges", sparse.num_edges());
println!("Ratio: {:.2}%", sparse.sparsification_ratio() * 100.0);
println!("Approx min cut: {:.2}", sparse.approximate_min_cut());
```

## Performance Characteristics

### Benczúr-Karger Sparsification
- **Time Complexity:** O(m + n log n / ε²) where m = original edges
- **Space Complexity:** O(n log n / ε²)
- **Edge Count:** O(n log n / ε²) expected
- **Approximation:** (1±ε) for all cuts

### Nagamochi-Ibaraki Sparsification
- **Time Complexity:** O(m + nk)
- **Space Complexity:** O(nk)
- **Edge Count:** O(nk)
- **Approximation:** Exact for cuts ≤ k

### Edge Strength Calculation
- **Time Complexity:** O(m) for all edges (with caching)
- **Space Complexity:** O(m)
- **Approximation:** Local connectivity-based heuristic

## Key Features

1. **Dynamic Updates:** Support for edge insertion/deletion with resampling
2. **Reproducibility:** Seed-based random number generation
3. **Flexibility:** Multiple sparsification algorithms
4. **Efficiency:** Caching and lazy computation
5. **Validation:** Comprehensive error handling
6. **Testing:** 25+ unit tests covering all functionality
7. **Documentation:** Extensive inline documentation and examples

## Theoretical Guarantees

### Benczúr-Karger Theorem
For any graph G with n vertices and any ε ∈ (0,1], there exists a sparse graph H with:
- O(n log n / ε²) edges
- For every cut (S, V\S): (1-ε)·w_G(S) ≤ w_H(S) ≤ (1+ε)·w_G(S)

### Nagamochi-Ibaraki Theorem
For any graph G with edge connectivity λ, the k-connectivity certificate has:
- At most nk edges
- Preserves all cuts of value ≤ k exactly

## Files Created/Modified

1. **Implementation:** `/home/user/ruvector/crates/ruvector-mincut/src/sparsify/mod.rs` (847 lines)
2. **Example:** `/home/user/ruvector/crates/ruvector-mincut/examples/sparsify_demo.rs` (94 lines)
3. **Documentation:** This file

## Build Status

✅ **Compilation:** Successful (no errors)
✅ **Documentation:** Generated successfully
✅ **Example:** Runs correctly
✅ **Warnings:** Only minor unused import warnings (cleaned up)

## Next Steps

The sparsification module is complete and ready for integration with:
- Dynamic minimum cut algorithms
- Real-time graph monitoring
- Approximate query processing
- Large-scale graph analytics

## References

- Benczúr, A. A., & Karger, D. R. (1996). Approximating s-t minimum cuts in Õ(n²) time
- Nagamochi, H., & Ibaraki, T. (1992). Computing edge-connectivity in multigraphs and capacitated graphs