# WAVE 26 P0.2: Batch Diversity Enforcement Implementation

## Overview
Added batch diversity enforcement to prevent sampling the same experience twice in consecutive batches.

## Implementation Changes

### 1. Added HashSet for Tracking Recently Sampled Indices

**File**: `/home/jgrusewski/Work/foxhunt/ml/src/dqn/prioritized_replay.rs`

#### Imports
```rust
use std::collections::HashSet;
```

#### Struct Fields
```rust
pub struct PrioritizedReplayBuffer {
    // ... existing fields ...
    /// Tracks recently sampled indices to prevent duplicates
    recently_sampled: Arc<Mutex<HashSet<usize>>>,
    /// Counter for cooldown management (cleared every 50 batches)
    sample_counter: AtomicUsize,
}
```

#### Constructor
```rust
pub fn new(config: PrioritizedReplayConfig) -> Result<Self, MLError> {
    // ... existing initialization ...
    Ok(Self {
        // ... existing fields ...
        recently_sampled: Arc::new(Mutex::new(HashSet::new())),
        sample_counter: AtomicUsize::new(0),
        config,
    })
}
```

### 2. Modified sample() Method

**Rejection Sampling with Fallback**:
```rust
pub fn sample(&self, batch_size: usize) -> Result<(Vec<Experience>, Vec<f32>, Vec<usize>), MLError> {
    // ... existing code ...

    let mut recently_sampled = self.recently_sampled.lock();

    for _ in 0..batch_size {
        // Try sampling with diversity enforcement (max 100 attempts)
        let mut attempts = 0;
        let max_attempts = 100;
        let idx = loop {
            let value = rng.gen::<f32>() * total_priority;
            let sampled_idx = tree.sample(value)?;

            // Check if this index was recently sampled
            if !recently_sampled.contains(&sampled_idx) {
                break sampled_idx;
            }

            attempts += 1;
            if attempts >= max_attempts {
                // Fallback: clear cooldown and use this sample
                recently_sampled.clear();
                break sampled_idx;
            }
        };

        // ... process experience ...
        recently_sampled.insert(idx);
    }

    // Increment sample counter and clear cooldown every 50 batches
    let sample_count = self.sample_counter.fetch_add(1, Ordering::Relaxed);
    if sample_count % 50 == 49 {
        recently_sampled.clear();
    }

    // ... return results ...
}
```

### 3. Updated clear() Method

```rust
pub fn clear(&self) {
    // ... existing clearing code ...

    self.sample_counter.store(0, Ordering::Release);

    // Clear diversity tracking
    {
        let mut recently_sampled = self.recently_sampled.lock();
        recently_sampled.clear();
    }

    // ... reset metrics ...
}
```

## Test Coverage

### Test 1: Basic Diversity Enforcement
```rust
#[test]
fn test_batch_diversity_no_duplicates() {
    // Tests that consecutive batches don't share indices
    // Verifies HashSet prevents duplicate sampling
}
```

### Test 2: Cooldown After 50 Batches
```rust
#[test]
fn test_batch_diversity_cooldown_cleared_after_50_batches() {
    // Verifies cooldown is cleared after 50 batches
    // Ensures long-running training doesn't accumulate all indices
}
```

### Test 3: Fallback on Exhaustion
```rust
#[test]
fn test_batch_diversity_fallback_on_exhaustion() {
    // Tests fallback when buffer size < 2*batch_size
    // Ensures graceful degradation instead of infinite loop
}
```

## Performance Characteristics

- **Space**: O(batch_size) for recently_sampled HashSet
- **Time**: O(1) expected per sample (rejection sampling), O(100) worst case
- **Fallback**: Clears cooldown after 100 failed attempts (prevents infinite loops)
- **Memory**: Cleared every 50 batches (prevents unbounded growth)

## Benefits

1. **No duplicate experiences** in consecutive batches
2. **Better gradient diversity** - each update uses unique experiences
3. **Graceful fallback** - handles small buffers without failing
4. **Automatic cleanup** - cooldown reset every 50 batches
5. **Minimal overhead** - HashSet lookups are O(1)

## Edge Cases Handled

1. **Small buffers**: Fallback clears cooldown if can't find unique samples
2. **Long training**: Automatic cooldown reset every 50 batches
3. **Thread safety**: Mutex protects HashSet from concurrent access
4. **Clear buffer**: Resets both counter and cooldown set