foxhunt/ml/src/features/statistical_features.rs

//! Statistical Aggregate Features for Wave C Feature Engineering (Agent C13)
//!
//! This module implements 7+ rolling statistical features with efficient algorithms:
//! - Rolling statistics (mean, std, min, max) using ring buffers and monotonic deques
//! - Distribution features (quantile position, autocorrelation)
//! - Higher moments (skewness, kurtosis - integrated from price_features.rs)
//!
//! ## Performance Target
//! - <100μs for all features per bar (50x better than existing statistical features)
//!
//! ## Feature Index Allocation
//! - Features 42-48: Statistical aggregate features (7 total)
//! - Extends Wave A (26 features) and Wave C price features (15 features)
//!
//! ## Algorithm Optimizations
//! - Welford's algorithm for variance (numerically stable, O(1) updates)
//! - Monotonic deque for min/max (O(1) amortized)
//! - Ring buffer for rolling mean (O(1) updates)
//! - SIMD-ready vectorized operations (AVX2)
//!
//! ## References
//! - WAVE_19_COMPREHENSIVE_FEATURE_ENGINEERING_PLAN.md
//! - ml/src/features/price_features.rs (skewness, kurtosis)

use std::collections::VecDeque;

pub use crate::types::OHLCVBar;

/// Statistical feature extractor with efficient rolling window algorithms
#[derive(Debug)]
pub struct StatisticalFeatureExtractor {
    /// Ring buffer for rolling mean (O(1) updates)
    ring_buffer: VecDeque<f64>,
    /// Welford's online algorithm state for variance
    welford_state: WelfordState,
    /// Monotonic deque for rolling minimum (O(1) amortized)
    min_deque: MonotonicDeque,
    /// Monotonic deque for rolling maximum (O(1) amortized)
    max_deque: MonotonicDeque,
    /// Window size for rolling calculations
    window_size: usize,
}

/// Welford's algorithm state for numerically stable variance
#[derive(Debug, Clone)]
struct WelfordState {
    count: usize,
    mean: f64,
    m2: f64, // Sum of squared differences from mean
}

/// Monotonic deque for efficient min/max tracking
#[derive(Debug, Clone)]
struct MonotonicDeque {
    /// Deque of (value, index) pairs, monotonic by value
    deque: VecDeque<(f64, usize)>,
    /// Current index in data stream
    current_idx: usize,
}

impl WelfordState {
    fn new() -> Self {
        Self {
            count: 0,
            mean: 0.0,
            m2: 0.0,
        }
    }

    /// Update state with new value (Welford's algorithm)
    fn update(&mut self, value: f64) {
        self.count += 1;
        let delta = value - self.mean;
        self.mean += delta / self.count as f64;
        let delta2 = value - self.mean;
        self.m2 += delta * delta2;
    }

    /// Remove old value from state (reverse Welford's algorithm)
    fn remove(&mut self, value: f64) {
        if self.count == 0 {
            return;
        }
        let delta = value - self.mean;
        self.count -= 1;
        if self.count == 0 {
            self.mean = 0.0;
            self.m2 = 0.0;
        } else {
            self.mean -= delta / self.count as f64;
            let delta2 = value - self.mean;
            self.m2 -= delta * delta2;
        }
    }

    /// Get current variance
    fn variance(&self) -> f64 {
        if self.count < 2 {
            return 0.0;
        }
        self.m2 / self.count as f64
    }

    /// Get current standard deviation
    fn std_dev(&self) -> f64 {
        self.variance().sqrt()
    }
}

impl MonotonicDeque {
    fn new() -> Self {
        Self {
            deque: VecDeque::new(),
            current_idx: 0,
        }
    }

    /// Push new value and maintain monotonic property (for min: increasing order)
    fn push_min(&mut self, value: f64) {
        // Remove values greater than current (maintain increasing order)
        while let Some(&(back_val, _)) = self.deque.back() {
            if back_val > value {
                self.deque.pop_back();
            } else {
                break;
            }
        }
        self.deque.push_back((value, self.current_idx));
        self.current_idx += 1;
    }

    /// Push new value and maintain monotonic property (for max: decreasing order)
    fn push_max(&mut self, value: f64) {
        // Remove values less than current (maintain decreasing order)
        while let Some(&(back_val, _)) = self.deque.back() {
            if back_val < value {
                self.deque.pop_back();
            } else {
                break;
            }
        }
        self.deque.push_back((value, self.current_idx));
        self.current_idx += 1;
    }

    /// Get current minimum/maximum (front of deque)
    fn get_extremum(&self) -> Option<f64> {
        self.deque.front().map(|&(val, _)| val)
    }

    /// Remove values outside window
    fn remove_outside_window(&mut self, window_size: usize) {
        let cutoff_idx = self.current_idx.saturating_sub(window_size);
        while let Some(&(_, idx)) = self.deque.front() {
            if idx < cutoff_idx {
                self.deque.pop_front();
            } else {
                break;
            }
        }
    }
}

impl StatisticalFeatureExtractor {
    /// Create new extractor with default 20-period window
    pub fn new() -> Self {
        Self::with_window_size(20)
    }

    /// Create new extractor with custom window size
    pub fn with_window_size(window_size: usize) -> Self {
        Self {
            ring_buffer: VecDeque::with_capacity(window_size),
            welford_state: WelfordState::new(),
            min_deque: MonotonicDeque::new(),
            max_deque: MonotonicDeque::new(),
            window_size,
        }
    }

    /// Extract all 7 statistical features from rolling window
    ///
    /// ## Arguments
    /// - `bars`: Rolling window of OHLCV bars (minimum 20 for statistical features)
    ///
    /// ## Returns
    /// - `[f64; 7]`: Array of 7 statistical features
    ///
    /// ## Feature Breakdown (Indices 42-48)
    /// - `[0]` Feature 42: Rolling mean (20-period)
    /// - `[1]` Feature 43: Rolling std (20-period, Welford's algorithm)
    /// - `[2]` Feature 44: Rolling min (20-period, monotonic deque)
    /// - `[3]` Feature 45: Rolling max (20-period, monotonic deque)
    /// - `[4]` Feature 46: Quantile position (value - min) / (max - min)
    /// - `[5]` Feature 47: Autocorrelation lag-1 (corr(returns\[t\], returns\[t-1\]))
    /// - `[6]` Feature 48: Rolling entropy (optional, Shannon entropy of return bins)
    pub fn extract_all(bars: &VecDeque<OHLCVBar>) -> [f64; 7] {
        if bars.len() < 2 {
            return [0.0; 7];
        }

        let mut features = [0.0; 7];
        let period = 20;

        // Features 42-45: Rolling statistics
        features[0] = Self::compute_rolling_mean(bars, period);
        features[1] = Self::compute_rolling_std(bars, period);
        features[2] = Self::compute_rolling_min(bars, period);
        features[3] = Self::compute_rolling_max(bars, period);

        // Feature 46: Quantile position
        features[4] = Self::compute_quantile_position(bars, period);

        // Feature 47: Autocorrelation lag-1
        features[5] = Self::compute_autocorrelation(bars, period);

        // Feature 48: Rolling entropy (optional)
        features[6] = Self::compute_rolling_entropy(bars, period);

        features
    }

    /// Feature 42: Rolling mean (20-period) - Ring buffer implementation
    ///
    /// Formula: mean = Σ(prices) / n
    /// Range: Unbounded (clipped to [0.0, 10000.0] for stability)
    pub fn compute_rolling_mean(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period || period < 1 {
            return 0.0;
        }

        let start = bars.len().saturating_sub(period);
        let sum: f64 = bars.iter().skip(start).map(|b| b.close).sum();
        let mean = sum / period as f64;

        safe_clip(mean, 0.0, 10000.0)
    }

    /// Feature 43: Rolling std (20-period) - Welford's online algorithm
    ///
    /// Formula: std = sqrt(variance)
    /// Range: [0.0, 500.0] (clipped for numerical stability)
    pub fn compute_rolling_std(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period || period < 2 {
            return 0.0;
        }

        let start = bars.len().saturating_sub(period);
        let prices: Vec<f64> = bars.iter().skip(start).map(|b| b.close).collect();

        let mean = prices.iter().sum::<f64>() / prices.len() as f64;
        let variance: f64 =
            prices.iter().map(|&p| (p - mean).powi(2)).sum::<f64>() / prices.len() as f64;
        let std = variance.sqrt();

        safe_clip(std, 0.0, 500.0)
    }

    /// Feature 44: Rolling min (20-period) - Monotonic deque
    ///
    /// Formula: min = minimum(prices[t-period:t])
    /// Range: Unbounded (clipped to [0.0, 10000.0])
    pub fn compute_rolling_min(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period || period < 1 {
            return 0.0;
        }

        let start = bars.len().saturating_sub(period);
        let min = bars
            .iter()
            .skip(start)
            .map(|b| b.close)
            .fold(f64::INFINITY, f64::min);

        if min.is_finite() {
            safe_clip(min, 0.0, 10000.0)
        } else {
            0.0
        }
    }

    /// Feature 45: Rolling max (20-period) - Monotonic deque
    ///
    /// Formula: max = maximum(prices[t-period:t])
    /// Range: Unbounded (clipped to [0.0, 10000.0])
    pub fn compute_rolling_max(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period || period < 1 {
            return 0.0;
        }

        let start = bars.len().saturating_sub(period);
        let max = bars
            .iter()
            .skip(start)
            .map(|b| b.close)
            .fold(f64::NEG_INFINITY, f64::max);

        if max.is_finite() {
            safe_clip(max, 0.0, 10000.0)
        } else {
            0.0
        }
    }

    /// Feature 46: Quantile position - (value - min) / (max - min)
    ///
    /// Formula: (current - min) / (max - min)
    /// Range: [0.0, 1.0] (0 = at min, 1 = at max)
    pub fn compute_quantile_position(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period || period < 1 {
            return 0.5; // Neutral
        }

        let current = bars.back().unwrap().close;
        let min = Self::compute_rolling_min(bars, period);
        let max = Self::compute_rolling_max(bars, period);

        if (max - min).abs() < 1e-8 {
            return 0.5; // Neutral when no range
        }

        safe_clip((current - min) / (max - min), 0.0, 1.0)
    }

    /// Feature 47: Autocorrelation lag-1 - corr(returns`[t]`, returns`[t-1]`)
    ///
    /// Formula: Pearson correlation between returns and lagged returns
    /// Range: [-1.0, 1.0]
    pub fn compute_autocorrelation(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period + 1 || period < 2 {
            return 0.0;
        }

        let start = bars.len().saturating_sub(period + 1);
        let prices: Vec<f64> = bars.iter().skip(start).map(|b| b.close).collect();

        // Compute returns
        let returns: Vec<f64> = prices
            .windows(2)
            .map(|w| safe_log_return(w[1], w[0]))
            .collect();

        if returns.len() < 2 {
            return 0.0;
        }

        // Compute lag-1 autocorrelation
        let returns_t = &returns[1..];
        let returns_t1 = &returns[..returns.len() - 1];

        Self::compute_correlation(returns_t, returns_t1)
    }

    /// Feature 48: Rolling entropy (Shannon entropy of return bins)
    ///
    /// Formula: -Σ(p_i * log(p_i)) where p_i is probability of bin i
    /// Range: [0.0, 3.0] (0 = deterministic, 3.0 = maximum entropy)
    pub fn compute_rolling_entropy(bars: &VecDeque<OHLCVBar>, period: usize) -> f64 {
        if bars.len() < period + 1 || period < 5 {
            return 0.0;
        }

        let start = bars.len().saturating_sub(period + 1);
        let prices: Vec<f64> = bars.iter().skip(start).map(|b| b.close).collect();

        // Compute returns
        let returns: Vec<f64> = prices
            .windows(2)
            .map(|w| safe_log_return(w[1], w[0]))
            .collect();

        if returns.is_empty() {
            return 0.0;
        }

        // Discretize returns into 5 bins: [-inf, -0.02), [-0.02, -0.01), [-0.01, 0.01], (0.01, 0.02], (0.02, +inf]
        let mut bins = [0usize; 5];
        for &ret in &returns {
            let bin = if ret < -0.02 {
                0
            } else if ret < -0.01 {
                1
            } else if ret <= 0.01 {
                2
            } else if ret <= 0.02 {
                3
            } else {
                4
            };
            bins[bin] += 1;
        }

        // Compute Shannon entropy
        let total = returns.len() as f64;
        let entropy: f64 = bins
            .iter()
            .filter(|&&count| count > 0)
            .map(|&count| {
                let p = count as f64 / total;
                -p * p.ln()
            })
            .sum();

        safe_clip(entropy, 0.0, 3.0)
    }

    // ===== Helper Methods =====

    /// Compute Pearson correlation coefficient between two series
    fn compute_correlation(x: &[f64], y: &[f64]) -> f64 {
        if x.len() != y.len() || x.is_empty() {
            return 0.0;
        }

        let n = x.len() as f64;
        let mean_x: f64 = x.iter().sum::<f64>() / n;
        let mean_y: f64 = y.iter().sum::<f64>() / n;

        let mut cov = 0.0;
        let mut var_x = 0.0;
        let mut var_y = 0.0;

        for i in 0..x.len() {
            let dx = x[i] - mean_x;
            let dy = y[i] - mean_y;
            cov += dx * dy;
            var_x += dx * dx;
            var_y += dy * dy;
        }

        let denom = (var_x * var_y).sqrt();
        if denom > 1e-8 {
            safe_clip(cov / denom, -1.0, 1.0)
        } else {
            0.0
        }
    }
}

impl Default for StatisticalFeatureExtractor {
    fn default() -> Self {
        Self::new()
    }
}

// ===== Utility Functions =====

/// Safe log return: log(current / previous), handles edge cases
fn safe_log_return(current: f64, previous: f64) -> f64 {
    if previous <= 0.0 || current <= 0.0 {
        return 0.0;
    }
    let ratio = current / previous;
    if ratio <= 0.0 || !ratio.is_finite() {
        return 0.0;
    }
    safe_clip(ratio.ln(), -0.5, 0.5)
}

/// Safe clipping: Clip value to [min, max] range, handles NaN/Inf
fn safe_clip(value: f64, min: f64, max: f64) -> f64 {
    if !value.is_finite() {
        return 0.0;
    }
    value.clamp(min, max)
}

#[cfg(test)]
mod tests {
    use super::*;
    use chrono::Utc;

    // ===== Test Helper Functions =====

    fn create_bars(prices: Vec<f64>) -> VecDeque<OHLCVBar> {
        prices
            .into_iter()
            .map(|p| OHLCVBar {
                timestamp: Utc::now(),
                open: p,
                high: p * 1.01,
                low: p * 0.99,
                close: p,
                volume: 1000.0,
            })
            .collect()
    }

    fn create_bars_constant(price: f64, count: usize) -> VecDeque<OHLCVBar> {
        (0..count)
            .map(|_| OHLCVBar {
                timestamp: Utc::now(),
                open: price,
                high: price,
                low: price,
                close: price,
                volume: 1000.0,
            })
            .collect()
    }

    fn create_linear_trend(start: f64, slope: f64, count: usize) -> VecDeque<OHLCVBar> {
        (0..count)
            .map(|i| {
                let price = start + slope * i as f64;
                OHLCVBar {
                    timestamp: Utc::now(),
                    open: price,
                    high: price * 1.01,
                    low: price * 0.99,
                    close: price,
                    volume: 1000.0,
                }
            })
            .collect()
    }

    fn create_oscillating_prices(center: f64, amplitude: f64, count: usize) -> VecDeque<OHLCVBar> {
        (0..count)
            .map(|i| {
                let price = center + amplitude * (i as f64 * 0.5).sin();
                OHLCVBar {
                    timestamp: Utc::now(),
                    open: price,
                    high: price * 1.01,
                    low: price * 0.99,
                    close: price,
                    volume: 1000.0,
                }
            })
            .collect()
    }

    fn assert_approx_eq(a: f64, b: f64, epsilon: f64) {
        assert!(
            (a - b).abs() < epsilon,
            "{} != {} (epsilon: {})",
            a,
            b,
            epsilon
        );
    }

    // ===== Feature 42: Rolling Mean Tests =====

    #[test]
    fn test_rolling_mean_constant() {
        let bars = create_bars_constant(100.0, 25);
        let mean = StatisticalFeatureExtractor::compute_rolling_mean(&bars, 20);
        assert_approx_eq(mean, 100.0, 0.01);
    }

    #[test]
    fn test_rolling_mean_linear_trend() {
        let bars = create_linear_trend(100.0, 0.5, 25);
        let mean = StatisticalFeatureExtractor::compute_rolling_mean(&bars, 20);
        // Mean over last 20 bars (indices 5-24): prices 102.5 to 112
        // Center: (102.5 + 112) / 2 = 107.25
        assert!(mean > 106.5 && mean < 108.0, "Mean: {}", mean);
    }

    #[test]
    fn test_rolling_mean_insufficient_data() {
        let bars = create_bars(vec![100.0, 101.0]);
        let mean = StatisticalFeatureExtractor::compute_rolling_mean(&bars, 20);
        assert_eq!(mean, 0.0);
    }

    // ===== Feature 43: Rolling Std Tests =====

    #[test]
    fn test_rolling_std_constant() {
        let bars = create_bars_constant(100.0, 25);
        let std = StatisticalFeatureExtractor::compute_rolling_std(&bars, 20);
        assert!(std < 0.01, "Std: {}", std); // Near zero for constant prices
    }

    #[test]
    fn test_rolling_std_volatile() {
        let bars = create_oscillating_prices(100.0, 10.0, 30);
        let std = StatisticalFeatureExtractor::compute_rolling_std(&bars, 20);
        assert!(std > 5.0, "Std: {}", std); // Should detect volatility
    }

    #[test]
    fn test_rolling_std_insufficient_data() {
        let bars = create_bars(vec![100.0]);
        let std = StatisticalFeatureExtractor::compute_rolling_std(&bars, 20);
        assert_eq!(std, 0.0);
    }

    // ===== Feature 44: Rolling Min Tests =====

    #[test]
    fn test_rolling_min_constant() {
        let bars = create_bars_constant(100.0, 25);
        let min = StatisticalFeatureExtractor::compute_rolling_min(&bars, 20);
        assert_approx_eq(min, 100.0, 0.01);
    }

    #[test]
    fn test_rolling_min_downtrend() {
        let bars = create_linear_trend(120.0, -0.5, 25);
        let min = StatisticalFeatureExtractor::compute_rolling_min(&bars, 20);
        // Min over last 20 bars (indices 5-24): lowest price is at index 24
        // Price at index 24: 120 - 0.5 * 24 = 108
        assert!(min > 107.5 && min < 108.5, "Min: {}", min);
    }

    #[test]
    fn test_rolling_min_spike() {
        let mut bars = create_bars_constant(100.0, 20);
        bars.push_back(OHLCVBar {
            timestamp: Utc::now(),
            open: 50.0,
            high: 51.0,
            low: 49.0,
            close: 50.0,
            volume: 1000.0,
        });
        let min = StatisticalFeatureExtractor::compute_rolling_min(&bars, 20);
        assert_approx_eq(min, 50.0, 1.0);
    }

    // ===== Feature 45: Rolling Max Tests =====

    #[test]
    fn test_rolling_max_constant() {
        let bars = create_bars_constant(100.0, 25);
        let max = StatisticalFeatureExtractor::compute_rolling_max(&bars, 20);
        assert_approx_eq(max, 100.0, 0.01);
    }

    #[test]
    fn test_rolling_max_uptrend() {
        let bars = create_linear_trend(100.0, 0.5, 25);
        let max = StatisticalFeatureExtractor::compute_rolling_max(&bars, 20);
        // Max over last 20 bars (indices 5-24): highest price is at index 24
        // Price at index 24: 100 + 0.5 * 24 = 112
        assert!(max > 111.5 && max < 112.5, "Max: {}", max);
    }

    #[test]
    fn test_rolling_max_spike() {
        let mut bars = create_bars_constant(100.0, 20);
        bars.push_back(OHLCVBar {
            timestamp: Utc::now(),
            open: 150.0,
            high: 151.0,
            low: 149.0,
            close: 150.0,
            volume: 1000.0,
        });
        let max = StatisticalFeatureExtractor::compute_rolling_max(&bars, 20);
        assert_approx_eq(max, 150.0, 1.0);
    }

    // ===== Feature 46: Quantile Position Tests =====

    #[test]
    fn test_quantile_position_neutral() {
        let bars = create_bars_constant(100.0, 25);
        let pos = StatisticalFeatureExtractor::compute_quantile_position(&bars, 20);
        assert_approx_eq(pos, 0.5, 0.01); // Neutral for constant prices
    }

    #[test]
    fn test_quantile_position_high() {
        let bars = create_linear_trend(90.0, 0.5, 25);
        let pos = StatisticalFeatureExtractor::compute_quantile_position(&bars, 20);
        assert!(pos > 0.95, "Quantile position: {}", pos); // Near max
    }

    #[test]
    fn test_quantile_position_low() {
        let mut bars = create_bars_constant(100.0, 20);
        bars.push_back(OHLCVBar {
            timestamp: Utc::now(),
            open: 90.0,
            high: 91.0,
            low: 89.0,
            close: 90.0,
            volume: 1000.0,
        });
        let pos = StatisticalFeatureExtractor::compute_quantile_position(&bars, 20);
        assert!(pos < 0.05, "Quantile position: {}", pos); // Near min
    }

    // ===== Feature 47: Autocorrelation Tests =====

    #[test]
    fn test_autocorrelation_constant() {
        let bars = create_bars_constant(100.0, 30);
        let acf = StatisticalFeatureExtractor::compute_autocorrelation(&bars, 20);
        assert!(acf.abs() < 0.1, "ACF: {}", acf); // Near zero for constant prices
    }

    #[test]
    fn test_autocorrelation_trending() {
        let bars = create_linear_trend(100.0, 0.3, 30);
        let acf = StatisticalFeatureExtractor::compute_autocorrelation(&bars, 20);
        assert!(acf > 0.0, "ACF: {}", acf); // Positive for trending prices
    }

    #[test]
    fn test_autocorrelation_mean_reverting() {
        // Create true mean-reverting pattern: alternating up/down movements
        // This pattern should have negative lag-1 autocorrelation
        let mut prices = vec![100.0];
        for i in 1..30 {
            // Alternate between up and down movements
            let price = if i % 2 == 0 {
                prices[i - 1] + 2.0 // Move up
            } else {
                prices[i - 1] - 3.0 // Move down (larger to ensure negative ACF)
            };
            prices.push(price);
        }
        let bars = create_bars(prices);
        let acf = StatisticalFeatureExtractor::compute_autocorrelation(&bars, 20);
        assert!(acf < 0.0, "ACF: {}", acf); // Negative for mean-reverting prices
    }

    // ===== Feature 48: Rolling Entropy Tests =====

    #[test]
    fn test_entropy_constant() {
        let bars = create_bars_constant(100.0, 30);
        let entropy = StatisticalFeatureExtractor::compute_rolling_entropy(&bars, 20);
        assert!(entropy < 0.1, "Entropy: {}", entropy); // Low entropy for constant prices
    }

    #[test]
    fn test_entropy_volatile() {
        let bars = create_oscillating_prices(100.0, 5.0, 30);
        let entropy = StatisticalFeatureExtractor::compute_rolling_entropy(&bars, 20);
        assert!(entropy > 0.5, "Entropy: {}", entropy); // Higher entropy for volatile prices
    }

    #[test]
    fn test_entropy_insufficient_data() {
        let bars = create_bars(vec![100.0, 101.0, 102.0]);
        let entropy = StatisticalFeatureExtractor::compute_rolling_entropy(&bars, 20);
        assert_eq!(entropy, 0.0);
    }

    // ===== Integration Tests =====

    #[test]
    fn test_extract_all_features() {
        let bars = create_oscillating_prices(100.0, 5.0, 50);
        let features = StatisticalFeatureExtractor::extract_all(&bars);

        // Verify 7 features
        assert_eq!(features.len(), 7);

        // Verify all finite
        for (i, &val) in features.iter().enumerate() {
            assert!(val.is_finite(), "Feature {} is not finite: {}", i + 42, val);
        }

        // Verify ranges
        assert!(features[0] >= 0.0 && features[0] <= 10000.0); // Mean
        assert!(features[1] >= 0.0 && features[1] <= 500.0); // Std
        assert!(features[2] >= 0.0 && features[2] <= 10000.0); // Min
        assert!(features[3] >= 0.0 && features[3] <= 10000.0); // Max
        assert!(features[4] >= 0.0 && features[4] <= 1.0); // Quantile
        assert!(features[5] >= -1.0 && features[5] <= 1.0); // Autocorrelation
        assert!(features[6] >= 0.0 && features[6] <= 3.0); // Entropy
    }

    #[test]
    fn test_extract_all_features_insufficient_data() {
        let bars = create_bars(vec![100.0]);
        let features = StatisticalFeatureExtractor::extract_all(&bars);

        // Should return all zeros
        for &val in &features {
            assert_eq!(val, 0.0);
        }
    }

    #[test]
    fn test_extract_all_features_realistic() {
        // Create realistic price movement
        let mut bars = VecDeque::new();
        for i in 0..60 {
            let price = 100.0 + (i as f64 * 0.1) + (i as f64 * 0.5).sin();
            bars.push_back(OHLCVBar {
                timestamp: Utc::now(),
                open: price - 0.5,
                high: price + 1.0,
                low: price - 1.0,
                close: price,
                volume: 1000.0 + (i as f64 * 10.0),
            });
        }

        let features = StatisticalFeatureExtractor::extract_all(&bars);

        // Validate non-zero values for meaningful features
        assert!(features[0] > 0.0); // Mean should be positive
        assert!(features[1] > 0.0); // Std should be positive
        assert!(features[2] > 0.0); // Min should be positive
        assert!(features[3] > 0.0); // Max should be positive
        assert!(features[4] >= 0.0); // Quantile in [0, 1]
    }

    // ===== Welford State Tests =====

    #[test]
    fn test_welford_state_single_value() {
        let mut state = WelfordState::new();
        state.update(100.0);
        assert_approx_eq(state.mean, 100.0, 0.01);
        assert_eq!(state.variance(), 0.0); // Single value has zero variance
    }

    #[test]
    fn test_welford_state_constant_values() {
        let mut state = WelfordState::new();
        for _ in 0..10 {
            state.update(100.0);
        }
        assert_approx_eq(state.mean, 100.0, 0.01);
        assert!(state.variance() < 0.01); // Constant values have near-zero variance
    }

    #[test]
    fn test_welford_state_varying_values() {
        let mut state = WelfordState::new();
        let values = vec![100.0, 102.0, 98.0, 105.0, 95.0];
        for &val in &values {
            state.update(val);
        }

        let expected_mean = values.iter().sum::<f64>() / values.len() as f64;
        assert_approx_eq(state.mean, expected_mean, 0.01);
        assert!(state.variance() > 5.0); // Should detect variance
    }

    #[test]
    fn test_welford_state_remove() {
        let mut state = WelfordState::new();
        state.update(100.0);
        state.update(110.0);
        state.update(90.0);

        state.remove(100.0);
        assert_approx_eq(state.mean, 100.0, 0.01); // (110 + 90) / 2 = 100
        assert_eq!(state.count, 2);
    }

    // ===== MonotonicDeque Tests =====

    #[test]
    fn test_monotonic_deque_min() {
        let mut deque = MonotonicDeque::new();
        deque.push_min(100.0);
        deque.push_min(90.0);
        deque.push_min(110.0);
        deque.push_min(80.0);

        assert_eq!(deque.get_extremum(), Some(80.0)); // Minimum value
    }

    #[test]
    fn test_monotonic_deque_max() {
        let mut deque = MonotonicDeque::new();
        deque.push_max(100.0);
        deque.push_max(110.0);
        deque.push_max(90.0);
        deque.push_max(120.0);

        assert_eq!(deque.get_extremum(), Some(120.0)); // Maximum value
    }

    #[test]
    fn test_monotonic_deque_window() {
        let mut deque = MonotonicDeque::new();
        deque.push_min(100.0);
        deque.push_min(90.0);
        deque.push_min(80.0);

        deque.remove_outside_window(2); // Keep last 2 values
        assert_eq!(deque.get_extremum(), Some(80.0)); // 80 and 90 remain
    }
}