// hakmem_p2.h - P² (P-square) Algorithm for Percentile Estimation
// Purpose: O(1) memory, O(1) update, 1-2% accuracy for p99 estimation
//
// Reference: Jain & Chlamtac (1985) "The P² algorithm for dynamic
//            calculation of quantiles and histograms without storing observations"
//
// License: MIT
// Date: 2025-10-21

#ifndef HAKMEM_P2_H
#define HAKMEM_P2_H

#include <stdint.h>

// ============================================================================
// P² Estimator (5 markers: min, p25, p50, p75, p99)
// ============================================================================

typedef struct {
    double q[5];      // Marker heights (quantile values)
    int n[5];         // Marker positions (1-based indices)
    double dn[5];     // Desired marker positions (increments)
    int count;        // Total samples processed
    double target_p;  // Target percentile (0.99 for p99)
} hak_p2_t;

// ============================================================================
// Public API
// ============================================================================

// Initialize P² estimator for target percentile (e.g., 0.99 for p99)
void hak_p2_init(hak_p2_t* p2, double target_p);

// Add a new sample value
void hak_p2_add(hak_p2_t* p2, double value);

// Get current percentile estimate
double hak_p2_get(hak_p2_t* p2);

// Reset estimator (start new window)
void hak_p2_reset(hak_p2_t* p2);

// Get sample count
int hak_p2_count(hak_p2_t* p2);

#endif // HAKMEM_P2_H