"""Shoreline Boundary Stress Parameter (SBSP)

Estimates inundation momentum flux and bore formation at the shoreline transition.

SBSP = Fr² · (H/H_ref) = (u²·H) / (g·H_ref²)
"""

import numpy as np

class ShorelineBoundaryStressParameter:
    """
    Shoreline Boundary Stress Parameter - Parameter 6 of 7
    
    Thresholds:
        SAFE:    SBSP < 0.3   (low inundation stress)
        MONITOR: 0.3 ≤ SBSP < 0.7 (moderate inundation)
        ALERT:   0.7 ≤ SBSP < 1.2 (high inundation)
        CRITICAL: SBSP ≥ 1.2   (supercritical bore)
    
    Run-up regression: R = 19.7 × SBSP − 2.1 [m]
    Pearson correlation r = 0.956
    """
    
    def __init__(self, g=9.81, H_ref=1.0):
        self.g = g
        self.H_ref = H_ref  # reference depth for normalization
        self.thresholds = {
            'safe': 0.3,
            'monitor': 0.7,
            'alert': 1.2,
            'critical': 1.2
        }
    
    def compute_froude(self, u, H):
        """Compute Froude number Fr = u/√(gH)
        
        Fr < 1: subcritical flow
        Fr = 1: critical flow
        Fr > 1: supercritical flow (bore)
        
        Args:
            u: flow velocity [m/s]
            H: water depth [m]
        
        Returns:
            Fr: Froude number
        """
        return u / np.sqrt(self.g * H)
    
    def compute_sbsp(self, u, H):
        """Compute Shoreline Boundary Stress Parameter
        
        SBSP = Fr² · (H/H_ref) = (u²·H) / (g·H_ref²)
        
        Args:
            u: flow velocity at shoreline [m/s]
            H: water depth at shoreline [m]
        
        Returns:
            dict with SBSP value, status, and components
        """
        Fr = self.compute_froude(u, H)
        sbsp = (u**2 * H) / (self.g * self.H_ref**2)
        
        return {
            'value': float(sbsp),
            'status': self.get_status(sbsp),
            'froude': float(Fr),
            'flow_regime': self.get_flow_regime(Fr),
            'velocity': float(u),
            'depth': float(H)
        }
    
    def get_status(self, sbsp):
        """Get alert status based on SBSP value"""
        if sbsp < self.thresholds['safe']:
            return 'SAFE'
        elif sbsp < self.thresholds['monitor']:
            return 'MONITOR'
        elif sbsp < self.thresholds['alert']:
            return 'ALERT'
        else:
            return 'CRITICAL'
    
    def get_flow_regime(self, Fr):
        """Get descriptive flow regime"""
        if Fr < 0.8:
            return "Subcritical flow"
        elif Fr < 1.0:
            return "Near-critical flow"
        elif Fr < 1.5:
            return "Supercritical flow (bore)"
        else:
            return "Highly supercritical bore"
    
    def estimate_runup(self, sbsp):
        """Estimate run-up from SBSP using validated regression
        
        R = 19.7 × SBSP − 2.1 [m]
        
        Args:
            sbsp: SBSP value
        
        Returns:
            estimated_runup: estimated run-up height [m]
        """
        runup = 19.7 * sbsp - 2.1
        return max(0, runup)
    
    def compute_momentum_flux(self, u, H):
        """Compute depth-integrated momentum flux
        
        M = ρ·H·u²
        
        Args:
            u: velocity [m/s]
            H: depth [m]
        
        Returns:
            M: momentum flux [kg·m/s² per m width]
        """
        rho = 1025  # seawater density
        return rho * H * u**2
    
    def compute_bore_characteristics(self, u, H, eta):
        """Compute hydraulic bore characteristics
        
        Args:
            u: velocity [m/s]
            H: depth [m]
            eta: wave height [m]
        
        Returns:
            dict with bore characteristics
        """
        Fr = self.compute_froude(u, H)
        sbsp = self.compute_sbsp(u, H)
        
        # Bore height ratio (Boussinesq)
        if Fr > 1:
            # Hydraulic jump / bore
            H2_H1 = 0.5 * (np.sqrt(1 + 8*Fr**2) - 1)
        else:
            H2_H1 = 1.0
        
        return {
            'froude': float(Fr),
            'sbsp': sbsp,
            'bore_formation': Fr > 1.0,
            'depth_ratio': float(H2_H1),
            'bore_height_m': float(H * (H2_H1 - 1)),
            'bore_celerity': float(u / (1 - 1/H2_H1)) if Fr > 1 else None
        }
    
    def validate_event(self, u, H, observed_runup):
        """Validate SBSP against observed run-up"""
        sbsp_result = self.compute_sbsp(u, H)
        predicted_runup = self.estimate_runup(sbsp_result['value'])
        
        error = abs(predicted_runup - observed_runup) / observed_runup * 100
        
        validation = {
            'sbsp': sbsp_result,
            'predicted_runup': float(predicted_runup),
            'observed_runup': observed_runup,
            'error_percent': float(error),
            'within_15_percent': error < 15,
            'bore': self.compute_bore_characteristics(u, H, observed_runup)
        }
        
        return validation