"""
Knife-edge diffraction geometry calculations.
"""

import math


def knife_edge_loss(
    d1_m: float,
    d2_m: float,
    h_m: float,
    wavelength_m: float,
) -> float:
    """
    Calculate diffraction loss over single knife edge.

    Args:
        d1_m: Distance TX to obstacle
        d2_m: Distance obstacle to RX
        h_m: Obstacle height above LOS (positive = above)
        wavelength_m: Signal wavelength

    Returns:
        Loss in dB (>= 0)
    """
    if d1_m <= 0 or d2_m <= 0 or wavelength_m <= 0:
        return 0.0

    v = h_m * math.sqrt(2 * (d1_m + d2_m) / (wavelength_m * d1_m * d2_m))

    if v < -0.78:
        L = 0.0
    elif v < 0:
        L = 6.02 + 9.11 * v - 1.27 * v ** 2
    elif v < 2.4:
        L = 6.02 + 9.11 * v + 1.65 * v ** 2
    else:
        L = 12.95 + 20 * math.log10(v)

    return max(0.0, L)