from __future__ import annotations

from dataclasses import dataclass
from math import exp


@dataclass(frozen=True)
class P833Coefficients:
    gamma_db_per_m: float
    max_attenuation_db: float


def p833_attenuation(
    depth_m: float,
    freq_hz: float,
    forest_type: str,
    coefficients: P833Coefficients | None = None,
) -> float:
    if depth_m <= 0:
        return 0.0
    if freq_hz <= 0:
        raise ValueError("freq_hz must be positive")

    if coefficients is None:
        raise NotImplementedError(
            "P.833 coefficients must be supplied from ITU-R tables for "
            f"forest_type={forest_type}"
        )
    coeff = coefficients
    if coeff.max_attenuation_db <= 0:
        raise ValueError("max_attenuation_db must be positive")
    return coeff.max_attenuation_db * (
        1 - exp(-(depth_m * coeff.gamma_db_per_m) / coeff.max_attenuation_db)
    )