from __future__ import annotations
from dataclasses import dataclass
from typing import Any

@dataclass(frozen=True)
class RetrainDecision:
    baseline_precision: float
    current_precision: float | None
    deterioration: float | None
    threshold: float
    should_flag: bool
    reason: str

    def as_dict(self) -> dict[str, Any]:
        return self.__dict__

def calculate_deterioration(baseline_precision: float, current_precision: float) -> float:
    if baseline_precision <= 0:
        raise ValueError("baseline_precision must be > 0")
    return (baseline_precision - current_precision) / baseline_precision

def evaluate_retrain_trigger(
    baseline_precision: float,
    current_precision: float | None,
    *,
    deterioration_threshold: float = 0.15,
) -> RetrainDecision:
    if not 0 < deterioration_threshold < 1:
        raise ValueError("deterioration_threshold must be between 0 and 1")
    if not 0 < baseline_precision <= 1:
        raise ValueError("baseline_precision must be in (0, 1]")
    if current_precision is None:
        return RetrainDecision(
            baseline_precision, None, None, deterioration_threshold, False,
            "No current 20-day precision available."
        )
    if not 0 <= current_precision <= 1:
        raise ValueError("current_precision must be between 0 and 1")

    deterioration = calculate_deterioration(baseline_precision, current_precision)
    should_flag = deterioration >= deterioration_threshold - 1e-12

    if should_flag:
        reason = (
            f"20-day precision deteriorated by {deterioration:.2%}, "
            f"meeting/exceeding the {deterioration_threshold:.2%} threshold."
        )
    elif deterioration > 0:
        reason = (
            f"20-day precision deteriorated by {deterioration:.2%}, "
            f"below the {deterioration_threshold:.2%} threshold."
        )
    else:
        reason = (
            f"20-day precision is not below baseline "
            f"(change={-deterioration:.2%})."
        )

    return RetrainDecision(
        baseline_precision, current_precision, deterioration,
        deterioration_threshold, should_flag, reason
    )