"""Constant-real-£ withdrawal (the classic 4% rule shape).

Withdraw `state.initial_withdrawal` in year 0, then keep that real-£
amount fixed for the rest of retirement. In a 4% / £1M setup the year-0
draw is £40k, then £40k real every year after. The strategy's
`initial_rate` is kept only as a fallback for callers that don't feed
`state.initial_withdrawal`.
"""
from __future__ import annotations

from fire_planner.strategies.base import StrategyState, WithdrawalStrategy

DEFAULT_INITIAL_RATE = 0.04


class TrinityStrategy(WithdrawalStrategy):
    name = "trinity"

    def __init__(self, initial_rate: float = DEFAULT_INITIAL_RATE) -> None:
        self.initial_rate = initial_rate

    def propose_withdrawal(self, state: StrategyState) -> float:
        if state.year_idx == 0:
            # Year 0 = the user's target spending. Falls back to
            # initial_rate × initial_portfolio if no target was provided
            # (zero or missing) for backwards compatibility.
            if state.initial_withdrawal > 0:
                return state.initial_withdrawal
            return state.initial_portfolio * self.initial_rate
        return state.last_withdrawal