"""Short-horizon reversal signal."""
import pandas as pd

from signals.base import AlphaSignal


class ReversalSignal(AlphaSignal):
    """Negative trailing return: oversold stocks score high.

    The signal is ``-close.pct_change(lookback)``, so a stock that fell over the
    lookback window gets a positive (bullish) score.
    """

    def __init__(self, lookback: int = 5):
        self.lookback = lookback

    def compute(self, df: pd.DataFrame) -> pd.Series:
        return -df["close"].pct_change(self.lookback)

    @property
    def name(self) -> str:
        return f"reversal_{self.lookback}d"