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

from signals.base import AlphaSignal


class MomentumSignal(AlphaSignal):
    """Positive trailing return: stocks that rose score high (momentum).

    The signal is ``close.pct_change(lookback)`` — opposite of ReversalSignal.
    """

    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"momentum_{self.lookback}d"