import numpy as np
import pandas as pd
from strategies.base import StockStrategy, mask_scores


class TrendFollowingStrategy(StockStrategy):
    """
    Per-stock trend following with momentum ranking.

    Two filters applied:
    1. Trend filter: only hold stocks trading above their own moving average
       (individual uptrend, not market-level timing)
    2. Momentum rank: among trending stocks, pick the top_n by 6-month return

    This avoids the Multi-Factor problem of all-or-nothing market timing
    while still providing downside protection at the individual stock level.
    """

    def __init__(self, ma_window: int = 150, momentum_period: int = 126, top_n: int = 20):
        self.ma_window = ma_window
        self.momentum_period = momentum_period
        self.top_n = top_n

    def generate_signals(self, data: pd.DataFrame, eligible: pd.DataFrame | None = None) -> pd.DataFrame:
        # Per-stock trend filter: price > MA
        ma = data.rolling(self.ma_window).mean()
        in_uptrend = data > ma

        # Momentum score among trending stocks
        momentum = data.pct_change(self.momentum_period, fill_method=None)
        # Mask out stocks not in uptrend
        momentum_filtered = momentum.where(in_uptrend, np.nan)

        # Rank and select top_n
        momentum_filtered = mask_scores(momentum_filtered, eligible)
        n_valid = momentum_filtered.notna().sum(axis=1)
        enough = n_valid >= 1

        rank = mask_scores(momentum_filtered, eligible).rank(axis=1, ascending=False, na_option="bottom")
        effective_n = n_valid.clip(upper=self.top_n)
        top_mask = (rank <= effective_n.values.reshape(-1, 1)) & enough.values.reshape(-1, 1)
        # Ensure we only pick stocks that are actually in uptrend
        top_mask = top_mask & in_uptrend

        raw = top_mask.astype(float)
        row_sums = raw.sum(axis=1).replace(0, np.nan)
        signals = raw.div(row_sums, axis=0).fillna(0.0)

        warmup = max(self.ma_window, self.momentum_period)
        signals.iloc[:warmup] = 0.0

        return signals.shift(1).fillna(0.0)