# indie:lang_version = 5
from indie import indicator, param, plot, color, MainContext, MutSeriesF
from indie.algorithms import Sma

@indicator('NVI/PVI Fosback', overlay_main_pane=False)
@param.str('mode', default='NVI', options=['NVI', 'PVI'], title='Indicator Type')
@param.int('ma_length', default=255, min=1, title='Signal Line Period (SMA)')
@param.float('start_val', default=100.0, title='Starting Value')
@plot.line(id='index_line', title='Index Value')
@plot.line(id='signal_line', color=color.GRAY, title='Signal MA')
@plot.fill('index_line', 'signal_line')
def Main(self, mode: str, ma_length: int, start_val: float):
    # Persistent series that carry values forward
    nvi = MutSeriesF.new(init=start_val)
    pvi = MutSeriesF.new(init=start_val)

    # Calculate price change
    if self.bar_index > 0 and self.close[1] != 0:
        price_change_pct = (self.close[0] - self.close[1]) / self.close[1]

        # NVI: update only when volume decreases
        if self.volume[0] < self.volume[1]:
            nvi[0] = nvi[1] * (1.0 + price_change_pct)
        else:
            nvi[0] = nvi[1]

        # PVI: update only when volume increases
        if self.volume[0] > self.volume[1]:
            pvi[0] = pvi[1] * (1.0 + price_change_pct)
        else:
            pvi[0] = pvi[1]

    # Mode selection
    index_val = nvi[0] if mode == 'NVI' else pvi[0]

    # Signal line
    index_series = MutSeriesF.new(index_val)
    sig_line = Sma.new(index_series, ma_length)[0]

    # Trend colors
    is_bull = index_val > sig_line
    line_col = color.GREEN if is_bull else color.RED
    fill_col = color.GREEN(0.1) if is_bull else color.RED(0.1)

    return (
        plot.Line(index_val, color=line_col),
        sig_line,
        plot.Fill(color=fill_col)
    )