import pandas as pd

class Taxes:
    def __init__(self, tax_rate=0.20):
        """
        tax_rate: flat tax rate on positive profits (e.g., 0.20 for 20%)
        """
        self.tax_rate = tax_rate

    def add_taxes_to_results_csv(self, input_csv, output_csv=None, profit_col='final_usd'):
        """
        Reads a backtest results CSV, adds tax columns, and writes to a new CSV.
        - input_csv: path to the input CSV file
        - output_csv: path to the output CSV file (if None, overwrite input)
        - profit_col: column name for profit (default: 'final_usd')
        """
        df = pd.read_csv(input_csv, delimiter=None)
        # Compute tax only on positive profits
        df['tax_paid'] = df[profit_col].apply(lambda x: self.tax_rate * x if x > 0 else 0)
        df['net_profit_after_tax'] = df[profit_col] - df['tax_paid']
        df['cumulative_tax_paid'] = df['tax_paid'].cumsum()
        if not output_csv:
            output_csv = input_csv
        df.to_csv(output_csv, index=False)
        return output_csv