"""Sample interface counters without summing physical and virtual interfaces."""
import re
from pathlib import Path


class NetworkRates:
    def __init__(self, root='/sys/class/net'):
        self.root = Path(root)
        self.previous = None

    def sample(self, interface, now):
        try:
            if not re.fullmatch(r'[\w.:-]{1,64}', interface):
                raise ValueError('invalid interface')
            directory = self.root / interface / 'statistics'
            rx = int((directory / 'rx_bytes').read_text())
            tx = int((directory / 'tx_bytes').read_text())
            if rx < 0 or tx < 0:
                raise ValueError('negative counter')
        except (OSError, ValueError, TypeError):
            self.previous = None
            return None, None
        previous = self.previous
        self.previous = interface, now, rx, tx
        if previous is None or previous[0] != interface or now <= previous[1]:
            return None, None
        elapsed = now - previous[1]
        return (None if rx < previous[2] else (rx - previous[2]) / elapsed,
                None if tx < previous[3] else (tx - previous[3]) / elapsed)