from time import time
from .types import FloatOrNone


class StopWatch:
    started: FloatOrNone = None
    stopped: FloatOrNone = None

    def __init__(self, auto_start=True):
        if auto_start:
            self.start()

    def start(self):
        self.started = time()
        self.stopped = None

    def stop(self) -> float:
        self.stopped = time()
        return self.elapsed()

    def elapsed(self) -> float:
        if self.stopped is None:
            return time() - self.started
        else:
            return self.stopped - self.started

    def __str__(self):
        units = ['s', 'ms', 'µs', 'ns']
        unit = 0
        elapsed = self.elapsed()
        while elapsed < 1:
            elapsed *= 1000
            unit += 1

        return "%1.2f%s" % (elapsed, units[unit])