class SlidingWindow:
    """Counts events in the last `span` seconds."""

    def __init__(self, span):
        self.span = span
        self.events = []

    def add(self, at):
        self.events.append(at)

    def prune(self, now):
        cutoff = now - self.span
        self.events = [e for e in self.events if e > cutoff]

    def count(self, now):
        self.prune(now)
        return len(self.events)