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 count(self, now):
        # Keeps events strictly newer than the cutoff.
        cutoff = now - self.span
        return len([e for e in self.events if e > cutoff])