import time


class DNSResolver:
    def __init__(self, zone=None, clock=time.time):
        self.zone = zone if zone is not None else {}
        self.clock = clock
        self._rr = {}                 # name -> next index

    def _lookup(self, name, seen=None):
        seen = seen or set()
        if name in seen:
            return []
        seen.add(name)
        records = self.zone.get(name, [])
        a_values = [r["value"] for r in records if r["type"] == "A"]
        if a_values:
            return a_values
        for r in records:
            if r["type"] == "CNAME":
                return self._lookup(r["value"], seen)
        return []

    def resolve(self, name):
        """Return the next A value round-robin, or None if there are none."""
        # TODO: ask self._lookup for this name's A values and answer None when
        # that list comes back empty. Otherwise pick the value sitting at the
        # counter self._rr holds for this name, wrapped by modulo so a short
        # list cycles, and bump that per-name counter so the next call moves on.
        pass