# Copyright (c) 2011 Google Inc. All rights reserved.
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class ExpectedFailures(object):
    def __init__(self):
        self._failures = set()
        self._is_trustworthy = True

    @classmethod
    def _has_failures(cls, results):
        return bool(results and results.failing_tests())

    @classmethod
    def _should_trust(cls, results):
        return bool(cls._has_failures(results) and results.failure_limit_count() and len(results.failing_tests()) < results.failure_limit_count())

    def failures_were_expected(self, results):
        if not self._is_trustworthy:
            return False
        if not self._should_trust(results):
            return False
        return set(results.failing_tests()) <= self._failures

    def unexpected_failures_observed(self, results):
        if not self._is_trustworthy:
            return None
        if not self._has_failures(results):
            return None
        return set(results.failing_tests()) - self._failures

    def update(self, results):
        if results:
            self._failures = set(results.failing_tests())
            self._is_trustworthy = self._should_trust(results)