def attack_batch(self, imgs, labs):
    """
    Run the attack on a batch of instance and labels.
    """

    def compare(x, y):
      if not isinstance(x, (float, int, np.int64)):
        x = np.copy(x)
        if self.TARGETED:
          x[y] -= self.CONFIDENCE
        else:
          x[y] += self.CONFIDENCE
        x = np.argmax(x)
      if self.TARGETED:
        return x == y
      else:
        return x != y

    batch_size = self.batch_size

    oimgs = np.clip(imgs, self.clip_min, self.clip_max)

    # re-scale instances to be within range [0, 1]
    imgs = (imgs - self.clip_min) / (self.clip_max - self.clip_min)
    imgs = np.clip(imgs, 0, 1)
    # now convert to [-1, 1]
    imgs = (imgs * 2) - 1
    # convert to tanh-space
    imgs = np.arctanh(imgs * .999999)

    # set the lower and upper bounds accordingly
    lower_bound = np.zeros(batch_size)
    CONST = np.ones(batch_size) * self.initial_const
    upper_bound = np.ones(batch_size) * 1e10

    # placeholders for the best l2, score, and instance attack found so far
    o_bestl2 = [1e10] * batch_size
    o_bestscore = [-1] * batch_size
    o_bestattack = np.copy(oimgs)

    for outer_step in range(self.BINARY_SEARCH_STEPS):
      # completely reset adam's internal state.
      self.sess.run(self.init)
      batch = imgs[:batch_size]
      batchlab = labs[:batch_size]

      bestl2 = [1e10] * batch_size
      bestscore = [-1] * batch_size
      _logger.debug("  Binary search step %s of %s",
                    outer_step, self.BINARY_SEARCH_STEPS)

      # The last iteration (if we run many steps) repeat the search once.
      if self.repeat and outer_step == self.BINARY_SEARCH_STEPS - 1:
        CONST = upper_bound

      # set the variables so that we don't have to send them over again
      self.sess.run(
          self.setup, {
              self.assign_timg: batch,
              self.assign_tlab: batchlab,
              self.assign_const: CONST
          })

      prev = 1e6
      for iteration in range(self.MAX_ITERATIONS):
        # perform the attack
        _, l, l2s, scores, nimg = self.sess.run([
            self.train, self.loss, self.l2dist, self.output,
            self.newimg
        ])

        if iteration % ((self.MAX_ITERATIONS // 10) or 1) == 0:
          _logger.debug(("    Iteration {} of {}: loss={:.3g} " +
                         "l2={:.3g} f={:.3g}").format(
                             iteration, self.MAX_ITERATIONS, l,
                             np.mean(l2s), np.mean(scores)))

        # check if we should abort search if we're getting nowhere.
        if self.ABORT_EARLY and \
           iteration % ((self.MAX_ITERATIONS // 10) or 1) == 0:
          if l > prev * .9999:
            msg = "    Failed to make progress; stop early"
            _logger.debug(msg)
            break
          prev = l

        # adjust the best result found so far
        for e, (l2, sc, ii) in enumerate(zip(l2s, scores, nimg)):
          lab = np.argmax(batchlab[e])
          if l2 < bestl2[e] and compare(sc, lab):
            bestl2[e] = l2
            bestscore[e] = np.argmax(sc)
          if l2 < o_bestl2[e] and compare(sc, lab):
            o_bestl2[e] = l2
            o_bestscore[e] = np.argmax(sc)
            o_bestattack[e] = ii

      # adjust the constant as needed
      for e in range(batch_size):
        if compare(bestscore[e], np.argmax(batchlab[e])) and \
           bestscore[e] != -1:
          # success, divide const by two
          upper_bound[e] = min(upper_bound[e], CONST[e])
          if upper_bound[e] < 1e9:
            CONST[e] = (lower_bound[e] + upper_bound[e]) / 2
        else:
          # failure, either multiply by 10 if no solution found yet
          #          or do binary search with the known upper bound
          lower_bound[e] = max(lower_bound[e], CONST[e])
          if upper_bound[e] < 1e9:
            CONST[e] = (lower_bound[e] + upper_bound[e]) / 2
          else:
            CONST[e] *= 10
      _logger.debug("  Successfully generated adversarial examples " +
                    "on {} of {} instances.".format(
                        sum(upper_bound < 1e9), batch_size))
      o_bestl2 = np.array(o_bestl2)
      mean = np.mean(np.sqrt(o_bestl2[o_bestl2 < 1e9]))
      _logger.debug("   Mean successful distortion: {:.4g}".format(mean))

    # return the best solution found
    o_bestl2 = np.array(o_bestl2)
    return o_bestattack