def segment(self, document):
        """
        document: list[str]
        return list[int],
            i-th element denotes whether exists a boundary right before paragraph i(0 indexed)
        """
        assert(len(document) > 0 and len([d for d in document if not isinstance(d, str)]) == 0)
        if len(document) < 3:
            return [1] + [0 for _ in range(len(document) - 1)]
        # step 1, preprocessing
        n = len(document)
        self.window = min(self.window, n)
        cnts = [Counter(self.tokenizer.tokenize(document[i])) for i in range(n)]

        # step 2, compute similarity matrix
        self.sim = np.zeros((n, n))
        for i in range(n):
            for j in range(i, n):
                self.sim[i][j] = cosine_sim(cnts[i], cnts[j])
                self.sim[j][i] = self.sim[i][j]

        # step 3, compute rank matrix & sum matrix
        self.rank = np.zeros((n, n))
        for i in range(n):
            for j in range(i, n):
                r1 = max(0, i - self.window + 1)
                r2 = min(n - 1, i + self.window - 1)
                c1 = max(0, j - self.window + 1)
                c2 = min(n - 1, j + self.window - 1)
                sublist = self.sim[r1:(r2 + 1), c1:(c2+1)].flatten()
                lowlist = [x for x in sublist if x < self.sim[i][j]]
                self.rank[i][j] = 1.0 * len(lowlist) / ((r2 - r1 + 1) * (c2 - c1 + 1))
                self.rank[j][i] = self.rank[i][j]

        self.sm = np.zeros((n, n))
        # O(n^4) solution
        # for i in xrange(n):
        #     for j in xrange(i, n):
        #         self.sm[i][j] = sum(self.rank[i:(j + 1), i:(j + 1)].flatten())
        #         self.sm[j][i] = self.sm[i][j]
        # O(n^2) solution
        prefix_sm = np.zeros((n, n))
        for i in range(n):
            for j in range(n):
                prefix_sm[i][j] = self.rank[i][j]
                if i - 1 >= 0: prefix_sm[i][j] += prefix_sm[i - 1][j]
                if j - 1 >= 0: prefix_sm[i][j] += prefix_sm[i][j - 1]
                if i - 1 >= 0 and j - 1 >= 0: prefix_sm[i][j] -= prefix_sm[i - 1][j - 1]
        for i in range(n):
            for j in range(i, n):
                if i == 0:
                    self.sm[i][j] = prefix_sm[j][j]
                else:
                    self.sm[i][j] = prefix_sm[j][j] - prefix_sm[i - 1][j] \
                                    - prefix_sm[j][i - 1] + prefix_sm[i - 1][i - 1]
                self.sm[j][i] = self.sm[i][j]

        # step 4, determine boundaries
        D = 1.0 * self.sm[0][n - 1] / (n * n)
        darr, region_arr, idx = [D], [Region(0, n - 1, self.sm)], []
        sum_region, sum_area = float(self.sm[0][n - 1]), float(n * n)
        for i in range(n - 1):
            mx, pos = -1e9, -1
            for j, region in enumerate(region_arr):
                if region.l == region.r:
                    continue
                region.split(self.sm)
                den = sum_area - region.area + region.lch.area + region.rch.area
                cur = (sum_region - region.tot + region.lch.tot + region.rch.tot) / den
                if cur > mx:
                    mx, pos = cur, j
            assert(pos >= 0)
            tmp = region_arr[pos]
            region_arr[pos] = tmp.rch
            region_arr.insert(pos, tmp.lch)
            sum_region += tmp.lch.tot + tmp.rch.tot - tmp.tot
            sum_area += tmp.lch.area + tmp.rch.area - tmp.area
            darr.append(sum_region / sum_area)
            idx.append(tmp.best_pos)

        dgrad = [(darr[i + 1] - darr[i]) for i in range(len(darr) - 1)]

        # optional step, smooth gradient
        smooth_dgrad = [dgrad[i] for i in range(len(dgrad))]
        if len(dgrad) > 1:
            smooth_dgrad[0] = (dgrad[0] * 2 + dgrad[1]) / 3.0
            smooth_dgrad[-1] = (dgrad[-1] * 2 + dgrad[-2]) / 3.0
        for i in range(1, len(dgrad) - 1):
            smooth_dgrad[i] = (dgrad[i - 1] + 2 * dgrad[i] + dgrad[i + 1]) / 4.0
        dgrad = smooth_dgrad

        avg, stdev = np.average(dgrad), np.std(dgrad)
        cutoff = avg + self.std_coeff * stdev
        assert(len(idx) == len(dgrad))
        above_cutoff_idx = [i for i in range(len(dgrad)) if dgrad[i] >= cutoff]
        if len(above_cutoff_idx) == 0: boundary = []
        else: boundary = idx[:max(above_cutoff_idx) + 1]
        ret = [0 for _ in range(n)]
        for i in boundary:
            ret[i] = 1
            # boundary should not be too close
            for j in range(i - 1, i + 2):
                if j >= 0 and j < n and j != i and ret[j] == 1:
                    ret[i] = 0
                    break
        return [1] + ret[:-1]
document: list[str]
        return list[int],
            i-th element denotes whether exists a boundary right before paragraph i(0 indexed)