private static List<Integer> getBucketMigrateCount(int numColumns, int numBucketBits) {
        List<Integer> ret = getBucketCount(numColumns++, numBucketBits);
        int cnt = getNumBuckets(numBucketBits) / numColumns;
        int rst = getNumBuckets(numBucketBits) % numColumns;
        for (int i = 0; i < numColumns - 1; ++i) {
            ret.set(i, ret.get(i) - (cnt + (i < rst ? 1 : 0)));
        }
        return ret;
    }
This method returns a list similar to {@link com.yahoo.vdslib.BucketDistribution#getBucketCount(int, int)}, except that the returned list
contains the number of buckets that will have to be migrated from each column if an additional column was added.

@param numColumns    The original number of columns.
@param numBucketBits The number of bits to use for bucket id.
@return The number of buckets to migrate, one value per column.
vdslib/src/main/java/com/yahoo/vdslib/BucketDistribution.java