func truno(t *testing.T, o *opt.Options, f func(h *dbHarness)) {
	for i := 0; i < 4; i++ {
		func() {
			switch i {
			case 0:
			case 1:
				if o == nil {
					o = &opt.Options{Filter: _bloom_filter}
				} else {
					old := o
					o = &opt.Options{}
					*o = *old
					o.Filter = _bloom_filter
				}
			case 2:
				if o == nil {
					o = &opt.Options{Compression: opt.NoCompression}
				} else {
					old := o
					o = &opt.Options{}
					*o = *old
					o.Compression = opt.NoCompression
				}
			}
			h := newDbHarnessWopt(t, o)
			defer h.close()
			switch i {
			case 3:
				h.reopenDB()
			}
			f(h)
		}()
	}
}

func (s *session) setOptions(o *opt.Options) {
	s.o = &opt.Options{}
	if o != nil {
		*s.o = *o
	}
	// Alternative filters.
	if filters := o.GetAltFilters(); len(filters) > 0 {
		s.o.AltFilters = make([]filter.Filter, len(filters))
		for i, filter := range filters {
			s.o.AltFilters[i] = &iFilter{filter}
		}
	}
	// Block cache.
	switch o.GetBlockCache() {
	case nil:
		s.o.BlockCache = cache.NewLRUCache(opt.DefaultBlockCacheSize)
	case opt.NoCache:
		s.o.BlockCache = nil
	}
	// Comparer.
	s.icmp = &iComparer{o.GetComparer()}
	s.o.Comparer = s.icmp
	// Filter.
	if filter := o.GetFilter(); filter != nil {
		s.o.Filter = &iFilter{filter}
	}
}

// NewWriter creates a new initialized table writer for the file.
//
// Table writer is not goroutine-safe.
func NewWriter(f io.Writer, o *opt.Options) *Writer {
	w := &Writer{
		writer:          f,
		cmp:             o.GetComparer(),
		filter:          o.GetFilter(),
		compression:     o.GetCompression(),
		blockSize:       o.GetBlockSize(),
		comparerScratch: make([]byte, 0),
	}
	// data block
	w.dataBlock.restartInterval = o.GetBlockRestartInterval()
	// The first 20-bytes are used for encoding block handle.
	w.dataBlock.scratch = w.scratch[20:]
	// index block
	w.indexBlock.restartInterval = 1
	w.indexBlock.scratch = w.scratch[20:]
	// filter block
	if w.filter != nil {
		w.filterBlock.generator = w.filter.NewGenerator()
		w.filterBlock.flush(0)
	}
	return w
}

// NewReader creates a new initialized table reader for the file.
// The cache is optional and can be nil.
func NewReader(f io.ReaderAt, size int64, cache cache.Namespace, o *opt.Options) *Reader {
	r := &Reader{
		reader:     f,
		cache:      cache,
		cmp:        o.GetComparer(),
		checksum:   o.GetStrict(opt.StrictBlockChecksum),
		strictIter: o.GetStrict(opt.StrictIterator),
	}
	if f == nil {
		r.err = errors.New("leveldb/table: Reader: nil file")
		return r
	}
	if size < footerLen {
		r.err = errors.New("leveldb/table: Reader: invalid table (file size is too small)")
		return r
	}
	var footer [footerLen]byte
	if _, err := r.reader.ReadAt(footer[:], size-footerLen); err != nil && err != io.EOF {
		r.err = fmt.Errorf("leveldb/table: Reader: invalid table (could not read footer): %v", err)
	}
	if string(footer[footerLen-len(magic):footerLen]) != magic {
		r.err = errors.New("leveldb/table: Reader: invalid table (bad magic number)")
		return r
	}
	// Decode the metaindex block handle.
	metaBH, n := decodeBlockHandle(footer[:])
	if n == 0 {
		r.err = errors.New("leveldb/table: Reader: invalid table (bad metaindex block handle)")
		return r
	}
	// Decode the index block handle.
	indexBH, n := decodeBlockHandle(footer[n:])
	if n == 0 {
		r.err = errors.New("leveldb/table: Reader: invalid table (bad index block handle)")
		return r
	}
	// Read index block.
	r.indexBlock, r.err = r.readBlock(indexBH, true)
	if r.err != nil {
		return r
	}
	// Read metaindex block.
	metaBlock, err := r.readBlock(metaBH, true)
	if err != nil {
		r.err = err
		return r
	}
	// Set data end.
	r.dataEnd = int64(metaBH.offset)
	metaIter := metaBlock.newIterator(nil)
	for metaIter.Next() {
		key := string(metaIter.Key())
		if !strings.HasPrefix(key, "filter.") {
			continue
		}
		fn := key[7:]
		var filter filter.Filter
		if f0 := o.GetFilter(); f0 != nil && f0.Name() == fn {
			filter = f0
		} else {
			for _, f0 := range o.GetAltFilters() {
				if f0.Name() == fn {
					filter = f0
					break
				}
			}
		}
		if filter != nil {
			filterBH, n := decodeBlockHandle(metaIter.Value())
			if n == 0 {
				continue
			}
			// Update data end.
			r.dataEnd = int64(filterBH.offset)
			filterBlock, err := r.readFilterBlock(filterBH, filter)
			if err != nil {
				continue
			}
			r.filterBlock = filterBlock
			break
		}
	}
	metaIter.Release()
	return r
}