// finishBlock finishes the current block and returns its block handle, which is // its offset and length in the table. func (w *Writer) finishBlock() (blockHandle, error) { // Write the restart points to the buffer. if w.nEntries == 0 { // Every block must have at least one restart point. w.restarts = w.restarts[:1] w.restarts[0] = 0 } tmp4 := w.tmp[:4] for _, x := range w.restarts { binary.LittleEndian.PutUint32(tmp4, x) w.buf.Write(tmp4) } binary.LittleEndian.PutUint32(tmp4, uint32(len(w.restarts))) w.buf.Write(tmp4) // Compress the buffer, discarding the result if the improvement // isn't at least 12.5%. b := w.buf.Bytes() w.tmp[0] = noCompressionBlockType if w.compression == db.SnappyCompression { compressed := snappy.Encode(w.compressedBuf, b) w.compressedBuf = compressed[:cap(compressed)] if len(compressed) < len(b)-len(b)/8 { w.tmp[0] = snappyCompressionBlockType b = compressed } } // Calculate the checksum. checksum := crc.New(b).Update(w.tmp[:1]).Value() binary.LittleEndian.PutUint32(w.tmp[1:5], checksum) // Write the bytes to the file. if _, err := w.writer.Write(b); err != nil { return blockHandle{}, err } if _, err := w.writer.Write(w.tmp[:5]); err != nil { return blockHandle{}, err } bh := blockHandle{w.offset, uint64(len(b))} w.offset += uint64(len(b)) + blockTrailerLen // Reset the per-block state. w.buf.Reset() w.nEntries = 0 w.restarts = w.restarts[:0] return bh, nil }
// readBlock reads and decompresses a block from disk into memory. func (r *Reader) readBlock(bh blockHandle) (block, error) { b := make([]byte, bh.length+blockTrailerLen) if _, err := r.file.ReadAt(b, int64(bh.offset)); err != nil { return nil, err } if r.verifyChecksums { checksum0 := binary.LittleEndian.Uint32(b[bh.length+1:]) checksum1 := crc.New(b[:bh.length+1]).Value() if checksum0 != checksum1 { return nil, errors.New("leveldb/table: invalid table (checksum mismatch)") } } switch b[bh.length] { case noCompressionBlockType: return b[:bh.length], nil case snappyCompressionBlockType: b, err := snappy.Decode(nil, b[:bh.length]) if err != nil { return nil, err } return b, nil } return nil, fmt.Errorf("leveldb/table: unknown block compression: %d", b[bh.length]) }