// 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])
}