Line data    Source code

       1             : // Copyright 2011 The LevelDB-Go and Pebble Authors. All rights reserved. Use
       2             : // of this source code is governed by a BSD-style license that can be found in
       3             : // the LICENSE file.
       4             : 
       5             : // Package crc implements the checksum algorithm used throughout pebble.
       6             : //
       7             : // The algorithm is CRC-32 with Castagnoli's polynomial, followed by a bit
       8             : // rotation and an additional delta. The additional processing is to lessen the
       9             : // probability of arbitrary key/value data coincidentally containing bytes that
      10             : // look like a checksum.
      11             : //
      12             : // To calculate the uint32 checksum of some data:
      13             : //
      14             : //      var u uint32 = crc.New(data).Value()
      15             : //
      16             : // In pebble, the uint32 value is then stored in little-endian format.
      17             : package crc // import "github.com/cockroachdb/pebble/internal/crc"
      18             : 
      19             : import "hash/crc32"
      20             : 
      21             : var table = crc32.MakeTable(crc32.Castagnoli)
      22             : 
      23             : // CRC is a small convenience wrapper for computing the CRC32 checksum used by
      24             : // pebble. This is the same algorithm as used by RocksDB.
      25             : type CRC uint32
      26             : 
      27             : // New returns the result of adding the bytes to the zero-value CRC.
      28           1 : func New(b []byte) CRC {
      29           1 :         return CRC(0).Update(b)
      30           1 : }
      31             : 
      32             : // Update returns the result of adding the bytes to the CRC.
      33           1 : func (c CRC) Update(b []byte) CRC {
      34           1 :         return CRC(crc32.Update(uint32(c), table, b))
      35           1 : }
      36             : 
      37             : // Value returns the cooked CRC value. The additional processing is to lessen
      38             : // the probability of arbitrary key/value data coincidentally containing bytes
      39             : // that look like a checksum.
      40           1 : func (c CRC) Value() uint32 {
      41           1 :         return uint32(c>>15|c<<17) + 0xa282ead8
      42           1 : }