java/src/main/com/google/appengine/api/files/RecordConstants.java

   1 // Copyright 2011 Google Inc. All Rights Reserved.
   2
   3 package com.google.appengine.api.files;
   4
   5 /**
   6  * A set of constants needed by {@link RecordReadChannelImpl} and {@link RecordWriteChannelImpl}.
   7  *
   8  */
   9 @Deprecated
  10 final class RecordConstants {
  11
  12   /**
  13    * Size of a block.
  14    */
  15   public static final int BLOCK_SIZE = 32 * 1024;
  16
  17   /**
  18    * Header length in data.
  19    */
  20   public static final int HEADER_LENGTH = 7;
  21
  22   /**
  23    *  CRC Mask. Comes from http://leveldb.googlecode.com/svn/trunk/util/crc32c.h
  24    */
  25   public static final int CRC_MASK_DELTA = 0xa282ead8;
  26
  27   /**
  28    * An enumerated type that describes the type of the record being stored.
  29    * These byte values must match those of the leveldb log format for
  30    * compatibility between different implementations
  31    * of the {@link RecordWriteChannelImpl}.
  32    *
  33    */
  34   public enum RecordType {
  35     NONE((byte) 0x00),
  36     FULL((byte) 0x01),
  37     FIRST((byte) 0x02),
  38     MIDDLE((byte) 0x03),
  39     LAST((byte) 0x04),
  40     UNKNOWN((byte) 0xFF);
  41
  42     private byte value;
  43
  44     private RecordType(byte value) {
  45       this.value = value;
  46     }
  47     /**
  48      * The byte value of the record type is written to the file as part of the
  49      * header.
  50      * @return the byte value of the record type.
  51      */
  52     public byte value() {
  53       return this.value;
  54     }
  55
  56     /**
  57      * Converts a byte value into a {@link RecordType} enum.
  58      * @param value the byte value of the {@link RecordType} you want.
  59      * @return a {@link RecordType} that corresponds to the inputed byte value.
  60      */
  61     public static RecordType get(byte value) {
  62       switch(value) {
  63         case 0x00: return NONE;
  64         case 0x01: return FULL;
  65         case 0x02: return FIRST;
  66         case 0x03: return MIDDLE;
  67         case 0x04: return LAST;
  68         default: return UNKNOWN;
  69       }
  70     }
  71   }
  72
  73   /**
  74    * Masks the crc.
  75    *
  76    * Motivation taken from leveldb:
  77    *    it is problematic to compute the CRC of a string that
  78    *    contains embedded CRCs.  Therefore we recommend that CRCs stored
  79    *    somewhere (e.g., in files) should be masked before being stored.
  80    * @param crc the crc to mask.
  81    * @return the masked crc.
  82    */
  83   public static long maskCrc(long crc) {
  84     return ((((crc >> 15) | (crc << 17)) + RecordConstants.CRC_MASK_DELTA) & 0xFFFFFFFFL);
  85   }
  86
  87   /**
  88    * Unmasks the crc.
  89    * @param maskedCrc a masked crc.
  90    * @return an unmasked crc.
  91    */
  92   public static long unmaskCrc(long maskedCrc) {
  93     long rot = (maskedCrc - CRC_MASK_DELTA) & 0xFFFFFFFFL;
  94     return (((rot >> 17) | (rot << 15)) & 0xFFFFFFFFL);
  95  }
  96 }