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