ORG.oclc.ecat.util
Class UCDecoder

java.lang.Object
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  +--ORG.oclc.ecat.util.CharacterDecoder
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        +--ORG.oclc.ecat.util.UCDecoder

public class UCDecoder

extends CharacterDecoder

This class implements a robust character decoder. The decoder will converted encoded text into binary data. The basic encoding unit is a 3 character atom. It encodes two bytes of data. Bytes are encoded into a 64 character set, the characters were chosen specifically because they appear in all codesets. We don't care what their numerical equivalent is because we use a character array to map them. This is like UUencoding with the dependency on ASCII removed. The three chars that make up an atom are encoded as follows:

      00xxxyyy 00axxxxx 00byyyyy
      00 = leading zeros, all values are 0 - 63
      xxxyyy - Top 3 bits of X, Top 3 bits of Y
      axxxxx - a = X parity bit, xxxxx lower 5 bits of X
      byyyyy - b = Y parity bit, yyyyy lower 5 bits of Y
 

The atoms are arranged into lines suitable for inclusion into an email message or text file. The number of bytes that are encoded per line is 48 which keeps the total line length under 80 chars) Each line has the form(

  *(LLSS)(DDDD)(DDDD)(DDDD)...(CRC)
  Where each (xxx) represents a three character atom.
  (LLSS) - 8 bit length (high byte), and sequence number
           modulo 256;
  (DDDD) - Data byte atoms, if length is odd, last data
           atom has (DD00) (high byte data, low byte 0)
  (CRC)  - 16 bit CRC for the line, includes length,
           sequence, and all data bytes. If there is a
           zero pad byte (odd length) it is _NOT_
           included in the CRC.
 

If an error is encountered during decoding this class throws a CEFormatException. The specific detail messages are:

    "UCDecoder: High byte parity error."
    "UCDecoder: Low byte parity error."
    "UCDecoder: Out of sequence line."
    "UCDecoder: CRC check failed."
 

See Also:

CharacterEncoder, UCEncoder

Constructor Summary

UCDecoder()

Methods inherited from class ORG.oclc.ecat.util.CharacterDecoder

decodeBuffer, decodeBuffer, decodeBuffer, readFully

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

UCDecoder

public UCDecoder()