rfc1505.txt

RFC 的详细文档！

    {
    uint i, j;
    long r;
        for (i = 0; i <= 255; i++) {
            r = i;
            for (j = 8; j > 0; j--) {
                if (r & 1)
                    r = (r >> 1) ^ CRCPOLY;
                else
                    r >>= 1;
            }
            crctable[i] = r;
        }
    }



    void PutXX(int c)           /* Translate and put xxcode */
    {
        c = xxcodes[c & 0x3F];
        if (++char_ct > CHARSLINE) {
            char_ct = 1;
            fputc('\n', out);
        }
        fputc(c, out);
        out_count++;
    }


    void PutBits(int c, int len)  /* Put rightmost "len" bits of "c" */
    {
        c <<= 16 - len;
        c &= 0xFFFF;
        putbuf |= (uint) c >> putlen;



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        c <<= 16 - putlen;
        c &= 0xFFFF;
        putlen += len;
        while (putlen >= 6) {
            PutXX(putbuf >> 10);
            putlen -= 6;
            putbuf <<= 6;
            putbuf &= 0xFFFF;
            putbuf |= (uint) c >> 10;
            c = 0;
            }
    }


    void EncodePosition(int ch) /* Encode offset position pointer */
    {
    int width;
    int prefix;
    int pwr;
        pwr = 1 << STRTP;
        for (width = STRTP; ch >= pwr; width += STEPP, pwr <<= 1)
            ch -= pwr;
        if ((prefix = width - STRTP) != 0)
            PutBits(0xffff, prefix);
        if (width < STOPP)
            width++;
        /* else if (width > STOPP)
        abort(); do nothing */
        PutBits(ch, width);
    }


    void EncodeLength(int ch)   /* Encode code length */
    {
    int width;
    int prefix;
    int pwr;
        pwr = 1 << STRTL;
        for (width = STRTL; ch >= pwr; width += STEPL, pwr <<= 1)
            ch -= pwr;
        if ((prefix = width - STRTL) != 0)
            PutBits(0xffff, prefix);
        if (width < STOPL)
            width++;
        /* else if (width > STOPL)
        abort(); do nothing */
        PutBits(ch, width);
    }



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    main(int ac, char **av)            /* main program */
    {
    uint r, s, i, c;
    uchar *p, *rp;
    int match_position;
    int match_length;
    int len;
    uint hash, h;

        if (ac < 3) {
            fprintf(stderr, "usage: juencode in out\n");
        return(1);
            }

        in = fopen(av[1], "rb");
        if (!in) {
            fprintf(stderr, "Can't open %s\n", av[1]);
        return(1);
            }

        out = fopen(av[2], "w");
        if (!out) {
            fprintf(stderr, "Can't open %s\n", av[2]);
            fclose(in);
        return(1);
            }

        char_ct = 0;
        in_count = 0;
        out_count = 0;
        putbuf = 0;
        putlen = 0;
        hash = 0;

        MakeCrctable();
        crc = CRC_MASK;

        fprintf(out, "* LZJU90 %s\n", av[1]);

        /* The hash table inititialization is somewhat arbitrary */
        for (i = 0; i < K; i++) hash_table[i] = i % N;

        r = 0;
        s = 0;

        /* Fill lookahead buffer */

        for (len = 0; len < F && (c = fgetc(in)) != EOF; len++) {



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            UPDATE_CRC(crc, c);
        in_count++;
        window_text[s++] = c;
        }


        while (len > 0) {
        /* look for match in window at hash position */
        h = ((((window_text[r] << 5) ^ window_text[r+1])
                << 5) ^ window_text[r+2]);
        p = window_text + hash_table[h % K];
        rp = window_text + r;
        for (i = 0, match_length = 0; i < F; i++) {
                if (*p++ != *rp++) break;
                match_length++;
                }
        match_position = r - hash_table[h % K];
        if (match_position <= 0) match_position += N;

        if (match_position > N - F - 2) match_length = 0;
        if (match_position > in_count - len - 2)
            match_length = 0; /* ! :-) */

        if (match_length > len)
            match_length = len;
        if (match_length < THRESHOLD) {
            EncodeLength(0);
            PutBits(window_text[r], 8);
            match_length = 1;
            }
        else {
            EncodeLength(match_length - THRESHOLD + 1);
            EncodePosition(match_position);
            }

        for (i = 0; i < match_length &&
                        (c = fgetc(in)) != EOF; i++) {
                UPDATE_CRC(crc, c);
                in_count++;
            window_text[s] = c;
                if (s < F - 1)
                window_text
                [s + N] = c;
            if (++s > N - 1) s = 0;
            hash = ((hash << 5) ^ window_text[r]);
            if (r > 1) hash_table[hash % K] = r - 2;
            if (++r > N - 1) r = 0;
            }



Costanzo, Robinson & Ullmann                                   [Page 32]

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        while (i++ < match_length) {
            if (++s > N - 1) s = 0;
            hash = ((hash << 5) ^ window_text[r]);
            if (r > 1) hash_table[hash % K] = r - 2;
            if (++r > N - 1 ) r = 0;
            len--;
                }
        }


        /* end compression indicator */
        EncodeLength(1);
        EncodePosition(0);
        PutBits(0, 7);

        fprintf(out, "\n* %lu %08lX\n", in_count, crc);
        fprintf(stderr, "Encoded %lu bytes to %lu symbols\n",
                in_count, out_count);

        fclose(in);
        fclose(out);

    return(0);
    }


5.3.2  Example LZJU90 Compressed Object

   The following is an example of an LZJU90 compressed object.  Using
   this as source for the program in section 5.3 will reveal what it is.

      Encoding: 7 LZJU90 Text

      * LZJU90 example
      8-mBtWA7WBVZ3dEBtnCNdU2WkE4owW+l4kkaApW+o4Ir0k33Ao4IE4kk
      bYtk1XY618NnCQl+OHQ61d+J8FZBVVCVdClZ2-LUI0v+I4EraItasHbG
      VVg7c8tdk2lCBtr3U86FZANVCdnAcUCNcAcbCMUCdicx0+u4wEETHcRM
      7tZ2-6Btr268-Eh3cUAlmBth2-IUo3As42laIE2Ao4Yq4G-cHHT-wCEU
      6tjBtnAci-I++
      * 190 081E2601











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6.  Alphabetical Listing of Defined Encodings


        Keyword         Description             Section  Reference(s)
        _______         ___________             _______  ____________

        EDIFACT         EDIFACT format          3.5
        EDI-X12         EDI X12 format          3.5      ANSI X12
        EVFU            FORTRAN format          3.4
        FS              File System format      3.6, 4
        Hex             Hex binary format       3.3
        LZJU90          LZJU90 format           3.7, 5
        LZW             LZW format              3.8
        Message         Encapsulated Message    3.2      STD 11, RFC 822
        PEM, PEM-Clear  Privacy Enhanced Mail   3.10     RFC 1421-1424
        PGP             Pretty Good Privacy     3.11
        Postscript      Postscript format       3.14     [8]
        Shar            Shell Archive format    3.15
        Signature       Signature               3.12
        Tar             Tar format              3.13
        Text            Text                    3.1      IS 10646
        uuencode        uuencode format         3.9
        URL             external URL-reference  3.16

7.  Security Considerations

   Security of content and the receiving (decoding) system is discussed
   in sections 3.10, 3.11, 3.15, and 4.2.10.  The considerations
   mentioned also apply to other encodings and attributes with similar
   functions.

8.  References

   [1] Robinson, D. and R. Ullmann, "Encoding Header Field for Internet
       Messages", RFC 1154, Prime Computer, Inc., April 1990.

   [2] Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", STD 11, RFC 822, University of Delaware, August 1982.

   [3] International Organization for Standardization, Information
       Technology -- Universal Coded Character Set (UCS).  ISO/IEC
       10646-1:1993, June 1993.

   [4] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part
       I: Message Encryption and Authentication Procedures" RFC 1421,
       IAB IRTF PSRG, IETF PEM WG, February 1993.





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   [5] Kent, S., "Privacy Enhancement for Internet Electronic Mail: Part
       II: Certificate-Based Key Management", RFC 1422, IAB IRTF PSRG,
       IETF PEM, BBN, February 1993.

   [6] Balenson, D., "Privacy Enhancement for Internet Electronic Mail:
       Part III: Algorithms, Modes, and Identifiers", RFC 1423, IAB IRTF
       PSRG, IETF PEM WG, TIS, February 1993.

   [7] Kaliski, B., "Privacy Enhancement for Internet Electronic Mail:
       Part IV: Key Certification and Related Services", RFC 1424, RSR
       Laboratories, February 1993.

   [8] Adobe Systems Inc., PostScript Language Reference Manual.  2nd
       Edition, 2nd Printing, January 1991.

   [9] Rose, M. and E. Steffererud, "Proposed Standard for Message
       Encapsulation", RFC 934, Delaware and NMA, January 1985.

  [10] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
       USC/Information Sciences Institute, August 1982.

  [11] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
       Extensions): Mechanisms for Specifying and Describing the Format
       of Internet Message Bodies", RFC 1341, Bellcore, Innosoft, June
       1992.

  [12] Borenstein, N., and M. Linimon, "Extension of MIME Content-Types
       to a New Medium", RFC 1437, 1 April 1993.

9.  Acknowledgements

   The authors would like to thank Robert Jung for his contributions to
   this work, in particular the public domain sample code for LZJU90.


















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RFC 1505                 Encoding Header Field               August 1993


10.  Authors' Addresses

   Albert K. Costanzo
   AKC Consulting Inc.
   P.O. Box 4031
   Roselle Park, NJ  07204-0531

   Phone: [phone]
   Email: [e-mail]


   David Robinson
   Computervision Corporation
   100 Crosby Drive
   Bedford, MA  017