rfc1505.txt

RFC 的详细文档！

    #define STRTL          0
    #define STEPL          1
    #define STOPL          7

    static FILE *in;
    static FILE *out;

    static int   getbuf;
    static int   getlen;
    static long  in_count;
    static long  out_count;
    static long  crc;
    static long  crctable[256];
    static uchar xxcodes[] =
    "+-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ\
    abcdefghijklmnopqrstuvwxyz";
    static uchar ddcodes[256];

    static uchar text[N];

    #define CRCPOLY         0xEDB88320
    #define CRC_MASK        0xFFFFFFFF
    #define UPDATE_CRC(crc, c)  \
            crc = crctable[((uchar)(crc) ^ (uchar)(c)) & 0xFF] \
                  ^ (crc >> 8)
    #define START_RECD      "* LZJU90"



    void MakeCrctable()     /* Initialize CRC-32 table */
    {
    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



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RFC 1505                 Encoding Header Field               August 1993


                    r >>= 1;
                }
            crctable[i] = r;
            }
    }



    int GetXX()             /* Get xxcode and translate */
    {
    int c;
        do {
            if ((c = fgetc(in)) == EOF)
                c = 0;
            } while (c == '\n');
        in_count++;
        return ddcodes[c];
    }



    int GetBit()            /* Get one bit from input buffer */
    {
    int c;
        while (getlen <= 0) {
            c = GetXX();
            getbuf |= c << (10-getlen);
            getlen += 6;
            }
        c = (getbuf & 0x8000) != 0;
        getbuf <<= 1;
        getbuf &= 0xFFFF;
        getlen--;
        return(c);
    }



    int GetBits(int len)        /* Get len bits */
    {
    int c;
        while (getlen <= 10) {
            c = GetXX();
            getbuf |= c << (10-getlen);
            getlen += 6;
            }
        if (getlen < len) {
            c = (uint)getbuf >> (16-len);



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            getbuf = GetXX();
            c |= getbuf >> (6+getlen-len);
            getbuf <<= (10+len-getlen);
            getbuf &= 0xFFFF;
            getlen -= len - 6;
            }
        else {
            c = (uint)getbuf >> (16-len);
            getbuf <<= len;
            getbuf &= 0xFFFF;
            getlen -= len;
            }
        return(c);
    }



    int DecodePosition()    /* Decode offset position pointer */
    {
    int c;
    int width;
    int plus;
    int pwr;
        plus = 0;
        pwr = 1 << STRTP;
        for (width = STRTP; width < STOPP; width += STEPP) {
            c = GetBit();
            if (c == 0)
                break;
            plus += pwr;
            pwr <<= 1;
            }
        if (width != 0)
            c = GetBits(width);
        c += plus;
        return(c);
    }



    int DecodeLength()      /* Decode code length */
    {
    int c;
    int width;
    int plus;
    int pwr;
        plus = 0;
        pwr = 1 << STRTL;



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        for (width = STRTL; width < STOPL; width += STEPL) {
            c = GetBit();
            if (c == 0)
                break;
            plus += pwr;
            pwr <<= 1;
            }
        if (width != 0)
            c = GetBits(width);
        c += plus;
    return(c);
    }


    void InitCodes()        /* Initialize decode table */
    {
    int i;
        for (i = 0; i < 256; i++) ddcodes[i] = 0;
        for (i = 0; i < 64; i++) ddcodes[xxcodes[i]] = i;
    return;
    }

    main(int ac, char **av)            /* main program */
    {
    int r;
    int j, k;
    int c;
    int pos;
    char buf[80];
    char name[3];
    long num, bytes;

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

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


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



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        return(1);
            }

        while (1) {
            if (fgets(buf, sizeof(buf), in) == NULL) {
                fprintf(stderr, "Unexpected EOF\n");
            return(1);
                }
            if (strncmp(buf, START_RECD, strlen(START_RECD)) == 0)
                break;
            }

        in_count = 0;
        out_count = 0;
        getbuf = 0;
        getlen = 0;

        InitCodes();
        MakeCrctable();

        crc = CRC_MASK;
        r = 0;

        while (feof(in) == 0) {
            c = DecodeLength();
            if (c == 0) {
                c = GetBits(8);
                UPDATE_CRC(crc, c);
                out_count++;
                text[r] = c;
                fputc(c, out);
                if (++r >= N)
                    r = 0;
                }

            else {
                pos = DecodePosition();
                if (pos == 0)
                    break;
                pos--;
                j = c + THRESHOLD - 1;
                pos = r - pos - 1;
                if (pos < 0)
                    pos += N;
                for (k = 0; k < j; k++) {
                    c = text[pos];
                    text[r] = c;
                    UPDATE_CRC(crc, c);



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RFC 1505                 Encoding Header Field               August 1993


                    out_count++;
                    fputc(c, out);
                    if (++r >= N)
                        r = 0;
                    if (++pos >= N)
                        pos = 0;
                    }
                }
            }

        fgetc(in); /* skip newline */

        if (fscanf(in, "* %ld %lX", &bytes, &num) != 2) {
            fprintf(stderr, "CRC record not found\n");
            return(1);
            }

        else if (crc != num) {
            fprintf(stderr,
                 "CRC error, expected %lX, found %lX\n",
                 crc, num);
            return(1);
            }

        else if (bytes != out_count) {
            fprintf(stderr,
                 "File size error, expected %lu, found %lu\n",
                 bytes, out_count);
        return(1);
            }

        else
            fprintf(stderr,
                 "File decoded to %lu bytes correctly\n",
                 out_count);

        fclose(in);
        fclose(out);
    return(0);
    }


5.3.1  An example of an Encoder

   Many algorithms are possible for the encoder, with different
   tradeoffs between speed, size, and complexity.  The following is a
   simple example program which is fairly efficient; more sophisticated
   implementations will run much faster, and in some cases produce



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RFC 1505                 Encoding Header Field               August 1993


   somewhat better compression.

   This example also shows that the encoder need not use the entire
   window available.  Not using the full window costs a small amount of
   compression, but can greatly increase the speed of some algorithms.

    /* LZJU 90 Encoding program */

    /* Written By Robert Jung and Robert Ullmann, 1990 and 1991. */

    /* This code is NOT COPYRIGHT, not protected. It is in the true
       Public Domain. */

    #include <stdio.h>

    typedef unsigned char uchar;
    typedef unsigned int  uint;

    #define N          24000    /* Size of window buffer */
    #define F            256   /* Size of look-ahead buffer */
    #define THRESHOLD      3
    #define K          16384    /* Size of hash table */

    #define STRTP          9
    #define STEPP          1
    #define STOPP         14

    #define STRTL          0
    #define STEPL          1
    #define STOPL          7

    #define CHARSLINE     78

    static FILE *in;
    static FILE *out;

    static int   putlen;
    static int   putbuf;
    static int   char_ct;
    static long  in_count;
    static long  out_count;
    static long  crc;
    static long  crctable[256];
    static uchar xxcodes[] =
    "+-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ\
    abcdefghijklmnopqrstuvwxyz";
    uchar window_text[N + F + 1];




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    /* text contains window, plus 1st F of window again
       (for comparisons) */

    uint hash_table[K];
    /* table of pointers into the text */

    #define CRCPOLY         0xEDB88320
    #define CRC_MASK        0xFFFFFFFF
    #define UPDATE_CRC(crc, c)  \
      crc = crctable[((uchar)(crc) ^ (uchar)(c)) & 0xFF] \
      ^ (crc >> 8)


    void MakeCrctable()     /* Initialize CRC-32 table */