locate/frcode.c

   1 /* frcode -- front-compress a sorted list
   2    Copyright (C) 1994 Free Software Foundation, Inc.
   3
   4    This program is free software; you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 2, or (at your option)
   7    any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program; if not, write to the Free Software
  16    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
  17
  18 /* Usage: frcode < sorted-list > compressed-list
  19
  20    Uses front compression (also known as incremental encoding);
  21    see ";login:", March 1983, p. 8.
  22
  23    The input is a sorted list of NUL-terminated strings.
  24    (FIXME newline-terminated, until we figure out how to sort
  25    NUL-terminated strings.)
  26
  27    The output entries are in the same order as the input;
  28    each entry consists of an offset-differential count byte
  29    (the additional number of characters of prefix of the preceding entry to
  30    use beyond the number that the preceding entry is using of its predecessor),
  31    followed by a null-terminated ASCII remainder.
  32
  33    If the offset-differential count is larger than can be stored
  34    in a byte (+/-127), the byte has the value LOCATEDB_ESCAPE
  35    and the count follows in a 2-byte word, with the high byte first
  36    (network byte order).
  37
  38    Example:
  39
  40    Input, with NULs changed to newlines:
  41    /usr/src
  42    /usr/src/cmd/aardvark.c
  43    /usr/src/cmd/armadillo.c
  44    /usr/tmp/zoo
  45
  46    Length of the longest prefix of the preceding entry to share:
  47    0 /usr/src
  48    8 /cmd/aardvark.c
  49    14 rmadillo.c
  50    5 tmp/zoo
  51
  52    Output, with NULs changed to newlines and count bytes made printable:
  53    0 LOCATE02
  54    0 /usr/src
  55    8 /cmd/aardvark.c
  56    6 rmadillo.c
  57    -9 tmp/zoo
  58
  59    (6 = 14 - 8, and -9 = 5 - 14)
  60
  61    Written by James A. Woods <jwoods@adobe.com>.
  62    Modified by David MacKenzie <djm@gnu.ai.mit.edu>.  */
  63
  64 #include <config.h>
  65 #include <stdio.h>
  66 #include <sys/types.h>
  67
  68 #if defined(HAVE_STRING_H) || defined(STDC_HEADERS)
  69 #include <string.h>
  70 #else
  71 #include <strings.h>
  72 #endif
  73
  74 #ifdef STDC_HEADERS
  75 #include <stdlib.h>
  76 #endif
  77
  78 #include "locatedb.h"
  79
  80 char *xmalloc ();
  81
  82 /* The name this program was run with.  */
  83 char *program_name;
  84
  85 /* Write out a 16-bit int, high byte first (network byte order).  */
  86
  87 static void
  88 put_short (c, fp)
  89      int c;
  90      FILE *fp;
  91 {
  92   putc (c >> 8, fp);
  93   putc (c, fp);
  94 }
  95
  96 /* Return the length of the longest common prefix of strings S1 and S2. */
  97
  98 static int
  99 prefix_length (s1, s2)
 100      char *s1, *s2;
 101 {
 102   register char *start;
 103
 104   for (start = s1; *s1 == *s2 && *s1 != '\0'; s1++, s2++)
 105     ;
 106   return s1 - start;
 107 }
 108
 109 void
 110 main (argc, argv)
 111      int argc;
 112      char **argv;
 113 {
 114   char *path;                   /* The current input entry.  */
 115   char *oldpath;                /* The previous input entry.  */
 116   size_t pathsize, oldpathsize; /* Amounts allocated for them.  */
 117   int count, oldcount, diffcount; /* Their prefix lengths & the difference. */
 118   int line_len;                 /* Length of input line.  */
 119
 120   program_name = argv[0];
 121
 122   pathsize = oldpathsize = 1026; /* Increased as necessary by getstr.  */
 123   path = xmalloc (pathsize);
 124   oldpath = xmalloc (oldpathsize);
 125
 126   /* Set to anything not starting with a slash, to force the first
 127      prefix count to 0.  */
 128   strcpy (oldpath, " ");
 129   oldcount = 0;
 130
 131   fwrite (LOCATEDB_MAGIC, sizeof (LOCATEDB_MAGIC), 1, stdout);
 132
 133   /* FIXME temporary: change the \n to \0 when we figure out how to sort
 134      null-terminated strings.  */
 135   while ((line_len = getstr (&path, &pathsize, stdin, '\n', 0)) > 0)
 136     {
 137       path[line_len - 1] = '\0'; /* FIXME temporary: nuke the newline.  */
 138
 139       count = prefix_length (oldpath, path);
 140       diffcount = count - oldcount;
 141       oldcount = count;
 142       /* If the difference is small, it fits in one byte;
 143          otherwise, two bytes plus a marker noting that fact.  */
 144       if (diffcount < -127 || diffcount > 127)
 145         {
 146           putc (LOCATEDB_ESCAPE, stdout);
 147           put_short (diffcount, stdout);
 148         }
 149       else
 150         putc (diffcount, stdout);
 151
 152       fputs (path + count, stdout);
 153       putc ('\0', stdout);
 154
 155       {
 156         /* Swap path and oldpath and their sizes.  */
 157         char *tmppath = oldpath;
 158         size_t tmppathsize = oldpathsize;
 159         oldpath = path;
 160         oldpathsize = pathsize;
 161         path = tmppath;
 162         pathsize = tmppathsize;
 163       }
 164     }
 165
 166   free (path);
 167   free (oldpath);
 168
 169   exit (0);
 170 }