gio/strinfo.c

   1 /*
   2  * Copyright © 2010 Codethink Limited
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2 of the licence, or (at your option) any later version.
   8  *
   9  * This library 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 GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General Public
  15  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  16  *
  17  * Author: Ryan Lortie <desrt@desrt.ca>
  18  */
  19
  20 #include <string.h>
  21 #include <glib.h>
  22
  23 /*
  24  * The string info map is an efficient data structure designed to be
  25  * used with a small set of items.  It is used by GSettings schemas for
  26  * three purposes:
  27  *
  28  *  1) Implement <choices> with a list of valid strings
  29  *
  30  *  2) Implement <alias> by mapping one string to another
  31  *
  32  *  3) Implement enumerated types by mapping strings to integer values
  33  *     (and back).
  34  *
  35  * The map is made out of an array of uint32s.  Each entry in the array
  36  * is an integer value, followed by a specially formatted string value:
  37  *
  38  *   The string starts with the byte 0xff or 0xfe, followed by the
  39  *   content of the string, followed by a nul byte, followed by
  40  *   additional nul bytes for padding, followed by a 0xff byte.
  41  *
  42  *   Padding is added so that the entire formatted string takes up a
  43  *   multiple of 4 bytes, and not less than 8 bytes.  The requirement
  44  *   for a string to take up 8 bytes is so that the scanner doesn't lose
  45  *   synch and mistake a string for an integer value.
  46  *
  47  * The first byte of the formatted string depends on if the integer is
  48  * an enum value (0xff) or an alias (0xfe).  If it is an alias then the
  49  * number refers to the word offset within the info map at which the
  50  * integer corresponding to the "target" value is stored.
  51  *
  52  * For example, consider the case of the string info map representing an
  53  * enumerated type of 'foo' (value 1) and 'bar' (value 2) and 'baz'
  54  * (alias for 'bar').  Note that string info maps are always little
  55  * endian.
  56  *
  57  * x01 x00 x00 x00   xff 'f' 'o' 'o'   x00 x00 x00 xff   x02 x00 x00 x00
  58  * xff 'b' 'a' 'r'   x00 x00 x00 xff   x03 x00 x00 x00   xfe 'b' 'a' 'z'
  59  * x00 x00 x00 xff
  60  *
  61  *
  62  * The operations that someone may want to perform with the map:
  63  *
  64  *   - lookup if a string is valid (and not an alias)
  65  *   - lookup the integer value for a enum 'nick'
  66  *   - lookup the integer value for the target of an alias
  67  *   - lookup an alias and convert it to its target string
  68  *   - lookup the enum nick for a given value
  69  *
  70  * In order to lookup if a string is valid, it is padded on either side
  71  * (as described) and scanned for in the array.  For example, you might
  72  * look for "foo":
  73  *
  74  *                   xff 'f' 'o' 'o'   x00 x00 x00 xff
  75  *
  76  * In order to lookup the integer value for a nick, the string is padded
  77  * on either side and scanned for in the array, as above.  Instead of
  78  * merely succeeding, we look at the integer value to the left of the
  79  * match.  This is the enum value.
  80  *
  81  * In order to lookup an alias and convert it to its target enum value,
  82  * the string is padded on either side (as described, with 0xfe) and
  83  * scanned for.  For example, you might look for "baz":
  84  *
  85  *                   xfe 'b' 'a' 'z'  x00 x00 x00 xff
  86  *
  87  * The integer immediately preceding the match then contains the offset
  88  * of the integer value of the target.  In our example, that's '3'.
  89  * This index is dereferenced to find the enum value of '2'.
  90  *
  91  * To convert the alias to its target string, 5 bytes just need to be
  92  * added past the start of the integer value to find the start of the
  93  * string.
  94  *
  95  * To lookup the enum nick for a given value, the value is searched for
  96  * in the array.  To ensure that the value isn't matching the inside of a
  97  * string, we must check that it is either the first item in the array or
  98  * immediately preceded by the byte 0xff.  It must also be immediately
  99  * followed by the byte 0xff.
 100  *
 101  * Because strings always take up a minimum of 2 words, because 0xff or
 102  * 0xfe never appear inside of a utf-8 string and because no two integer
 103  * values ever appear in sequence, the only way we can have the
 104  * sequence:
 105  *
 106  *     xff __ __ __ __ xff (or 0xfe)
 107  *
 108  * is in the event of an integer nested between two strings.
 109  *
 110  * For implementation simplicity/efficiency, strings may not be more
 111  * than 65 characters in length (ie: 17 32bit words after padding).
 112  *
 113  * In the event that we are doing <choices> (ie: not an enum type) then
 114  * the value of each choice is set to zero and ignored.
 115  */
 116
 117 #define STRINFO_MAX_WORDS   17
 118 G_GNUC_UNUSED static guint
 119 strinfo_string_to_words (const gchar *string,
 120                          guint32     *words,
 121                          gboolean     alias)
 122 {
 123   guint n_words;
 124   gsize size;
 125
 126   size = strlen (string);
 127
 128   n_words = MAX (2, (size + 6) >> 2);
 129
 130   if (n_words > STRINFO_MAX_WORDS)
 131     return FALSE;
 132
 133   words[0] = GUINT32_TO_LE (alias ? 0xfe : 0xff);
 134   words[n_words - 1] = GUINT32_TO_BE (0xff);
 135   memcpy (((gchar *) words) + 1, string, size + 1);
 136
 137   return n_words;
 138 }
 139
 140 G_GNUC_UNUSED static gint
 141 strinfo_scan (const guint32 *strinfo,
 142               guint          length,
 143               const guint32 *words,
 144               guint          n_words)
 145 {
 146   guint i = 0;
 147
 148   if (length < n_words)
 149     return -1;
 150
 151   while (i <= length - n_words)
 152     {
 153       guint j = 0;
 154
 155       for (j = 0; j < n_words; j++)
 156         if (strinfo[i + j] != words[j])
 157           break;
 158
 159       if (j == n_words)
 160         return i;   /* match */
 161
 162       /* skip at least one word, continue */
 163       i += j ? j : 1;
 164     }
 165
 166   return -1;
 167 }
 168
 169 G_GNUC_UNUSED static gint
 170 strinfo_find_string (const guint32 *strinfo,
 171                      guint          length,
 172                      const gchar   *string,
 173                      gboolean       alias)
 174 {
 175   guint32 words[STRINFO_MAX_WORDS];
 176   guint n_words;
 177
 178   if (length == 0)
 179     return -1;
 180
 181   n_words = strinfo_string_to_words (string, words, alias);
 182
 183   return strinfo_scan (strinfo + 1, length - 1, words, n_words);
 184 }
 185
 186 G_GNUC_UNUSED static gint
 187 strinfo_find_integer (const guint32 *strinfo,
 188                       guint          length,
 189                       guint32        value)
 190 {
 191   guint i;
 192
 193   for (i = 0; i < length; i++)
 194     if (strinfo[i] == GUINT32_TO_LE (value))
 195       {
 196         const guchar *charinfo = (const guchar *) &strinfo[i];
 197
 198         /* make sure it has 0xff on either side */
 199         if ((i == 0 || charinfo[-1] == 0xff) && charinfo[4] == 0xff)
 200           return i;
 201       }
 202
 203   return -1;
 204 }
 205
 206 G_GNUC_UNUSED static gboolean
 207 strinfo_is_string_valid (const guint32 *strinfo,
 208                          guint          length,
 209                          const gchar   *string)
 210 {
 211   return strinfo_find_string (strinfo, length, string, FALSE) != -1;
 212 }
 213
 214 G_GNUC_UNUSED static gboolean
 215 strinfo_enum_from_string (const guint32 *strinfo,
 216                           guint          length,
 217                           const gchar   *string,
 218                           guint         *result)
 219 {
 220   gint index;
 221
 222   index = strinfo_find_string (strinfo, length, string, FALSE);
 223
 224   if (index < 0)
 225     return FALSE;
 226
 227   *result = GUINT32_FROM_LE (strinfo[index]);
 228   return TRUE;
 229 }
 230
 231 G_GNUC_UNUSED static const gchar *
 232 strinfo_string_from_enum (const guint32 *strinfo,
 233                           guint          length,
 234                           guint          value)
 235 {
 236   gint index;
 237
 238   index = strinfo_find_integer (strinfo, length, value);
 239
 240   if (index < 0)
 241     return NULL;
 242
 243   return 1 + (const gchar *) &strinfo[index + 1];
 244 }
 245
 246 G_GNUC_UNUSED static const gchar *
 247 strinfo_string_from_alias (const guint32 *strinfo,
 248                            guint          length,
 249                            const gchar   *alias)
 250 {
 251   gint index;
 252
 253   index = strinfo_find_string (strinfo, length, alias, TRUE);
 254
 255   if (index < 0)
 256     return NULL;
 257
 258   return 1 + (const gchar *) &strinfo[GUINT32_TO_LE (strinfo[index]) + 1];
 259 }
 260
 261 G_GNUC_UNUSED static GVariant *
 262 strinfo_enumerate (const guint32 *strinfo,
 263                    guint          length)
 264 {
 265   GVariantBuilder builder;
 266   const gchar *ptr, *end;
 267
 268   ptr = (gpointer) strinfo;
 269   end = ptr + 4 * length;
 270
 271   ptr += 4;
 272
 273   g_variant_builder_init (&builder, G_VARIANT_TYPE_STRING_ARRAY);
 274
 275   while (ptr < end)
 276     {
 277       /* don't include aliases */
 278       if (*ptr == '\xff')
 279         g_variant_builder_add (&builder, "s", ptr + 1);
 280
 281       /* find the end of this string */
 282       ptr = memchr (ptr, '\xff', end - ptr);
 283       g_assert (ptr != NULL);
 284
 285       /* skip over the int to the next string */
 286       ptr += 5;
 287     }
 288
 289   return g_variant_builder_end (&builder);
 290 }
 291
 292 G_GNUC_UNUSED static void
 293 strinfo_builder_append_item (GString     *builder,
 294                              const gchar *string,
 295                              guint        value)
 296 {
 297   guint32 words[STRINFO_MAX_WORDS];
 298   guint n_words;
 299
 300   value = GUINT32_TO_LE (value);
 301
 302   n_words = strinfo_string_to_words (string, words, FALSE);
 303   g_string_append_len (builder, (void *) &value, sizeof value);
 304   g_string_append_len (builder, (void *) words, 4 * n_words);
 305 }
 306
 307 G_GNUC_UNUSED static gboolean
 308 strinfo_builder_append_alias (GString     *builder,
 309                               const gchar *alias,
 310                               const gchar *target)
 311 {
 312   guint32 words[STRINFO_MAX_WORDS];
 313   guint n_words;
 314   guint value;
 315   gint index;
 316
 317   index = strinfo_find_string ((const guint32 *) builder->str,
 318                                builder->len / 4, target, FALSE);
 319
 320   if (index == -1)
 321     return FALSE;
 322
 323   value = GUINT32_TO_LE (index);
 324
 325   n_words = strinfo_string_to_words (alias, words, TRUE);
 326   g_string_append_len (builder, (void *) &value, sizeof value);
 327   g_string_append_len (builder, (void *) words, 4 * n_words);
 328
 329   return TRUE;
 330 }
 331
 332 G_GNUC_UNUSED static gboolean
 333 strinfo_builder_contains (GString     *builder,
 334                           const gchar *string)
 335 {
 336   return strinfo_find_string ((const guint32 *) builder->str,
 337                               builder->len / 4, string, FALSE) != -1 ||
 338          strinfo_find_string ((const guint32 *) builder->str,
 339                               builder->len / 4, string, TRUE) != -1;
 340 }
 341
 342 G_GNUC_UNUSED static gboolean
 343 strinfo_builder_contains_value (GString *builder,
 344                                 guint    value)
 345 {
 346   return strinfo_string_from_enum ((const guint32 *) builder->str,
 347                                    builder->len / 4, value) != NULL;
 348 }