prefix_string.c revision 299742
1/* prefix_string.c --- implement strings based on a prefix tree 2 * 3 * ==================================================================== 4 * Licensed to the Apache Software Foundation (ASF) under one 5 * or more contributor license agreements. See the NOTICE file 6 * distributed with this work for additional information 7 * regarding copyright ownership. The ASF licenses this file 8 * to you under the Apache License, Version 2.0 (the 9 * "License"); you may not use this file except in compliance 10 * with the License. You may obtain a copy of the License at 11 * 12 * http://www.apache.org/licenses/LICENSE-2.0 13 * 14 * Unless required by applicable law or agreed to in writing, 15 * software distributed under the License is distributed on an 16 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY 17 * KIND, either express or implied. See the License for the 18 * specific language governing permissions and limitations 19 * under the License. 20 * ==================================================================== 21 */ 22 23#include <assert.h> 24#include "private/svn_string_private.h" 25 26/* A node in the tree represents a common prefix. The root node is the 27 * empty prefix. Nodes may have up to 256 sub-nodes, each starting with 28 * a different character (possibly '\0'). 29 * 30 * The nodes in the tree store only up to 8 chars of the respective common 31 * prefix, i.e. longer common prefixes must be drawn out over multiple 32 * hierarchy levels. This is a space <-> efficiency trade-off. 33 * 34 * Strings are the leaf nodes in the tree and use a specialized, smaller 35 * data structure. They may add 0 to 7 extra chars to the prefix. Both 36 * data types can be discerned by the last char in the data buffer. This 37 * must be 0 for strings (leaves) and non-0 otherwise. Please note that 38 * ordinary nodes have a length information so that no terminating 0 is 39 * required for them. 40 */ 41 42/* forward declaration */ 43typedef struct node_t node_t; 44 45/* String type and tree leaf. 46 */ 47struct svn_prefix_string__t 48{ 49 /* mandatory prefix */ 50 node_t *prefix; 51 52 /* 0 ..7 chars to add the the prefix. NUL-terminated. */ 53 char data[8]; 54}; 55 56/* A node inside the tree, i.e. not a string and not a leaf (unless this is 57 * the root node). 58 * 59 * Note: keep the ordering to minimize size / alignment overhead on 64 bit 60 * machines. 61 */ 62struct node_t 63{ 64 /* pointer to the parent prefix plus the 1 .. 8 extra chars. 65 * Only the root will provide 0 extra chars. */ 66 svn_prefix_string__t key; 67 68 /* Length of the prefix from the root down to and including this one. 69 * 0 for the root node. Only then will key.prefix be NULL. */ 70 apr_uint32_t length; 71 72 /* Number of entries used in SUB_NODES. */ 73 apr_uint32_t sub_node_count; 74 75 /* The sub-nodes, ordered by first char. node_t and svn_prefix_string__t 76 * may be mixed here. May be NULL. 77 * The number of allocated entries is always a power-of-two and only 78 * given implicitly by SUB_NODE_COUNT. */ 79 struct node_t **sub_nodes; 80}; 81 82/* The actual tree structure. */ 83struct svn_prefix_tree__t 84{ 85 /* the common tree root (represents the empty prefix). */ 86 node_t *root; 87 88 /* all sub-nodes & strings will be allocated from this pool */ 89 apr_pool_t *pool; 90}; 91 92/* Return TRUE, iff NODE is a leaf node. 93 */ 94static svn_boolean_t 95is_leaf(node_t *node) 96{ 97 return node->key.data[7] == 0; 98} 99 100/* Ensure that the sub-nodes array of NODE within TREE has at least one 101 * unused entry. Re-allocate as necessary. 102 */ 103static void 104auto_realloc_sub_nodes(svn_prefix_tree__t *tree, 105 node_t *node) 106{ 107 if (node->sub_node_count & (node->sub_node_count - 1)) 108 return; 109 110 if (node->sub_node_count == 0) 111 { 112 node->sub_nodes = apr_pcalloc(tree->pool, sizeof(*node->sub_nodes)); 113 } 114 else 115 { 116 node_t **sub_nodes 117 = apr_pcalloc(tree->pool, 118 2 * node->sub_node_count * sizeof(*sub_nodes)); 119 memcpy(sub_nodes, node->sub_nodes, 120 node->sub_node_count * sizeof(*sub_nodes)); 121 node->sub_nodes = sub_nodes; 122 } 123} 124 125/* Given the COUNT pointers in the SUB_NODES array, return the location at 126 * which KEY is either located or would be inserted. 127 */ 128static int 129search_lower_bound(node_t **sub_nodes, 130 unsigned char key, 131 int count) 132{ 133 int lower = 0; 134 int upper = count - 1; 135 136 /* Binary search for the lowest position at which to insert KEY. */ 137 while (lower <= upper) 138 { 139 int current = lower + (upper - lower) / 2; 140 141 if ((unsigned char)sub_nodes[current]->key.data[0] < key) 142 lower = current + 1; 143 else 144 upper = current - 1; 145 } 146 147 return lower; 148} 149 150svn_prefix_tree__t * 151svn_prefix_tree__create(apr_pool_t *pool) 152{ 153 svn_prefix_tree__t *tree = apr_pcalloc(pool, sizeof(*tree)); 154 tree->pool = pool; 155 156 tree->root = apr_pcalloc(pool, sizeof(*tree->root)); 157 tree->root->key.data[7] = '\xff'; 158 159 return tree; 160} 161 162svn_prefix_string__t * 163svn_prefix_string__create(svn_prefix_tree__t *tree, 164 const char *s) 165{ 166 svn_prefix_string__t *new_string; 167 apr_size_t len = strlen(s); 168 node_t *node = tree->root; 169 node_t *new_node; 170 int idx; 171 172 /* walk the existing tree until we either find S or the node at which S 173 * has to be inserted */ 174 while (TRUE) 175 { 176 node_t *sub_node; 177 int match = 1; 178 179 /* index of the matching sub-node */ 180 idx = node->sub_node_count 181 ? search_lower_bound(node->sub_nodes, 182 (unsigned char)s[node->length], 183 node->sub_node_count) 184 : 0; 185 186 /* any (partially) matching sub-nodes? */ 187 if (idx == (int)node->sub_node_count 188 || node->sub_nodes[idx]->key.data[0] != s[node->length]) 189 break; 190 191 sub_node = node->sub_nodes[idx]; 192 193 /* fully matching sub-node? */ 194 if (is_leaf(sub_node)) 195 { 196 if (strcmp(sub_node->key.data, s + node->length) == 0) 197 return &sub_node->key; 198 } 199 else 200 { 201 apr_size_t sub_node_len = sub_node->length - node->length; 202 if (strncmp(sub_node->key.data, s + node->length, 203 sub_node_len) == 0) 204 { 205 node = sub_node; 206 continue; 207 } 208 } 209 210 /* partial match -> split */ 211 while (sub_node->key.data[match] == s[node->length + match]) 212 ++match; 213 214 new_node = apr_pcalloc(tree->pool, sizeof(*new_node)); 215 new_node->key = sub_node->key; 216 new_node->length = node->length + match; 217 new_node->key.data[7] = '\xff'; 218 new_node->sub_node_count = 1; 219 new_node->sub_nodes = apr_palloc(tree->pool, sizeof(node_t *)); 220 new_node->sub_nodes[0] = sub_node; 221 222 memmove(sub_node->key.data, sub_node->key.data + match, 8 - match); 223 224 /* replace old sub-node with new one and continue lookup */ 225 sub_node->key.prefix = new_node; 226 node->sub_nodes[idx] = new_node; 227 node = new_node; 228 } 229 230 /* add sub-node(s) and final string */ 231 while (node->length + 7 < len) 232 { 233 new_node = apr_pcalloc(tree->pool, sizeof(*new_node)); 234 new_node->key.prefix = node; 235 new_node->length = node->length + 8; 236 memcpy(new_node->key.data, s + node->length, 8); 237 238 auto_realloc_sub_nodes(tree, node); 239 memmove(node->sub_nodes + idx + 1, node->sub_nodes + idx, 240 (node->sub_node_count - idx) * sizeof(node_t *)); 241 242 /* replace old sub-node with new one and continue lookup */ 243 node->sub_nodes[idx] = new_node; 244 node->sub_node_count++; 245 node = new_node; 246 idx = 0; 247 } 248 249 new_string = apr_pcalloc(tree->pool, sizeof(*new_string)); 250 new_string->prefix = node; 251 memcpy(new_string->data, s + node->length, len - node->length); 252 253 auto_realloc_sub_nodes(tree, node); 254 memmove(node->sub_nodes + idx + 1, node->sub_nodes + idx, 255 (node->sub_node_count - idx) * sizeof(node_t *)); 256 257 node->sub_nodes[idx] = (node_t *)new_string; 258 node->sub_node_count++; 259 return new_string; 260} 261 262svn_string_t * 263svn_prefix_string__expand(const svn_prefix_string__t *s, 264 apr_pool_t *pool) 265{ 266 apr_size_t s_len = strlen(s->data); 267 apr_size_t len = s->prefix->length + s_len; 268 char *buffer = apr_palloc(pool, len + 1); 269 270 svn_string_t *result = apr_pcalloc(pool, sizeof(*result)); 271 result->data = buffer; 272 result->len = len; 273 buffer[len] = '\0'; 274 275 while (s->prefix) 276 { 277 memcpy(buffer + s->prefix->length, s->data, len - s->prefix->length); 278 len = s->prefix->length; 279 s = &s->prefix->key; 280 } 281 282 return result; 283} 284 285int 286svn_prefix_string__compare(const svn_prefix_string__t *lhs, 287 const svn_prefix_string__t *rhs) 288{ 289 const node_t *lhs_parent = lhs->prefix; 290 const node_t *rhs_parent = rhs->prefix; 291 292 if (lhs == rhs) 293 return 0; 294 295 /* find the common root */ 296 while (lhs_parent != rhs_parent) 297 { 298 if (lhs_parent->length <= rhs_parent->length) 299 { 300 rhs = &rhs_parent->key; 301 rhs_parent = rhs_parent->key.prefix; 302 } 303 else if (rhs_parent->length <= lhs_parent->length) 304 { 305 lhs = &lhs_parent->key; 306 lhs_parent = lhs_parent->key.prefix; 307 } 308 309 /* same tree? */ 310 assert(lhs_parent && rhs_parent); 311 } 312 313 /* at the common root, strings will differ in the first follow-up char */ 314 return (int)(unsigned char)lhs->data[0] - (int)(unsigned char)rhs->data[0]; 315} 316