倒排索引

#include <stdio.h>
#include <stdlib.h>
 
char chr_legal[] = "abcdefghijklmnopqrstuvwxyz0123456789_-./";
int chr_idx[256] = {0};
char idx_chr[256] = {0};
 
#define FNAME 0
typedef struct trie_t *trie, trie_t;
struct trie_t {
  trie next[sizeof(chr_legal)]; /* next letter; slot 0 is for file name */
  int eow;
};
 
trie trie_new() { return calloc(sizeof(trie_t), 1); }
 
#define find_word(r, w) trie_trav(r, w, 1)
/* tree traversal: returns node if end of word and matches string, optionally
 * create node if doesn't exist
 */
trie trie_trav(trie root, char * str, int no_create)
{
  int c;
  while (root) {
  if ((c = str[0]) == '\0') {
  if (!root->eow && no_create) return 0;
  break;
  }
  if (! (c = chr_idx[c]) ) {
  str++;
  continue;
  }
  
  if (!root->next[c]) {
  if (no_create) return 0;
  root->next[c] = trie_new();
  }
  root = root->next[c];
  str++;
  }
  return root;
}
 
/* complete traversal of whole tree, calling callback at each end of word node.
 * similar method can be used to free nodes, had we wanted to do that.
 */
int trie_all(trie root, char path[], int depth, int (*callback)(char *))
{
  int i;
  if (root->eow && !callback(path)) return 0;
  
  for (i = 1; i < sizeof(chr_legal); i++) {
  if (!root->next[i]) continue;
  
  path[depth] = idx_chr[i];
  path[depth + 1] = '\0';
  if (!trie_all(root->next[i], path, depth + 1, callback))
  return 0;
  }
  return 1;
}
 
void add_index(trie root, char *word, char *fname)
{
  trie x = trie_trav(root, word, 0);
  x->eow = 1;
  
  if (!x->next[FNAME])
  x->next[FNAME] = trie_new();
  x = trie_trav(x->next[FNAME], fname, 0);
  x->eow = 1;
}
 
int print_path(char *path)
{
  printf(" %s", path);
  return 1;
}
 
/* pretend we parsed text files and got lower cased words: dealing *
 * with text file is a whole other animal and would make code too long */
char *files[] = { "f1.txt", "source/f2.txt", "other_file" };
char *text[][5] ={{ "it", "is", "what", "it", "is" },
  { "what", "is", "it", 0 },
  { "it", "is", "a", "banana", 0 }};
 
trie init_tables()
{
  int i, j;
  trie root = trie_new();
  for (i = 0; i < sizeof(chr_legal); i++) {
  chr_idx[(int)chr_legal[i]] = i + 1;
  idx_chr[i + 1] = chr_legal[i];
  }
 
/* Enable USE_ADVANCED_FILE_HANDLING to use advanced file handling.
 * You need to have files named like above files[], with words in them
 * like in text[][]. Case doesn't matter (told you it's advanced).
 */
#define USE_ADVANCED_FILE_HANDLING 0
#if USE_ADVANCED_FILE_HANDLING
  void read_file(char * fname) {
  char cmd[1024];
  char word[1024];
  sprintf(cmd, "perl -p -e 'while(/(\\w+)/g) {print lc($1),\"\\n\"}' %s", fname);
  FILE *in = popen(cmd, "r");
  while (!feof(in)) {
  fscanf(in, "%1000s", word);
  add_index(root, word, fname);
  }
  pclose(in);
  };
  
  read_file("f1.txt");
  read_file("source/f2.txt");
  read_file("other_file");
#else
  for (i = 0; i < 3; i++) {
  for (j = 0; j < 5; j++) {
  if (!text[i][j]) break;
  add_index(root, text[i][j], files[i]);
  }
  }
#endif /*USE_ADVANCED_FILE_HANDLING*/
  
  return root;
}
 
void search_index(trie root, char *word)
{
  char path[1024];
  printf("Search for \"%s\": ", word);
  trie found = find_word(root, word);
  
  if (!found) printf("not found\n");
  else {
  trie_all(found->next[FNAME], path, 0, print_path);
  printf("\n");
  }
}
 
int main()
{
  trie root = init_tables();
  
  search_index(root, "what");
  search_index(root, "is");
  search_index(root, "banana");
  search_index(root, "boo");
  return 0;

}