An Inverted Index is a data structure used to create full text search.



Task

Given a set of text files, implement a program to create an inverted index.

Also create a user interface to do a search using that inverted index which returns a list of files that contain the query term / terms.

The search index can be in memory.




#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, const 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, const char *word, const 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 */
const char *files[] = { "f1.txt", "source/f2.txt", "other_file" };
const 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(const 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, const 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;
}