mod_speling.c

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#define WANT_BASENAME_MATCH
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 * on public domain software written at the National Center for
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#include "httpd.h"
#include "http_core.h"
#include "http_config.h"
#include "http_log.h"

/* mod_speling.c - by Alexei Kosut <[e-mail]> June, 1996
 *
 * This module is transparent, and simple. It attempts to correct
 * misspellings of URLs that users might have entered, namely by checking
 * capitalizations. If it finds a match, it sends a redirect.
 *
 * 08-Aug-1997 <[e-mail]>
 * o Upgraded module interface to apache_1.3a2-dev API (more NULL's in
 *   speling_module).
 * o Integrated tcsh's "spelling correction" routine which allows one
 *   misspelling (character insertion/omission/typo/transposition).
 *   Rewrote it to ignore case as well. This ought to catch the majority
 *   of misspelled requests.
 * o Commented out the second pass where files' suffixes are stripped.
 *   Given the better hit rate of the first pass, this rather ugly
 *   (request index.html, receive index.db ?!?!) solution can be
 *   omitted.
 * o wrote a "kind of" html page for mod_speling
 *
 * Activate it with "CheckSpelling On"
 */

MODULE_VAR_EXPORT module speling_module;

typedef struct {
    int enabled;
} spconfig;

/*
 * Create a configuration specific to this module for a server or directory
 * location, and fill it with the default settings.
 *
 * The API says that in the absence of a merge function, the record for the
 * closest ancestor is used exclusively.  That's what we want, so we don't
 * bother to have such a function.
 */

static void *mkconfig(pool *p)
{
    spconfig *cfg = ap_pcalloc(p, sizeof(spconfig));

    cfg->enabled = 0;
    return cfg;
}

/*
 * Respond to a callback to create configuration record for a server or
 * vhost environment.
 */
static void *create_mconfig_for_server(pool *p, server_rec *s)
{
    return mkconfig(p);
}

/*
 * Respond to a callback to create a config record for a specific directory.
 */
static void *create_mconfig_for_directory(pool *p, char *dir)
{
    return mkconfig(p);
}

/*
 * Handler for the CheckSpelling directive, which is FLAG.
 */
static const char *set_speling(cmd_parms *cmd, void *mconfig, int arg)
{
    spconfig *cfg = (spconfig *) mconfig;

    cfg->enabled = arg;
    return NULL;
}

/*
 * Define the directives specific to this module.  This structure is referenced
 * later by the 'module' structure.
 */
static const command_rec speling_cmds[] =
{
    { "CheckSpelling", set_speling, NULL, OR_OPTIONS, FLAG,
      "whether or not to fix miscapitalized/misspelled requests" },
    { NULL }
};

typedef enum {
    SP_IDENTICAL = 0,
    SP_MISCAPITALIZED = 1,
    SP_TRANSPOSITION = 2,
    SP_MISSINGCHAR = 3,
    SP_EXTRACHAR = 4,
    SP_SIMPLETYPO = 5,
    SP_VERYDIFFERENT = 6
} sp_reason;

static const char *sp_reason_str[] =
{
    "identical",
    "miscapitalized",
    "transposed characters",
    "character missing",
    "extra character",
    "mistyped character",
    "common basename",
};

typedef struct {
    const char *name;
    sp_reason quality;
} misspelled_file;

/*
 * spdist() is taken from Kernighan & Pike,
 *  _The_UNIX_Programming_Environment_
 * and adapted somewhat to correspond better to psychological reality.
 * (Note the changes to the return values)
 *
 * According to Pollock and Zamora, CACM April 1984 (V. 27, No. 4),
 * page 363, the correct order for this is:
 * OMISSION = TRANSPOSITION > INSERTION > SUBSTITUTION
 * thus, it was exactly backwards in the old version. -- PWP
 *
 * This routine was taken out of tcsh's spelling correction code
 * (tcsh-6.07.04) and re-converted to apache data types ("char" type
 * instead of tcsh's NLS'ed "Char"). Plus it now ignores the case
 * during comparisons, so is a "approximate strcasecmp()".
 * NOTE that is still allows only _one_ real "typo",
 * it does NOT try to correct multiple errors.
 */

static sp_reason spdist(const char *s, const char *t)
{
    for (; ap_tolower(*s) == ap_tolower(*t); t++, s++) {
        if (*t == '\0') {
            return SP_MISCAPITALIZED;   /* exact match (sans case) */
	}
    }
    if (*s) {
        if (*t) {
            if (s[1] && t[1] && ap_tolower(*s) == ap_tolower(t[1])
		&& ap_tolower(*t) == ap_tolower(s[1])
		&& strcasecmp(s + 2, t + 2) == 0) {
                return SP_TRANSPOSITION;        /* transposition */
	    }
            if (strcasecmp(s + 1, t + 1) == 0) {
                return SP_SIMPLETYPO;   /* 1 char mismatch */
	    }
        }
        if (strcasecmp(s + 1, t) == 0) {
            return SP_EXTRACHAR;        /* extra character */
	}
    }
    if (*t && strcasecmp(s, t + 1) == 0) {
        return SP_MISSINGCHAR;  /* missing character */
    }
    return SP_VERYDIFFERENT;    /* distance too large to fix. */
}

static int sort_by_quality(const void *left, const void *rite)
{
    return (int) (((misspelled_file *) left)->quality)
        - (int) (((misspelled_file *) rite)->quality);
}

static int check_speling(request_rec *r)
{
    spconfig *cfg;
    char *good, *bad, *postgood, *url;
    int filoc, dotloc, urlen, pglen;
    DIR *dirp;
    struct DIR_TYPE *dir_entry;
    array_header *candidates = NULL;

    cfg = ap_get_module_config(r->per_dir_config, &speling_module);
    if (!cfg->enabled) {
        return DECLINED;
    }

    /* We only want to worry about GETs */
    if (r->method_number != M_GET) {
        return DECLINED;
    }

    /* We've already got a file of some kind or another */
    if (r->proxyreq || (r->finfo.st_mode != 0)) {
        return DECLINED;
    }

    /* This is a sub request - don't mess with it */
    if (r->main) {
        return DECLINED;
    }

    /*
     * The request should end up looking like this:
     * r->uri: /correct-url/mispelling/more
     * r->filename: /correct-file/mispelling r->path_info: /more
     *
     * So we do this in steps. First break r->filename into two pieces
     */

    filoc = ap_rind(r->filename, '/');
    /*
     * Don't do anything if the request doesn't contain a slash, or
     * requests "/" 
     */
    if (filoc == -1 || strcmp(r->uri, "/") == 0) {
        return DECLINED;
    }