ext/sdbm/_sdbm.c

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/*
 * sdbm - ndbm work-alike hashed database library
 * based on Per-Aake Larson's Dynamic Hashing algorithms. BIT 18 (1978).
 * author: oz@nexus.yorku.ca
 * status: public domain.
 *
 * core routines
 */

#include "ruby/config.h"
#include "ruby/defines.h"

#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif

#include "sdbm.h"

/*
 * sdbm - ndbm work-alike hashed database library
 * tuning and portability constructs [not nearly enough]
 * author: oz@nexus.yorku.ca
 */

#define BYTESIZ         8

#ifdef BSD42
#define SEEK_SET        L_SET
#define memset(s,c,n)   bzero((s), (n))         /* only when c is zero */
#define memcpy(s1,s2,n) bcopy((s2), (s1), (n))
#define memcmp(s1,s2,n) bcmp((s1),(s2),(n))
#endif

/*
 * important tuning parms (hah)
 */

#ifndef SEEDUPS
#define SEEDUPS 1       /* always detect duplicates */
#endif
#ifndef BADMESS
#define BADMESS 1       /* generate a message for worst case:
                           cannot make room after SPLTMAX splits */
#endif

/*
 * misc
 */
#ifdef DEBUG
#define debug(x)        printf x
#else
#define debug(x)
#endif

#ifdef BIG_E
#define GET_SHORT(p, i) (((unsigned)((unsigned char *)(p))[(i)*2] << 8) + (((unsigned char *)(p))[(i)*2 + 1]))
#define PUT_SHORT(p, i, s) (((unsigned char *)(p))[(i)*2] = (unsigned char)((s) >> 8), ((unsigned char *)(p))[(i)*2 + 1] = (unsigned char)(s))
#else
#define GET_SHORT(p, i) ((p)[(i)])
#define PUT_SHORT(p, i, s)      ((p)[(i)] = (s))
#endif

/*#include "pair.h"*/
static int   fitpair proto((char *, int));
static void  putpair proto((char *, datum, datum));
static datum getpair proto((char *, datum));
static int   delpair proto((char *, datum));
static int   chkpage proto((char *));
static datum getnkey proto((char *, int));
static void  splpage proto((char *, char *, long));
#if SEEDUPS
static int   duppair proto((char *, datum));
#endif

#include <stdio.h>
#include <stdlib.h>
#ifdef DOSISH
#include <io.h>
#endif
#include <sys/types.h>
#include <sys/stat.h>
#ifdef BSD42
#include <sys/file.h>
#else
#include <fcntl.h>
/*#include <memory.h>*/
#endif
#ifndef O_BINARY
#define O_BINARY        0
#endif

#include <errno.h>
#ifndef EPERM
#define EPERM   EACCES
#endif
#include <string.h>

#ifdef __STDC__
#include <stddef.h>
#endif

#ifndef NULL
#define NULL    0
#endif

/*
 * externals
 */
#if !defined(__sun) && !defined(_WIN32) && !defined(__CYGWIN__) && !defined(errno)
extern int errno;
#endif

/*
 * forward
 */
static int getdbit proto((DBM *, long));
static int setdbit proto((DBM *, long));
static int getpage proto((DBM *, long));
static datum getnext proto((DBM *));
static int makroom proto((DBM *, long, int));

/*
 * useful macros
 */
#define bad(x)          ((x).dptr == NULL || (x).dsize < 0)
#define exhash(item)    sdbm_hash((item).dptr, (item).dsize)
#define ioerr(db)       ((db)->flags |= DBM_IOERR)

#define OFF_PAG(off)    (long) (off) * PBLKSIZ
#define OFF_DIR(off)    (long) (off) * DBLKSIZ

static long masks[] = {
        000000000000L, 000000000001L, 000000000003L,
        000000000007L, 000000000017L, 000000000037L,
        000000000077L, 000000000177L, 000000000377L,
        000000000777L, 000000001777L, 000000003777L,
        000000007777L, 000000017777L, 000000037777L,
        000000077777L, 000000177777L, 000000377777L,
        000000777777L, 000001777777L, 000003777777L,
        000007777777L, 000017777777L, 000037777777L,
        000077777777L, 000177777777L, 000377777777L,
        000777777777L, 001777777777L, 003777777777L,
        007777777777L, 017777777777L
};

datum nullitem = {NULL, 0};

DBM *
sdbm_open(register char *file, register int flags, register int mode)
{
        register DBM *db;
        register char *dirname;
        register char *pagname;
        register size_t n;

        if (file == NULL || !*file)
                return errno = EINVAL, (DBM *) NULL;
/*
 * need space for two separate filenames
 */
        n = strlen(file) * 2 + strlen(DIRFEXT) + strlen(PAGFEXT) + 2;

        if ((dirname = malloc(n)) == NULL)
                return errno = ENOMEM, (DBM *) NULL;
/*
 * build the file names
 */
        dirname = strcat(strcpy(dirname, file), DIRFEXT);
        pagname = strcpy(dirname + strlen(dirname) + 1, file);
        pagname = strcat(pagname, PAGFEXT);

        db = sdbm_prep(dirname, pagname, flags, mode);
        free((char *) dirname);
        return db;
}

static int
fd_set_cloexec(int fd)
{
  /* MinGW don't have F_GETFD and FD_CLOEXEC.  [ruby-core:40281] */
#ifdef F_GETFD
    int flags, ret;
    flags = fcntl(fd, F_GETFD); /* should not fail except EBADF. */
    if (flags == -1) {
        return -1;
    }
    if (2 < fd) {
        if (!(flags & FD_CLOEXEC)) {
            flags |= FD_CLOEXEC;
            ret = fcntl(fd, F_SETFD, flags);
            if (ret == -1) {
                return -1;
            }
        }
    }
#endif
    return 0;
}

DBM *
sdbm_prep(char *dirname, char *pagname, int flags, int mode)
{
        register DBM *db;
        struct stat dstat;

        if ((db = (DBM *) malloc(sizeof(DBM))) == NULL)
                return errno = ENOMEM, (DBM *) NULL;

        db->pagf = -1;
        db->dirf = -1;
        db->flags = 0;
        db->hmask = 0;
        db->blkptr = 0;
        db->keyptr = 0;
/*
 * adjust user flags so that WRONLY becomes RDWR,
 * as required by this package. Also set our internal
 * flag for RDONLY.
 */
        if (flags & O_WRONLY)
                flags = (flags & ~O_WRONLY) | O_RDWR;
        if (flags & O_RDONLY)
                db->flags = DBM_RDONLY;
/*
 * open the files in sequence, and stat the dirfile.
 * If we fail anywhere, undo everything, return NULL.
 */
        flags |= O_BINARY;
#ifdef O_CLOEXEC
        flags |= O_CLOEXEC;
#endif

        if ((db->pagf = open(pagname, flags, mode)) == -1) goto err;
        if (fd_set_cloexec(db->pagf) == -1) goto err;
        if ((db->dirf = open(dirname, flags, mode)) == -1) goto err;
        if (fd_set_cloexec(db->dirf) == -1) goto err;
/*
 * need the dirfile size to establish max bit number.
 */
        if (fstat(db->dirf, &dstat) == -1) goto err;
/*
 * zero size: either a fresh database, or one with a single,
 * unsplit data page: dirpage is all zeros.
 */
        db->dirbno = (!dstat.st_size) ? 0 : -1;
        db->pagbno = -1;
        db->maxbno = dstat.st_size * (long) BYTESIZ;

        (void) memset(db->pagbuf, 0, PBLKSIZ);
        (void) memset(db->dirbuf, 0, DBLKSIZ);
/*
 * success
 */
        return db;

    err:
        if (db->pagf != -1)
                (void) close(db->pagf);
        if (db->dirf != -1)
                (void) close(db->dirf);
        free((char *) db);
        return (DBM *) NULL;
}

void
sdbm_close(register DBM *db)
{
        if (db == NULL)
                errno = EINVAL;
        else {
                (void) close(db->dirf);
                (void) close(db->pagf);
                free((char *) db);
        }
}

datum
sdbm_fetch(register DBM *db, datum key)
{
        if (db == NULL || bad(key))
                return errno = EINVAL, nullitem;

        if (getpage(db, exhash(key)))
                return getpair(db->pagbuf, key);

        return ioerr(db), nullitem;
}

int
sdbm_delete(register DBM *db, datum key)
{
        if (db == NULL || bad(key))
                return errno = EINVAL, -1;
        if (sdbm_rdonly(db))
                return errno = EPERM, -1;

        if (getpage(db, exhash(key))) {
                if (!delpair(db->pagbuf, key))
                        return -1;
/*
 * update the page file
 */
                if (lseek(db->pagf, OFF_PAG(db->pagbno), SEEK_SET) < 0
                    || write(db->pagf, db->pagbuf, PBLKSIZ) < 0)
                        return ioerr(db), -1;

                return 0;
        }

        return ioerr(db), -1;
}

int
sdbm_store(register DBM *db, datum key, datum val, int flags)
{
        int need;
        register long hash;

        if (db == NULL || bad(key))
                return errno = EINVAL, -1;
        if (sdbm_rdonly(db))
                return errno = EPERM, -1;

        need = key.dsize + val.dsize;
/*
 * is the pair too big (or too small) for this database ??
 */
        if (need < 0 || need > PAIRMAX)
                return errno = EINVAL, -1;

        if (getpage(db, (hash = exhash(key)))) {
/*
 * if we need to replace, delete the key/data pair
 * first. If it is not there, ignore.
 */
                if (flags == DBM_REPLACE)
                        (void) delpair(db->pagbuf, key);
#if SEEDUPS
                else if (duppair(db->pagbuf, key))
                        return 1;
#endif
/*
 * if we do not have enough room, we have to split.
 */
                if (!fitpair(db->pagbuf, need))
                        if (!makroom(db, hash, need))
                                return ioerr(db), -1;
/*
 * we have enough room or split is successful. insert the key,
 * and update the page file.
 */
                (void) putpair(db->pagbuf, key, val);

                if (lseek(db->pagf, OFF_PAG(db->pagbno), SEEK_SET) < 0
                    || write(db->pagf, db->pagbuf, PBLKSIZ) < 0)
                        return ioerr(db), -1;
        /*
         * success
         */
                return 0;
        }

        return ioerr(db), -1;
}

/*
 * makroom - make room by splitting the overfull page
 * this routine will attempt to make room for SPLTMAX times before
 * giving up.
 */
static int
makroom(register DBM *db, long int hash, int need)
{
        long newp;
        char twin[PBLKSIZ];
#if defined _WIN32 && !defined __CYGWIN__
        char zer[PBLKSIZ];
        long oldtail;
#endif
        char *pag = db->pagbuf;
        char *new = twin;
        register int smax = SPLTMAX;

        do {
/*
 * split the current page
 */
                (void) splpage(pag, new, db->hmask + 1);
/*
 * address of the new page
 */
                newp = (hash & db->hmask) | (db->hmask + 1);
                debug(("newp: %ld\n", newp));
/*
 * write delay, read avoidance/cache shuffle:
 * select the page for incoming pair: if key is to go to the new page,
 * write out the previous one, and copy the new one over, thus making
 * it the current page. If not, simply write the new page, and we are
 * still looking at the page of interest. current page is not updated
 * here, as sdbm_store will do so, after it inserts the incoming pair.
 */

#if defined _WIN32 && !defined __CYGWIN__
        /*
         * Fill hole with 0 if made it.
         * (hole is NOT read as 0)
         */
        oldtail = lseek(db->pagf, 0L, SEEK_END);
        memset(zer, 0, PBLKSIZ);
        while (OFF_PAG(newp) > oldtail) {
                if (lseek(db->pagf, 0L, SEEK_END) < 0 ||
                    write(db->pagf, zer, PBLKSIZ) < 0) {

                        return 0;
                }
                oldtail += PBLKSIZ;
        }
#endif

                if (hash & (db->hmask + 1)) {
                        if (lseek(db->pagf, OFF_PAG(db->pagbno), SEEK_SET) < 0
                            || write(db->pagf, db->pagbuf, PBLKSIZ) < 0)
                                return 0;
                        db->pagbno = newp;
                        (void) memcpy(pag, new, PBLKSIZ);
                }
                else if (lseek(db->pagf, OFF_PAG(newp), SEEK_SET) < 0
                         || write(db->pagf, new, PBLKSIZ) < 0)
                        return 0;

                if (!setdbit(db, db->curbit))
                        return 0;
/*
 * see if we have enough room now
 */
                if (fitpair(pag, need))
                        return 1;
/*
 * try again... update curbit and hmask as getpage would have
 * done. because of our update of the current page, we do not
 * need to read in anything. BUT we have to write the current
 * [deferred] page out, as the window of failure is too great.
 */
                db->curbit = 2 * db->curbit +
                        ((hash & (db->hmask + 1)) ? 2 : 1);
                db->hmask |= (db->hmask + 1);

                if (lseek(db->pagf, OFF_PAG(db->pagbno), SEEK_SET) < 0
                    || write(db->pagf, db->pagbuf, PBLKSIZ) < 0)
                        return 0;

        } while (--smax);
/*
 * if we are here, this is real bad news. After SPLTMAX splits,
 * we still cannot fit the key. say goodnight.
 */
#if BADMESS
        (void) (write(2, "sdbm: cannot insert after SPLTMAX attempts.\n", 44) < 0);
#endif
        return 0;

}

/*
 * the following two routines will break if
 * deletions aren't taken into account. (ndbm bug)
 */
datum
sdbm_firstkey(register DBM *db)
{
        if (db == NULL)
                return errno = EINVAL, nullitem;
/*
 * start at page 0
 */
        (void) memset(db->pagbuf, 0, PBLKSIZ);
        if (lseek(db->pagf, OFF_PAG(0), SEEK_SET) < 0
            || read(db->pagf, db->pagbuf, PBLKSIZ) < 0)
                return ioerr(db), nullitem;
        db->pagbno = 0;
        db->blkptr = 0;
        db->keyptr = 0;

        return getnext(db);
}

datum
sdbm_nextkey(register DBM *db)
{
        if (db == NULL)
                return errno = EINVAL, nullitem;
        return getnext(db);
}

/*
 * all important binary trie traversal
 */
static int
getpage(register DBM *db, register long int hash)
{
        register int hbit;
        register long dbit;
        register long pagb;

        dbit = 0;
        hbit = 0;
        while (dbit < db->maxbno && getdbit(db, dbit))
                dbit = 2 * dbit + ((hash & ((long) 1 << hbit++)) ? 2 : 1);

        debug(("dbit: %d...", dbit));

        db->curbit = dbit;
        db->hmask = masks[hbit];

        pagb = hash & db->hmask;
/*
 * see if the block we need is already in memory.
 * note: this lookaside cache has about 10% hit rate.
 */
        if (pagb != db->pagbno) {
/*
 * note: here, we assume a "hole" is read as 0s.
 * if not, must zero pagbuf first.
 */
                (void) memset(db->pagbuf, 0, PBLKSIZ);

                if (lseek(db->pagf, OFF_PAG(pagb), SEEK_SET) < 0
                    || read(db->pagf, db->pagbuf, PBLKSIZ) < 0)
                        return 0;
                if (!chkpage(db->pagbuf)) {
                        return 0;
                }
                db->pagbno = pagb;

                debug(("pag read: %d\n", pagb));
        }
        return 1;
}

static int
getdbit(register DBM *db, register long int dbit)
{
        register long c;
        register long dirb;

        c = dbit / BYTESIZ;
        dirb = c / DBLKSIZ;

        if (dirb != db->dirbno) {
                if (lseek(db->dirf, OFF_DIR(dirb), SEEK_SET) < 0
                    || read(db->dirf, db->dirbuf, DBLKSIZ) < 0)
                        return 0;
                db->dirbno = dirb;

                debug(("dir read: %d\n", dirb));
        }

        return db->dirbuf[c % DBLKSIZ] & (1 << (dbit % BYTESIZ));
}

static int
setdbit(register DBM *db, register long int dbit)
{
        register long c;
        register long dirb;

        c = dbit / BYTESIZ;
        dirb = c / DBLKSIZ;

        if (dirb != db->dirbno) {
                if (lseek(db->dirf, OFF_DIR(dirb), SEEK_SET) < 0
                    || read(db->dirf, db->dirbuf, DBLKSIZ) < 0)
                        return 0;
                db->dirbno = dirb;

                debug(("dir read: %d\n", dirb));
        }

        db->dirbuf[c % DBLKSIZ] |= (1 << (dbit % BYTESIZ));

        if (dbit >= db->maxbno)
                db->maxbno += (long) DBLKSIZ * BYTESIZ;

        if (lseek(db->dirf, OFF_DIR(dirb), SEEK_SET) < 0
            || write(db->dirf, db->dirbuf, DBLKSIZ) < 0)
                return 0;

        return 1;
}

/*
 * getnext - get the next key in the page, and if done with
 * the page, try the next page in sequence
 */
static datum
getnext(register DBM *db)
{
        datum key;

        for (;;) {
                db->keyptr++;
                key = getnkey(db->pagbuf, db->keyptr);
                if (key.dptr != NULL)
                        return key;
/*
 * we either run out, or there is nothing on this page..
 * try the next one... If we lost our position on the
 * file, we will have to seek.
 */
                db->keyptr = 0;
                if (db->pagbno != db->blkptr++)
                        if (lseek(db->pagf, OFF_PAG(db->blkptr), SEEK_SET) < 0)
                                break;
                db->pagbno = db->blkptr;
                if (read(db->pagf, db->pagbuf, PBLKSIZ) <= 0)
                        break;
                if (!chkpage(db->pagbuf)) {
                        break;
                }
        }

        return ioerr(db), nullitem;
}

/* pair.c */
/*
 * sdbm - ndbm work-alike hashed database library
 * based on Per-Aake Larson's Dynamic Hashing algorithms. BIT 18 (1978).
 * author: oz@nexus.yorku.ca
 * status: public domain.
 *
 * page-level routines
 */

#ifndef BSD42
/*#include <memory.h>*/
#endif

#define exhash(item)    sdbm_hash((item).dptr, (item).dsize)

/*
 * forward
 */
static int seepair proto((char *, int, char *, int));

/*
 * page format:
 *      +------------------------------+
 * ino  | n | keyoff | datoff | keyoff |
 *      +------------+--------+--------+
 *      | datoff | - - - ---->         |
 *      +--------+---------------------+
 *      |        F R E E A R E A       |
 *      +--------------+---------------+
 *      |  <---- - - - | data          |
 *      +--------+-----+----+----------+
 *      |  key   | data     | key      |
 *      +--------+----------+----------+
 *
 * calculating the offsets for free area:  if the number
 * of entries (ino[0]) is zero, the offset to the END of
 * the free area is the block size. Otherwise, it is the
 * nth (ino[ino[0]]) entry's offset.
 */

static int
fitpair(char *pag, int need)
{
        register int n;
        register int off;
        register int free;
        register short *ino = (short *) pag;

        off = ((n = GET_SHORT(ino,0)) > 0) ? GET_SHORT(ino,n) : PBLKSIZ;
        free = off - (n + 1) * (int)sizeof(short);
        need += 2 * (int)sizeof(short);

        debug(("free %d need %d\n", free, need));

        return need <= free;
}

static void
putpair(char *pag, datum key, datum val)
{
        register int n;
        register int off;
        register short *ino = (short *) pag;

        off = ((n = GET_SHORT(ino,0)) > 0) ? GET_SHORT(ino,n) : PBLKSIZ;
/*
 * enter the key first
 */
        off -= key.dsize;
        if (key.dsize)
                (void) memcpy(pag + off, key.dptr, key.dsize);
        PUT_SHORT(ino,n + 1,off);
/*
 * now the data
 */
        off -= val.dsize;
        if (val.dsize)
                (void) memcpy(pag + off, val.dptr, val.dsize);
        PUT_SHORT(ino,n + 2,off);
/*
 * adjust item count
 */
        PUT_SHORT(ino,0,GET_SHORT(ino,0) + 2);
}

static datum
getpair(char *pag, datum key)
{
        register int i;
        register int n;
        datum val;
        register short *ino = (short *) pag;

        if ((n = GET_SHORT(ino,0)) == 0)
                return nullitem;

        if ((i = seepair(pag, n, key.dptr, key.dsize)) == 0)
                return nullitem;

        val.dptr = pag + GET_SHORT(ino,i + 1);
        val.dsize = GET_SHORT(ino,i) - GET_SHORT(ino,i + 1);
        return val;
}

#if SEEDUPS
static int
duppair(char *pag, datum key)
{
        register short *ino = (short *) pag;
        return GET_SHORT(ino,0) > 0 &&
                   seepair(pag, GET_SHORT(ino,0), key.dptr, key.dsize) > 0;
}
#endif

static datum
getnkey(char *pag, int num)
{
        datum key;
        register int off;
        register short *ino = (short *) pag;

        num = num * 2 - 1;
        if (GET_SHORT(ino,0) == 0 || num > GET_SHORT(ino,0))
                return nullitem;

        off = (num > 1) ? GET_SHORT(ino,num - 1) : PBLKSIZ;

        key.dptr = pag + GET_SHORT(ino,num);
        key.dsize = off - GET_SHORT(ino,num);

        return key;
}

static int
delpair(char *pag, datum key)
{
        register int n;
        register int i;
        register short *ino = (short *) pag;

        if ((n = GET_SHORT(ino,0)) == 0)
                return 0;

        if ((i = seepair(pag, n, key.dptr, key.dsize)) == 0)
                return 0;
/*
 * found the key. if it is the last entry
 * [i.e. i == n - 1] we just adjust the entry count.
 * hard case: move all data down onto the deleted pair,
 * shift offsets onto deleted offsets, and adjust them.
 * [note: 0 < i < n]
 */
        if (i < n - 1) {
                register int m;
                register char *dst = pag + (i == 1 ? PBLKSIZ : GET_SHORT(ino,i - 1));
                register char *src = pag + GET_SHORT(ino,i + 1);
                register ptrdiff_t   zoo = dst - src;

                debug(("free-up %"PRIdPTRDIFF" ", zoo));
/*
 * shift data/keys down
 */
                m = GET_SHORT(ino,i + 1) - GET_SHORT(ino,n);
#ifdef DUFF
#define MOVB    *--dst = *--src

                if (m > 0) {
                        register int loop = (m + 8 - 1) >> 3;

                        switch (m & (8 - 1)) {
                        case 0: do {
                                MOVB;   case 7: MOVB;
                        case 6: MOVB;   case 5: MOVB;
                        case 4: MOVB;   case 3: MOVB;
                        case 2: MOVB;   case 1: MOVB;
                                } while (--loop);
                        }
                }
#else
#ifdef MEMMOVE
                memmove(dst, src, m);
#else
                while (m--)
                        *--dst = *--src;
#endif
#endif
/*
 * adjust offset index up
 */
                while (i < n - 1) {
                        PUT_SHORT(ino,i, GET_SHORT(ino,i + 2) + zoo);
                        i++;
                }
        }
        PUT_SHORT(ino, 0, GET_SHORT(ino, 0) - 2);
        return 1;
}

/*
 * search for the key in the page.
 * return offset index in the range 0 < i < n.
 * return 0 if not found.
 */
static int
seepair(char *pag, register int n, register char *key, register int siz)
{
        register int i;
        register int off = PBLKSIZ;
        register short *ino = (short *) pag;

        for (i = 1; i < n; i += 2) {
                if (siz == off - GET_SHORT(ino,i) &&
                    memcmp(key, pag + GET_SHORT(ino,i), siz) == 0)
                        return i;
                off = GET_SHORT(ino,i + 1);
        }
        return 0;
}

static void
splpage(char *pag, char *new, long int sbit)
{
        datum key;
        datum val;

        register int n;
        register int off = PBLKSIZ;
        char cur[PBLKSIZ];
        register short *ino = (short *) cur;

        (void) memcpy(cur, pag, PBLKSIZ);
        (void) memset(pag, 0, PBLKSIZ);
        (void) memset(new, 0, PBLKSIZ);

        n = GET_SHORT(ino,0);
        for (ino++; n > 0; ino += 2) {
                key.dptr = cur + GET_SHORT(ino,0);
                key.dsize = off - GET_SHORT(ino,0);
                val.dptr = cur + GET_SHORT(ino,1);
                val.dsize = GET_SHORT(ino,0) - GET_SHORT(ino,1);
/*
 * select the page pointer (by looking at sbit) and insert
 */
                (void) putpair((exhash(key) & sbit) ? new : pag, key, val);

                off = GET_SHORT(ino,1);
                n -= 2;
        }

        debug(("%d split %d/%d\n", ((short *) cur)[0] / 2,
               ((short *) new)[0] / 2,
               ((short *) pag)[0] / 2));
}

/*
 * check page sanity:
 * number of entries should be something
 * reasonable, and all offsets in the index should be in order.
 * this could be made more rigorous.
 */
static int
chkpage(char *pag)
{
        register int n;
        register int off;
        register short *ino = (short *) pag;

        if ((n = GET_SHORT(ino,0)) < 0 || n > PBLKSIZ / (int)sizeof(short))
                return 0;

        if (n > 0) {
                off = PBLKSIZ;
                for (ino++; n > 0; ino += 2) {
                        if (GET_SHORT(ino,0) > off || GET_SHORT(ino,1) > off ||
                            GET_SHORT(ino,1) > GET_SHORT(ino,0))
                                return 0;
                        off = GET_SHORT(ino,1);
                        n -= 2;
                }
        }
        return 1;
}

/* hash.c */
/*
 * sdbm - ndbm work-alike hashed database library
 * based on Per-Aake Larson's Dynamic Hashing algorithms. BIT 18 (1978).
 * author: oz@nexus.yorku.ca
 * status: public domain. keep it that way.
 *
 * hashing routine
 */

/*
 * polynomial conversion ignoring overflows
 * [this seems to work remarkably well, in fact better
 * then the ndbm hash function. Replace at your own risk]
 * use: 65599   nice.
 *      65587   even better.
 */
long
sdbm_hash(register char *str, register int len)
{
        register unsigned long n = 0;

#ifdef DUFF

#define HASHC   n = *str++ + 65599 * n

        if (len > 0) {
                register int loop = (len + 8 - 1) >> 3;

                switch(len & (8 - 1)) {
                case 0: do {
                        HASHC;  case 7: HASHC;
                case 6: HASHC;  case 5: HASHC;
                case 4: HASHC;  case 3: HASHC;
                case 2: HASHC;  case 1: HASHC;
                        } while (--loop);
                }

        }
#else
        while (len--)
                n = ((*str++) & 255) + 65587L * n;
#endif
        return n;
}

---