libdb2/btree/bt_split.c

/*-
 * Copyright (c) 1990, 1993, 1994
 *  The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Mike Olson.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *  This product includes software developed by the University of
 *  California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)bt_split.c  8.10 (Berkeley) 1/9/95";
#endif /* LIBC_SCCS and not lint */

#include <sys/types.h>

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "db-int.h"
#include "btree.h"

static int   bt_broot __P((BTREE *, PAGE *, PAGE *, PAGE *));
static PAGE *bt_page
            __P((BTREE *, PAGE *, PAGE **, PAGE **, indx_t *, size_t));
static int   bt_preserve __P((BTREE *, db_pgno_t));
static PAGE *bt_psplit
            __P((BTREE *, PAGE *, PAGE *, PAGE *, indx_t *, size_t));
static PAGE *bt_root
            __P((BTREE *, PAGE *, PAGE **, PAGE **, indx_t *, size_t));
static int   bt_rroot __P((BTREE *, PAGE *, PAGE *, PAGE *));
static recno_t   rec_total __P((PAGE *));

#ifdef STATISTICS
u_long  bt_rootsplit, bt_split, bt_sortsplit, bt_pfxsaved;
#endif

/*
 * __BT_SPLIT -- Split the tree.
 *
 * Parameters:
 *  t:  tree
 *  sp: page to split
 *  key:    key to insert
 *  data:   data to insert
 *  flags:  BIGKEY/BIGDATA flags
 *  ilen:   insert length
 *  skip:   index to leave open
 *
 * Returns:
 *  RET_ERROR, RET_SUCCESS
 */
int
__bt_split(t, sp, key, data, flags, ilen, argskip)
    BTREE *t;
    PAGE *sp;
    const DBT *key, *data;
    int flags;
    size_t ilen;
    u_int32_t argskip;
{
    BINTERNAL *bi;
    BLEAF *bl, *tbl;
    DBT a, b;
    EPGNO *parent;
    PAGE *h, *l, *r, *lchild, *rchild;
    indx_t nxtindex;
    u_int16_t skip;
    u_int32_t n, nbytes, nksize;
    int parentsplit;
    char *dest;

    /*
     * Split the page into two pages, l and r.  The split routines return
     * a pointer to the page into which the key should be inserted and with
     * skip set to the offset which should be used.  Additionally, l and r
     * are pinned.
     */
    skip = argskip;
    h = sp->pgno == P_ROOT ?
        bt_root(t, sp, &l, &r, &skip, ilen) :
        bt_page(t, sp, &l, &r, &skip, ilen);
    if (h == NULL)
        return (RET_ERROR);

    /*
     * Insert the new key/data pair into the leaf page.  (Key inserts
     * always cause a leaf page to split first.)
     */
    h->linp[skip] = h->upper -= ilen;
    dest = (char *)h + h->upper;
    if (F_ISSET(t, R_RECNO))
        WR_RLEAF(dest, data, flags)
    else
        WR_BLEAF(dest, key, data, flags)

    /* If the root page was split, make it look right. */
    if (sp->pgno == P_ROOT &&
        (F_ISSET(t, R_RECNO) ?
        bt_rroot(t, sp, l, r) : bt_broot(t, sp, l, r)) == RET_ERROR)
        goto err2;

    /*
     * Now we walk the parent page stack -- a LIFO stack of the pages that
     * were traversed when we searched for the page that split.  Each stack
     * entry is a page number and a page index offset.  The offset is for
     * the page traversed on the search.  We've just split a page, so we
     * have to insert a new key into the parent page.
     *
     * If the insert into the parent page causes it to split, may have to
     * continue splitting all the way up the tree.  We stop if the root
     * splits or the page inserted into didn't have to split to hold the
     * new key.  Some algorithms replace the key for the old page as well
     * as the new page.  We don't, as there's no reason to believe that the
     * first key on the old page is any better than the key we have, and,
     * in the case of a key being placed at index 0 causing the split, the
     * key is unavailable.
     *
     * There are a maximum of 5 pages pinned at any time.  We keep the left
     * and right pages pinned while working on the parent.   The 5 are the
     * two children, left parent and right parent (when the parent splits)
     * and the root page or the overflow key page when calling bt_preserve.
     * This code must make sure that all pins are released other than the
     * root page or overflow page which is unlocked elsewhere.
     */
    while ((parent = BT_POP(t)) != NULL) {
        lchild = l;
        rchild = r;

        /* Get the parent page. */
        if ((h = mpool_get(t->bt_mp, parent->pgno, 0)) == NULL)
            goto err2;

        /*
         * The new key goes ONE AFTER the index, because the split
         * was to the right.
         */
        skip = parent->index + 1;

        /*
         * Calculate the space needed on the parent page.
         *
         * Prefix trees: space hack when inserting into BINTERNAL
         * pages.  Retain only what's needed to distinguish between
         * the new entry and the LAST entry on the page to its left.
         * If the keys compare equal, retain the entire key.  Note,
         * we don't touch overflow keys, and the entire key must be
         * retained for the next-to-left most key on the leftmost
         * page of each level, or the search will fail.  Applicable
         * ONLY to internal pages that have leaf pages as children.
         * Further reduction of the key between pairs of internal
         * pages loses too much information.
         */
        switch (rchild->flags & P_TYPE) {
        case P_BINTERNAL:
            bi = GETBINTERNAL(rchild, 0);
            nbytes = NBINTERNAL(bi->ksize);
            break;
        case P_BLEAF:
            bl = GETBLEAF(rchild, 0);
            nbytes = NBINTERNAL(bl->ksize);
            if (t->bt_pfx && !(bl->flags & P_BIGKEY) &&
                (h->prevpg != P_INVALID || skip > 1)) {
                tbl = GETBLEAF(lchild, NEXTINDEX(lchild) - 1);
                a.size = tbl->ksize;
                a.data = tbl->bytes;
                b.size = bl->ksize;
                b.data = bl->bytes;
                nksize = t->bt_pfx(&a, &b);
                n = NBINTERNAL(nksize);
                if (n < nbytes) {
#ifdef STATISTICS
                    bt_pfxsaved += nbytes - n;
#endif
                    nbytes = n;
                } else
                    nksize = 0;
            } else
                nksize = 0;
            break;
        case P_RINTERNAL:
        case P_RLEAF:
            nbytes = NRINTERNAL;
            break;
        default:
            abort();
        }

        /* Split the parent page if necessary or shift the indices. */
        if (h->upper - h->lower < nbytes + sizeof(indx_t)) {
            sp = h;
            h = h->pgno == P_ROOT ?
                bt_root(t, h, &l, &r, &skip, nbytes) :
                bt_page(t, h, &l, &r, &skip, nbytes);
            if (h == NULL)
                goto err1;
            parentsplit = 1;
        } else {
            if (skip < (nxtindex = NEXTINDEX(h)))
                memmove(h->linp + skip + 1, h->linp + skip,
                    (nxtindex - skip) * sizeof(indx_t));
            h->lower += sizeof(indx_t);
            parentsplit = 0;
        }

        /* Insert the key into the parent page. */
        switch (rchild->flags & P_TYPE) {
        case P_BINTERNAL:
            h->linp[skip] = h->upper -= nbytes;
            dest = (char *)h + h->linp[skip];
            memmove(dest, bi, nbytes);
            ((BINTERNAL *)dest)->pgno = rchild->pgno;
            break;
        case P_BLEAF:
            h->linp[skip] = h->upper -= nbytes;
            dest = (char *)h + h->linp[skip];
            WR_BINTERNAL(dest, nksize ? nksize : bl->ksize,
                rchild->pgno, bl->flags & P_BIGKEY);
            memmove(dest, bl->bytes, nksize ? nksize : bl->ksize);
            if (bl->flags & P_BIGKEY &&
                bt_preserve(t, *(db_pgno_t *)bl->bytes) == RET_ERROR)
                goto err1;
            break;
        case P_RINTERNAL:
            /*
             * Update the left page count.  If split
             * added at index 0, fix the correct page.
             */
            if (skip > 0)
                dest = (char *)h + h->linp[skip - 1];
            else
                dest = (char *)l + l->linp[NEXTINDEX(l) - 1];
            ((RINTERNAL *)dest)->nrecs = rec_total(lchild);
            ((RINTERNAL *)dest)->pgno = lchild->pgno;

            /* Update the right page count. */
            h->linp[skip] = h->upper -= nbytes;
            dest = (char *)h + h->linp[skip];
            ((RINTERNAL *)dest)->nrecs = rec_total(rchild);
            ((RINTERNAL *)dest)->pgno = rchild->pgno;
            break;
        case P_RLEAF:
            /*
             * Update the left page count.  If split
             * added at index 0, fix the correct page.
             */
            if (skip > 0)
                dest = (char *)h + h->linp[skip - 1];
            else
                dest = (char *)l + l->linp[NEXTINDEX(l) - 1];
            ((RINTERNAL *)dest)->nrecs = NEXTINDEX(lchild);
            ((RINTERNAL *)dest)->pgno = lchild->pgno;

            /* Update the right page count. */
            h->linp[skip] = h->upper -= nbytes;
            dest = (char *)h + h->linp[skip];
            ((RINTERNAL *)dest)->nrecs = NEXTINDEX(rchild);
            ((RINTERNAL *)dest)->pgno = rchild->pgno;
            break;
        default:
            abort();
        }

        /* Unpin the held pages. */
        if (!parentsplit) {
            mpool_put(t->bt_mp, h, MPOOL_DIRTY);
            break;
        }

        /* If the root page was split, make it look right. */
        if (sp->pgno == P_ROOT &&
            (F_ISSET(t, R_RECNO) ?
            bt_rroot(t, sp, l, r) : bt_broot(t, sp, l, r)) == RET_ERROR)
            goto err1;

        mpool_put(t->bt_mp, lchild, MPOOL_DIRTY);
        mpool_put(t->bt_mp, rchild, MPOOL_DIRTY);
    }

    /* Unpin the held pages. */
    mpool_put(t->bt_mp, l, MPOOL_DIRTY);
    mpool_put(t->bt_mp, r, MPOOL_DIRTY);

    /* Clear any pages left on the stack. */
    return (RET_SUCCESS);

    /*
     * If something fails in the above loop we were already walking back
     * up the tree and the tree is now inconsistent.  Nothing much we can
     * do about it but release any memory we're holding.
     */
err1:   mpool_put(t->bt_mp, lchild, MPOOL_DIRTY);
    mpool_put(t->bt_mp, rchild, MPOOL_DIRTY);

err2:   mpool_put(t->bt_mp, l, 0);
    mpool_put(t->bt_mp, r, 0);
    __dbpanic(t->bt_dbp);
    return (RET_ERROR);
}

/*
 * BT_PAGE -- Split a non-root page of a btree.
 *
 * Parameters:
 *  t:  tree
 *  h:  root page
 *  lp: pointer to left page pointer
 *  rp: pointer to right page pointer
 *  skip:   pointer to index to leave open
 *  ilen:   insert length
 *
 * Returns:
 *  Pointer to page in which to insert or NULL on error.
 */
static PAGE *
bt_page(t, h, lp, rp, skip, ilen)
    BTREE *t;
    PAGE *h, **lp, **rp;
    indx_t *skip;
    size_t ilen;
{
    PAGE *l, *r, *tp;
    db_pgno_t npg;

#ifdef STATISTICS
    ++bt_split;
#endif
    /* Put the new right page for the split into place. */
    if ((r = __bt_new(t, &npg)) == NULL)
        return (NULL);
    r->pgno = npg;
    r->lower = BTDATAOFF;
    r->upper = t->bt_psize;
    r->nextpg = h->nextpg;
    r->prevpg = h->pgno;
    r->flags = h->flags & P_TYPE;

    /*
     * If we're splitting the last page on a level because we're appending
     * a key to it (skip is NEXTINDEX()), it's likely that the data is
     * sorted.  Adding an empty page on the side of the level is less work
     * and can push the fill factor much higher than normal.  If we're
     * wrong it's no big deal, we'll just do the split the right way next
     * time.  It may look like it's equally easy to do a similar hack for
     * reverse sorted data, that is, split the tree left, but it's not.
     * Don't even try.
     */
    if (h->nextpg == P_INVALID && *skip == NEXTINDEX(h)) {
#ifdef STATISTICS
        ++bt_sortsplit;
#endif
        h->nextpg = r->pgno;
        r->lower = BTDATAOFF + sizeof(indx_t);
        *skip = 0;
        *lp = h;
        *rp = r;
        return (r);
    }

    /* Put the new left page for the split into place. */
    if ((l = (PAGE *)malloc(t->bt_psize)) == NULL) {
        mpool_put(t->bt_mp, r, 0);
        return (NULL);
    }
#if 1 /* def PURIFY */
    memset(l, 0xff, t->bt_psize);
#endif
    l->pgno = h->pgno;
    l->nextpg = r->pgno;
    l->prevpg = h->prevpg;
    l->lower = BTDATAOFF;
    l->upper = t->bt_psize;
    l->flags = h->flags & P_TYPE;

    /* Fix up the previous pointer of the page after the split page. */
    if (h->nextpg != P_INVALID) {
        if ((tp = mpool_get(t->bt_mp, h->nextpg, 0)) == NULL) {
            free(l);
            /* XXX mpool_free(t->bt_mp, r->pgno); */
            return (NULL);
        }
        tp->prevpg = r->pgno;
        mpool_put(t->bt_mp, tp, MPOOL_DIRTY);
    }

    /*
     * Split right.  The key/data pairs aren't sorted in the btree page so
     * it's simpler to copy the data from the split page onto two new pages
     * instead of copying half the data to the right page and compacting
     * the left page in place.  Since the left page can't change, we have
     * to swap the original and the allocated left page after the split.
     */
    tp = bt_psplit(t, h, l, r, skip, ilen);

    /* Move the new left page onto the old left page. */
    memmove(h, l, t->bt_psize);
    if (tp == l)
        tp = h;
    free(l);

    *lp = h;
    *rp = r;
    return (tp);
}

/*
 * BT_ROOT -- Split the root page of a btree.
 *
 * Parameters:
 *  t:  tree
 *  h:  root page
 *  lp: pointer to left page pointer
 *  rp: pointer to right page pointer
 *  skip:   pointer to index to leave open
 *  ilen:   insert length
 *
 * Returns:
 *  Pointer to page in which to insert or NULL on error.
 */
static PAGE *
bt_root(t, h, lp, rp, skip, ilen)
    BTREE *t;
    PAGE *h, **lp, **rp;
    indx_t *skip;
    size_t ilen;
{
    PAGE *l, *r, *tp;
    db_pgno_t lnpg, rnpg;

#ifdef STATISTICS
    ++bt_split;
    ++bt_rootsplit;
#endif
    /* Put the new left and right pages for the split into place. */
    if ((l = __bt_new(t, &lnpg)) == NULL ||
        (r = __bt_new(t, &rnpg)) == NULL)
        return (NULL);
    l->pgno = lnpg;
    r->pgno = rnpg;
    l->nextpg = r->pgno;
    r->prevpg = l->pgno;
    l->prevpg = r->nextpg = P_INVALID;
    l->lower = r->lower = BTDATAOFF;
    l->upper = r->upper = t->bt_psize;
    l->flags = r->flags = h->flags & P_TYPE;

    /* Split the root page. */
    tp = bt_psplit(t, h, l, r, skip, ilen);

    *lp = l;
    *rp = r;
    return (tp);
}

/*
 * BT_RROOT -- Fix up the recno root page after it has been split.
 *
 * Parameters:
 *  t:  tree
 *  h:  root page
 *  l:  left page
 *  r:  right page
 *
 * Returns:
 *  RET_ERROR, RET_SUCCESS
 */
static int
bt_rroot(t, h, l, r)
    BTREE *t;
    PAGE *h, *l, *r;
{
    char *dest;

    /* Insert the left and right keys, set the header information. */
    h->linp[0] = h->upper = t->bt_psize - NRINTERNAL;
    dest = (char *)h + h->upper;
    WR_RINTERNAL(dest,
        l->flags & P_RLEAF ? NEXTINDEX(l) : rec_total(l), l->pgno);

    h->linp[1] = h->upper -= NRINTERNAL;
    dest = (char *)h + h->upper;
    WR_RINTERNAL(dest,
        r->flags & P_RLEAF ? NEXTINDEX(r) : rec_total(r), r->pgno);

    h->lower = BTDATAOFF + 2 * sizeof(indx_t);

    /* Unpin the root page, set to recno internal page. */
    h->flags &= ~P_TYPE;
    h->flags |= P_RINTERNAL;
    mpool_put(t->bt_mp, h, MPOOL_DIRTY);

    return (RET_SUCCESS);
}

/*
 * BT_BROOT -- Fix up the btree root page after it has been split.
 *
 * Parameters:
 *  t:  tree
 *  h:  root page
 *  l:  left page
 *  r:  right page
 *
 * Returns:
 *  RET_ERROR, RET_SUCCESS
 */
static int
bt_broot(t, h, l, r)
    BTREE *t;
    PAGE *h, *l, *r;
{
    BINTERNAL *bi;
    BLEAF *bl;
    u_int32_t nbytes;
    char *dest;

    /*
     * If the root page was a leaf page, change it into an internal page.
     * We copy the key we split on (but not the key's data, in the case of
     * a leaf page) to the new root page.
     *
     * The btree comparison code guarantees that the left-most key on any
     * level of the tree is never used, so it doesn't need to be filled in.
     */
    nbytes = NBINTERNAL(0);
    h->linp[0] = h->upper = t->bt_psize - nbytes;
    dest = (char *)h + h->upper;
    WR_BINTERNAL(dest, 0, l->pgno, 0);

    switch (h->flags & P_TYPE) {
    case P_BLEAF:
        bl = GETBLEAF(r, 0);
        nbytes = NBINTERNAL(bl->ksize);
        h->linp[1] = h->upper -= nbytes;
        dest = (char *)h + h->upper;
        WR_BINTERNAL(dest, bl->ksize, r->pgno, 0);
        memmove(dest, bl->bytes, bl->ksize);

        /*
         * If the key is on an overflow page, mark the overflow chain
         * so it isn't deleted when the leaf copy of the key is deleted.
         */
        if (bl->flags & P_BIGKEY &&
            bt_preserve(t, *(db_pgno_t *)bl->bytes) == RET_ERROR)
            return (RET_ERROR);
        break;
    case P_BINTERNAL:
        bi = GETBINTERNAL(r, 0);
        nbytes = NBINTERNAL(bi->ksize);
        h->linp[1] = h->upper -= nbytes;
        dest = (char *)h + h->upper;
        memmove(dest, bi, nbytes);
        ((BINTERNAL *)dest)->pgno = r->pgno;
        break;
    default:
        abort();
    }

    /* There are two keys on the page. */
    h->lower = BTDATAOFF + 2 * sizeof(indx_t);

    /* Unpin the root page, set to btree internal page. */
    h->flags &= ~P_TYPE;
    h->flags |= P_BINTERNAL;
    mpool_put(t->bt_mp, h, MPOOL_DIRTY);

    return (RET_SUCCESS);
}

/*
 * BT_PSPLIT -- Do the real work of splitting the page.
 *
 * Parameters:
 *  t:  tree
 *  h:  page to be split
 *  l:  page to put lower half of data
 *  r:  page to put upper half of data
 *  pskip:  pointer to index to leave open
 *  ilen:   insert length
 *
 * Returns:
 *  Pointer to page in which to insert.
 */
static PAGE *
bt_psplit(t, h, l, r, pskip, ilen)
    BTREE *t;
    PAGE *h, *l, *r;
    indx_t *pskip;
    size_t ilen;
{
    BINTERNAL *bi;
    BLEAF *bl;
    CURSOR *c;
    RLEAF *rl;
    PAGE *rval;
    void *src;
    indx_t full, half, nxt, off, skip, top, used;
    u_int32_t nbytes;
    int bigkeycnt, isbigkey;

    /*
     * Split the data to the left and right pages.  Leave the skip index
     * open.  Additionally, make some effort not to split on an overflow
     * key.  This makes internal page processing faster and can save
     * space as overflow keys used by internal pages are never deleted.
     */
    bigkeycnt = 0;
    skip = *pskip;
    full = t->bt_psize - BTDATAOFF;
    half = full / 2;
    used = 0;
    for (nxt = off = 0, top = NEXTINDEX(h); nxt < top; ++off) {
        if (skip == off) {
            nbytes = ilen;
            isbigkey = 0;       /* XXX: not really known. */
        } else
            switch (h->flags & P_TYPE) {
            case P_BINTERNAL:
                src = bi = GETBINTERNAL(h, nxt);
                nbytes = NBINTERNAL(bi->ksize);
                isbigkey = bi->flags & P_BIGKEY;
                break;
            case P_BLEAF:
                src = bl = GETBLEAF(h, nxt);
                nbytes = NBLEAF(bl);
                isbigkey = bl->flags & P_BIGKEY;
                break;
            case P_RINTERNAL:
                src = GETRINTERNAL(h, nxt);
                nbytes = NRINTERNAL;
                isbigkey = 0;
                break;
            case P_RLEAF:
                src = rl = GETRLEAF(h, nxt);
                nbytes = NRLEAF(rl);
                isbigkey = 0;
                break;
            default:
                abort();
            }

        /*
         * If the key/data pairs are substantial fractions of the max
         * possible size for the page, it's possible to get situations
         * where we decide to try and copy too much onto the left page.
         * Make sure that doesn't happen.
         */
        if ((skip <= off && used + nbytes + sizeof(indx_t) >= full)
            || nxt == top - 1) {
            --off;
            break;
        }

        /* Copy the key/data pair, if not the skipped index. */
        if (skip != off) {
            ++nxt;

            l->linp[off] = l->upper -= nbytes;
            memmove((char *)l + l->upper, src, nbytes);
        }

        used += nbytes + sizeof(indx_t);
        if (used >= half) {
            if (!isbigkey || bigkeycnt == 3)
                break;
            else
                ++bigkeycnt;
        }
    }

    /*
     * Off is the last offset that's valid for the left page.
     * Nxt is the first offset to be placed on the right page.
     */
    l->lower += (off + 1) * sizeof(indx_t);

    /*
     * If splitting the page that the cursor was on, the cursor has to be
     * adjusted to point to the same record as before the split.  If the
     * cursor is at or past the skipped slot, the cursor is incremented by
     * one.  If the cursor is on the right page, it is decremented by the
     * number of records split to the left page.
     */
    c = &t->bt_cursor;
    if (F_ISSET(c, CURS_INIT) && c->pg.pgno == h->pgno) {
        if (c->pg.index >= skip)
            ++c->pg.index;
        if (c->pg.index < nxt)          /* Left page. */
            c->pg.pgno = l->pgno;
        else {                  /* Right page. */
            c->pg.pgno = r->pgno;
            c->pg.index -= nxt;
        }
    }

    /*
     * If the skipped index was on the left page, just return that page.
     * Otherwise, adjust the skip index to reflect the new position on
     * the right page.
     */
    if (skip <= off) {
        skip = (indx_t)-1;
        rval = l;
    } else {
        rval = r;
        *pskip -= nxt;
    }

    for (off = 0; nxt < top; ++off) {
        if (skip == nxt) {
            ++off;
            skip = (indx_t)-1;
        }
        switch (h->flags & P_TYPE) {
        case P_BINTERNAL:
            src = bi = GETBINTERNAL(h, nxt);
            nbytes = NBINTERNAL(bi->ksize);
            break;
        case P_BLEAF:
            src = bl = GETBLEAF(h, nxt);
            nbytes = NBLEAF(bl);
            break;
        case P_RINTERNAL:
            src = GETRINTERNAL(h, nxt);
            nbytes = NRINTERNAL;
            break;
        case P_RLEAF:
            src = rl = GETRLEAF(h, nxt);
            nbytes = NRLEAF(rl);
            break;
        default:
            abort();
        }
        ++nxt;
        r->linp[off] = r->upper -= nbytes;
        memmove((char *)r + r->upper, src, nbytes);
    }
    r->lower += off * sizeof(indx_t);

    /* If the key is being appended to the page, adjust the index. */
    if (skip == top)
        r->lower += sizeof(indx_t);

    return (rval);
}

/*
 * BT_PRESERVE -- Mark a chain of pages as used by an internal node.
 *
 * Chains of indirect blocks pointed to by leaf nodes get reclaimed when the
 * record that references them gets deleted.  Chains pointed to by internal
 * pages never get deleted.  This routine marks a chain as pointed to by an
 * internal page.
 *
 * Parameters:
 *  t:  tree
 *  pg: page number of first page in the chain.
 *
 * Returns:
 *  RET_SUCCESS, RET_ERROR.
 */
static int
bt_preserve(t, pg)
    BTREE *t;
    db_pgno_t pg;
{
    PAGE *h;

    if ((h = mpool_get(t->bt_mp, pg, 0)) == NULL)
        return (RET_ERROR);
    h->flags |= P_PRESERVE;
    mpool_put(t->bt_mp, h, MPOOL_DIRTY);
    return (RET_SUCCESS);
}

/*
 * REC_TOTAL -- Return the number of recno entries below a page.
 *
 * Parameters:
 *  h:  page
 *
 * Returns:
 *  The number of recno entries below a page.
 *
 * XXX
 * These values could be set by the bt_psplit routine.  The problem is that the
 * entry has to be popped off of the stack etc. or the values have to be passed
 * all the way back to bt_split/bt_rroot and it's not very clean.
 */
static recno_t
rec_total(h)
    PAGE *h;
{
    recno_t recs;
    indx_t nxt, top;

    for (recs = 0, nxt = 0, top = NEXTINDEX(h); nxt < top; ++nxt)
        recs += GETRINTERNAL(h, nxt)->nrecs;
    return (recs);
}