[apple/libc.git] / db / btree / FreeBSD / 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.
 * 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  8.9 (Berkeley) 7/26/94";
#endif /* LIBC_SCCS and not lint */
#include <sys/cdefs.h>
__FBSDID("$FreeBSD: src/lib/libc/db/btree/bt_split.c,v 1.12 2009/03/28 05:45:29 delphij Exp $");

#include <sys/types.h>

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

#include <db.h>
#include "btree.h"

static int       bt_broot(BTREE *, PAGE *, PAGE *, PAGE *);
static PAGE     *bt_page(BTREE *, PAGE *, PAGE **, PAGE **, indx_t *, size_t);
static int       bt_preserve(BTREE *, pgno_t);
static PAGE     *bt_psplit(BTREE *, PAGE *, PAGE *, PAGE *, indx_t *, size_t);
static PAGE     *bt_root(BTREE *, PAGE *, PAGE **, PAGE **, indx_t *, size_t);
static int       bt_rroot(BTREE *, PAGE *, PAGE *, PAGE *);
static recno_t   rec_total(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(BTREE *t, PAGE *sp, const DBT *key, const DBT *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 ((u_int32_t)(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, *(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(BTREE *t, PAGE *h, PAGE **lp, PAGE **rp, indx_t *skip, size_t ilen)
{
        PAGE *l, *r, *tp;
        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 *)calloc(1, t->bt_psize)) == NULL) {
                mpool_put(t->bt_mp, r, 0);
                return (NULL);
        }
        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(BTREE *t, PAGE *h, PAGE **lp, PAGE **rp, indx_t *skip, size_t ilen)
{
        PAGE *l, *r, *tp;
        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(BTREE *t, PAGE *h, PAGE *l, PAGE *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(BTREE *t, PAGE *h, PAGE *l, PAGE *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, *(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(BTREE *t, PAGE *h, PAGE *l, PAGE *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 = MAX_PAGE_OFFSET;
                rval = l;
        } else {
                rval = r;
                *pskip -= nxt;
        }

        for (off = 0; nxt < top; ++off) {
                if (skip == nxt) {
                        ++off;
                        skip = MAX_PAGE_OFFSET;
                }
                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(BTREE *t, 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(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);
}