## root/libdb/bt_split.c(50.html)

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## DEFINITIONS

This source file includes following definitions.

1. __bt_split
2. bt_page
3. bt_root
4. bt_rroot
5. bt_broot
6. bt_psplit
7. bt_preserve
8. rec_total

    /*-
     * 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. 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 */

    #ifdef HAVE_CONFIG_H
    #include <config.h>
    #endif
    #include <sys/types.h>
    #ifdef HAVE_LIMITS_H
    #include <limits.h>
    #endif
    #include <stdio.h>
    #ifdef STDC_HEADERS
    #include <stdlib.h>
    #endif
    #ifdef HAVE_STRING_H
    #include <string.h>
    #else
    #include <strings.h>
    #endif

    #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 *);

    u_long  bt_rootsplit, bt_split, bt_sortsplit, bt_pfxsaved;

    /*
     * __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 = NULL;
            BLEAF *bl = NULL, *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 = 0;
            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, *(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;
            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);
            }
    #ifdef 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;
            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, *(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 = NULL;
            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 = 0;
                    rval = l;
            } else {
                    rval = r;
                    *pskip -= nxt;
            }

            for (off = 0; nxt < top; ++off) {
                    if (skip == nxt) {
                            ++off;
                            skip = 0;
                    }
                    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;
            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);
    }

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