bt_split.c

Go to the documentation of this file.

/*-
 * 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 8.9 (Berkeley) 7/26/94";
#endif /* LIBC_SCCS and not lint */

#include <sys/types.h>

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

#include "../include/db.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 *, 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 = 0;
   BLEAF *bl = 0, *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 ((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(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 = 0;
   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);
}