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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    8.9 (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 <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;
      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, *(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;
      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 >= 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;
            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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