btprev.cpp

来自「CBASE v1.01 采用Borland公司TC++编写的数据库管理源程序库」· C++ 代码 · 共 102 行

/*	Copyright (c) 1989 Citadel	*/
/*	   All Rights Reserved    	*/

/* #ident	"@(#)btprev.c	1.4 - 90/06/20" */

#include <blkio.h>
#include <errno.h>

/* local headers */
#include "btree_.h"

/*man---------------------------------------------------------------------------
NAME
     btprev - previous btree key

SYNOPSIS
     #include <btree.h>

     int btprev(btp)
     btree_t *btp;

DESCRIPTION
     The btprev function retreats the cursor of btree btp to the
     previous key.  If cursor is currently null, it will be moved to
     the last key.  If the cursor is currently on the last key, it
     will move to null.  If the tree is empty, the cursor will remain
     on null.

     btprev will fail if one or more of the following is true:

     [EINVAL]       btp is not a valid btree pointer.
     [BTELOCK]      btp is not locked.
     [BTENOPEN]     btp is not open.

SEE ALSO
     btcursor, btfirst, btlast, btnext.

DIAGNOSTICS
     Upon successful completion, a value of 0 is returned.  Otherwise,
     a value of -1 is returned, and errno set to indicate the error.

------------------------------------------------------------------------------*/
int btprev(btree_t *btp)
{
	int terrno = 0;		/* tmp errno */

	/* validate arguments */
	if (!bt_valid(btp)) {
		errno = EINVAL;
		return -1;
	}

	/* check if not open */
	if (!(btp->flags & BTOPEN)) {
		errno = BTENOPEN;
		return -1;
	}

	/* check locks */
	if (!(btp->flags & BTLOCKS)) {
		errno = BTELOCK;
		return -1;
	}

	/* move cursor to previous key in current node */
	if (btp->cbtpos.node != NIL) {
		if (--btp->cbtpos.key > 0) {
			errno = 0;
			return 0;
		}
	}

	/* move cursor to null */
	if (btp->cbtpos.node == btp->bthdr.first) {
		btp->cbtpos.node = NIL;
		btp->cbtpos.key = 0;
		bt_ndinit(btp, btp->cbtnp);
		errno = 0;
		return 0;
	}

	/* move cursor to last key in prev node */
	if (btp->cbtpos.node == NIL) {
		btp->cbtpos.node = btp->bthdr.last;
	} else {
		btp->cbtpos.node = btp->cbtnp->lsib;
	}
	if (bt_ndget(btp, btp->cbtpos.node, btp->cbtnp) == -1) {
		BTEPRINT;
		terrno = errno;
		btp->cbtpos.node = NIL;
		btp->cbtpos.key = 0;
		bt_ndinit(btp, btp->cbtnp);
		errno = terrno;
		return -1;
	}
	btp->cbtpos.key = btp->cbtnp->n;

	errno = 0;
	return 0;
}