/*-------------------------------------------------------------------------
 *
 * off.h--
 *    POSTGRES disk "offset" definitions.
 *
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 * $Id: off.h,v 1.1 1996/08/28 01:58:18 scrappy Exp $
 *
 *-------------------------------------------------------------------------
 */
#ifndef	OFF_H
#define OFF_H

#include "c.h"
#include "machine.h"		/* for BLCKSZ */
#include "storage/itemid.h"

/*
 * OffsetNumber:
 *
 * this is a 1-based index into the linp (ItemIdData) array in the
 * header of each disk page.
 */
typedef uint16			OffsetNumber;

#define InvalidOffsetNumber	((OffsetNumber) 0)
#define FirstOffsetNumber	((OffsetNumber) 1)
#define	MaxOffsetNumber		((OffsetNumber) (BLCKSZ / sizeof(ItemIdData)))
#define	OffsetNumberMask	(0xffff)		/* valid uint16 bits */

/* ----------------
 *	support macros
 * ----------------
 */

/*
 * OffsetNumberIsValid --
 *	True iff the offset number is valid.
 */
#define OffsetNumberIsValid(offsetNumber) \
    ((bool) ((offsetNumber != InvalidOffsetNumber) && \
	     (offsetNumber <= MaxOffsetNumber)))

/*
 * OffsetNumberNext --
 * OffsetNumberPrev --
 *	Increments/decrements the argument.  These macros look pointless
 *	but they help us disambiguate the different manipulations on
 *	OffsetNumbers (e.g., sometimes we substract one from an
 *	OffsetNumber to move back, and sometimes we do so to form a
 *	real C array index).
 */
#define OffsetNumberNext(offsetNumber) \
    ((OffsetNumber) (1 + (offsetNumber)))
#define OffsetNumberPrev(offsetNumber) \
    ((OffsetNumber) (-1 + (offsetNumber)))

#endif	/* OFF_H */
