/*
 * window_gettupleslot
 *	Fetch the pos'th tuple of the current partition into the slot,
 *	using the winobj's read pointer
 *
 * Returns true if successful, false if no such row
 */
static bool
window_gettupleslot(WindowObject winobj, int64 pos, TupleTableSlot *slot)
{
	WindowAggState *winstate = winobj->winstate;
	MemoryContext oldcontext;

	/* often called repeatedly in a row */
	CHECK_FOR_INTERRUPTS();

	/* Don't allow passing -1 to spool_tuples here */
	if (pos < 0)
		return false;

	/* If necessary, fetch the tuple into the spool */
	spool_tuples(winstate, pos);

	if (pos >= winstate->spooled_rows)
		return false;

	if (pos < winobj->markpos)
		elog(ERROR, "cannot fetch row before WindowObject's mark position");

	oldcontext = MemoryContextSwitchTo(winstate->ss.ps.ps_ExprContext->ecxt_per_query_memory);

	tuplestore_select_read_pointer(winstate->buffer, winobj->readptr);

	/*
	 * Advance or rewind until we are within one tuple of the one we want.
	 */
	if (winobj->seekpos < pos - 1)
	{
		if (!tuplestore_skiptuples(winstate->buffer,
								   pos - 1 - winobj->seekpos,
								   true))
			elog(ERROR, "unexpected end of tuplestore");
		winobj->seekpos = pos - 1;
	}
	else if (winobj->seekpos > pos + 1)
	{
		if (!tuplestore_skiptuples(winstate->buffer,
								   winobj->seekpos - (pos + 1),
								   false))
			elog(ERROR, "unexpected end of tuplestore");
		winobj->seekpos = pos + 1;
	}
	else if (winobj->seekpos == pos)
	{
		/*
		 * There's no API to refetch the tuple at the current position.  We
		 * have to move one tuple forward, and then one backward.  (We don't
		 * do it the other way because we might try to fetch the row before
		 * our mark, which isn't allowed.)  XXX this case could stand to be
		 * optimized.
		 */
		tuplestore_advance(winstate->buffer, true);
		winobj->seekpos++;
	}

	/*
	 * Now we should be on the tuple immediately before or after the one we
	 * want, so just fetch forwards or backwards as appropriate.
	 */
	if (winobj->seekpos > pos)
	{
		if (!tuplestore_gettupleslot(winstate->buffer, false, true, slot))
			elog(ERROR, "unexpected end of tuplestore");
		winobj->seekpos--;
	}
	else
	{
		if (!tuplestore_gettupleslot(winstate->buffer, true, true, slot))
			elog(ERROR, "unexpected end of tuplestore");
		winobj->seekpos++;
	}

	Assert(winobj->seekpos == pos);

	MemoryContextSwitchTo(oldcontext);

	return true;
}