-- =============================================
-- Description:	Invoice backdating (ADR 0032). Returns the date an invoice may be backdated to
--				(the last working day Mon-Fri of the previous month, Europe/Berlin), or NULL when
--				backdating is not allowed:
--				  * @authuser needs fds_inv > 2, and
--				  * no invoice may have been finalized after that date yet (invoice numbers are
--				    issued in date order; DateFinalized is stored in UTC, hence the 2h margin).
--				Single source for fds__setInvoiceFinal (@backdate = 1) and the preview dialog.
--				The check is only race-free inside fds__setInvoiceFinal, which serializes all
--				finalizations with a transaction-owned app lock; the preview call is advisory.
-- =============================================
CREATE FUNCTION [dbo].[fds__fn_invoiceBackdate]
(
	@authuser varchar(25)
)
RETURNS date
AS
BEGIN
	IF [dbo].[fis_getModuleAuth]('fds_inv', @authuser) <= 2
		RETURN NULL;

	DECLARE @today date = CAST(SYSDATETIMEOFFSET() AT TIME ZONE 'W. Europe Standard Time' AS date);
	DECLARE @eom date = EOMONTH(@today, -1);
	-- DATEFIRST-independent weekday: 0 = Monday ... 5 = Saturday, 6 = Sunday
	DECLARE @bd date = DATEADD(day, CASE DATEDIFF(day, '19000101', @eom) % 7 WHEN 5 THEN -1 WHEN 6 THEN -2 ELSE 0 END, @eom);

	IF EXISTS (SELECT 0 FROM [dbo].[fds__invoices]
				WHERE [DateFinalized] IS NOT NULL
					AND [DateFinalized] >= DATEADD(HOUR, -2, CAST(DATEADD(DAY, 1, @bd) AS datetime)))
		RETURN NULL;

	RETURN @bd;
END