Detecting the previous row in Power Query - handling a text file

Hi,

I need to import more rows from several stock info text files, treating them inside power query and then producing some KPIs.

I need to manage a granularity at day and hour level.

The record layout for a text file is:

WH_SITE YYYYMM B_DDhh-E_DDhh MAIN_VALUE OVR OB_DDhh-OE_DDhh VALUE_TO_OVERRIDE OVR OB_DDhh-OE_DDhh VALUE_TO_OVERRIDE,

where

WH_SITE = Warehouse location code

YYYYMM = validity forecast year and month

B_DDhh = validity begin forecast day and hour

E_DDhh = validity end forecast day and hour

MAIN_VALUE = initial stock value, valid between B_DDhh and E_DDhh

OVR = optional section to override the initial value for a specific validity period (I could have 0 or 1 or max 5 OVR sections)

OB_DDhh-OE_DDhh = begin and end of a validity period for a stock value to override

VALUE_TO_OVERRIDE = stock value that overrides the initial stock value for a specific validity period

Here a record sample:

SITE01 202104 0709-0718 5000 OVR 0712-0713 4500 OVR 0715-0716 4200

I've applied several transformation steps inside Power Query, obtaining a such result:

but I need to obtain a such result, before using some DAX logics to obtain the final KPIs:

The override operation occurs after the related end of the validity period.

The override sections are optional, up to five ones.

I thought to detect the previous row to understand the passage from the old stock value to the new stock value to override. I cannot adding an index, I need to handle some millions of rows (I've expanded the validity period to split in date + time, hour for hour), perhaps I need to search the previous row respect to the WS_SITE and the VALIDITY_DATETIME less one hour.

Any suggests to me, please? Thanks