Increment value based on grouped field

Question

I've got a dataset of buildings roof segments in shapefiles. I need to sequentially number the roof segments from 1 to n of each building. Is there a way to do it with the QGIS field calculator?
Actual:

Expected:

It's pretty similar to that question on arcpy: Auto increment by group in ArcGIS ModelBuilder

MrXsquared · Accepted Answer

You can use this expression:
array_find(array_agg("BUILDING"||'_'||-$id,"BUILDING"),"BUILDING"||'_'||-$id)+1

Result:

Increment value based on grouped field

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