Power query, iterate over the column records to apply a custom cumulative calculation - powerquery

Question

Using Power Query in Excel. I am trying to implement a custom column that would iteratively calculate the row based on the previous row's value of the same column.
I have a 3 column table and the 4th column will be the calculation column that I am failing to implement.
The calculation is very easy to apply in Excel which goes as follows:
Formula in cell D3 --> = =IF(A3=1,C3+6.4,IF(C3+D2>=12.8,12.8,IF(C3+D2<=1.28,1.28,C3+D2)))
The same formula is applied to the whole column by dragging.
The idea behind it:
For each category, I have an index column starting from 1,
If Index = 1, then Calculation is Value + 6.4,
else if Value + Value(previous row Custom cumulative) >= 12.8 then 12.8
else if Value + Value(previous row Custom cumulative) <= 1.28 then 1.28
else Value + Value(previous row Custom cumulative)
So, the calculation is a cumulative sum with an upper and lower cap built into it.
How can I implement this in Power Query and M-Language?
I really appreciate your help!
I have tried to use List.Generate and List.Accumulate features, however, I was stuck with creating records that has values from multiple columns in it.

Answer 1

Try this
(edited to make more efficient with single pass process)
let Source = Excel.CurrentWorkbook(){[Name="Table15"]}[Content],
process = (zzz as list) => let x= List.Accumulate( zzz,{0},( state, current ) =>
if List.Last(state) =0 then List.Combine ({state,{6.4+current}}) else
if List.Last(state)+current >=12.8 then List.Combine ({state,{12.8}}) else
if List.Last(state)+current <=1.28 then List.Combine ({state,{1.28}}) else
List.Combine ({state,{List.Last(state)+current}})
) in x,
#"Grouped Rows" = Table.Group(Source, {"Category"}, {{"data", each
let a=process(_[Values])
in Table.AddColumn(_, "Custom Cumulative", each a{[Index]}), type table }}),
#"Expanded data" = Table.ExpandTableColumn(#"Grouped Rows", "data", {"Index", "Values", "Custom Cumulative"}, {"Index", "Values", "Custom Cumulative"})
in #"Expanded data"