Calculate Difference previous date in Power Query ( M ) (only)

Find below 2 queries.

The first query is the proposed solution.

This includes a function which you can't step through in the query editor.

The second query is an amended vesion for 1 product, with some lines commented out and some other lines added, so you can see the steps that are part of the function in the first query.

FYI: I first created the query for 1 product and turned this into a function for the final solution.

I tested with about 25,000 rows of data in Excel and it runs for about 2 seconds (!).

Proposed solution:

let
    Source = TestData,
    #"Grouped Rows" = Table.Group(Source, {"Product"}, {{"AllData", each fnPriceDifference(_), type table}}),

    fnPriceDifference = (MyTable as table) as table =>
    let
        #"Sorted Rows" = Table.Buffer(Table.Sort(MyTable,{{"date", Order.Descending}})),
        #"Added Index" = Table.AddIndexColumn(#"Sorted Rows", "Index", 0, 1),
        #"Added Index1" = Table.AddIndexColumn(#"Added Index", "Index.1", 1, 1),
        #"Merged Queries" = Table.NestedJoin(#"Added Index1",{"Index.1"},#"Added Index1",{"Index"},"PreviousDate",JoinKind.LeftOuter),
        #"Expanded PreviousDate" = Table.ExpandTableColumn(#"Merged Queries", "PreviousDate", {"Cost"}, {"PreviousDate.Cost"}),
        #"Added Custom" = Table.AddColumn(#"Expanded PreviousDate", "Difference", each if [PreviousDate.Cost] = null then null else [Cost] - [PreviousDate.Cost]),
        #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Index", "Index.1", "PreviousDate.Cost"})
    in
        #"Removed Columns",

    #"Expanded AllData" = Table.ExpandTableColumn(#"Grouped Rows", "AllData", {"date", "Campaign", "Cost", "id", "Difference"}, {"date", "Campaign", "Cost", "id", "Difference"})
in
    #"Expanded AllData"

Code for 1 product only (just to see all steps):

let
    Source = TestData,
//    #"Grouped Rows" = Table.Group(Source, {"Product"}, {{"AllData", each fnPriceDifference(_), type table}}),
    #"Grouped Rows" = Table.Group(Source, {"Product"}, {{"AllData", each _, type table}}),
    MyTable = #"Grouped Rows"{[Product="A"]}[AllData],

//    fnPriceDifference = (MyTable as table) as table =>
//    let
        #"Sorted Rows" = Table.Buffer(Table.Sort(MyTable,{{"date", Order.Descending}})),
        #"Added Index" = Table.AddIndexColumn(#"Sorted Rows", "Index", 0, 1),
        #"Added Index1" = Table.AddIndexColumn(#"Added Index", "Index.1", 1, 1),
        #"Merged Queries" = Table.NestedJoin(#"Added Index1",{"Index.1"},#"Added Index1",{"Index"},"PreviousDate",JoinKind.LeftOuter),
        #"Expanded PreviousDate" = Table.ExpandTableColumn(#"Merged Queries", "PreviousDate", {"Cost"}, {"PreviousDate.Cost"}),
        #"Added Custom" = Table.AddColumn(#"Expanded PreviousDate", "Difference", each if [PreviousDate.Cost] = null then null else [Cost] - [PreviousDate.Cost]),
        #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Index", "Index.1", "PreviousDate.Cost"})
//    in
//        #"Removed Columns",

//    #"Expanded AllData" = Table.ExpandTableColumn(#"Grouped Rows", "AllData", {"date", "Campaign", "Cost", "id", "Difference"}, {"date", "Campaign", "Cost", "id", "Difference"})
in
//    #"Expanded AllData"
    #"Removed Columns"