fix: rounding error due to stock change multiplier (#151)

pviotti · Aug 31, 2024 · 12ece83 · 12ece83
1 parent 61fd95f
commit 12ece83
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diff --git a/src/Degiro.Account/Degiro.fs b/src/Degiro.Account/Degiro.fs
@@ -253,8 +253,26 @@ module Account =
                         ISIN = stockChange.IsinAfter
                         Value = x.Value * (decimal stockChange.Multiplier)
                         Quantity = x.Quantity / stockChange.Multiplier })
+                |> List.sortByDescending (fun x -> x.Date)
+
+            // Note: If the total quantity of buys is one unit less than the quantity sold,
+            // then it's due to a rounding error in the stock change multiplier.
+            // So we arbitrarily add one unit to the last buy transaction before the stock change.
+            let totQuantityBuys =
+                buysPrecedingSell @ buysPrecedingSellBeforeStockChange |> List.sumBy _.Quantity
+
+            let buysPrecedingSellBeforeStockChangeRounded =
+                if totQuantityBuys - sellTxn.Quantity = -1 then
+                    buysPrecedingSellBeforeStockChange
+                    |> List.mapi (fun index buy ->
+                        if index = 0 then
+                            { buy with Quantity = buy.Quantity + 1 }
+                        else
+                            buy)
+                else
+                    buysPrecedingSellBeforeStockChange
 
-            buysPrecedingSell @ buysPrecedingSellBeforeStockChange
+            buysPrecedingSell @ buysPrecedingSellBeforeStockChangeRounded
             |> List.sortByDescending (fun x -> x.Date)
         else
             buysPrecedingSell
@@ -271,7 +289,8 @@ module Account =
             if quantityToSell = 0 then
                 totBuyPrice
             elif List.isEmpty buys && quantityToSell <> 0 then // Should not happen
-                failwithf $"could not find buy transactions for remaining {quantityToSell} sells of %A{sellTxn}"
+                failwithf
+                    $"could not find buy transactions for remaining {quantityToSell} stock unit(s) sold as part of %A{sellTxn}"
             else
                 let currBuy = List.head buys
 

diff --git a/test/AccountTests.fs b/test/AccountTests.fs
@@ -499,6 +499,70 @@ module AccountTests =
         getSellsEarnings sellTxns txns splits |> should equal [ expectedEarning ]
 
 
+    [<Test>]
+    let ``Get earnings of a stock that had a split with ISIN change and multiplier rounding error`` () =
+        let testRows =
+            header
+            + """
+        29-07-2024,15:30,29-07-2024,ACME INC,NEWISIN12345,FX Debit,1.0842,USD,-9.02,USD,-0.01,96908937-369f-46fe-be23-d9807ad1550d
+        29-07-2024,15:30,29-07-2024,ACME INC,NEWISIN12345,FX Credit,,EUR,8.31,EUR,4364.27,96908937-369f-46fe-be23-d9807ad1550d
+        29-07-2024,15:30,29-07-2024,ACME INC,NEWISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-2.00,EUR,4355.96,96908937-369f-46fe-be23-d9807ad1550d
+        29-07-2024,15:30,29-07-2024,ACME INC,NEWISIN12345,Sell 25 ACME Inc@0.3606 USD (NEWISIN12345),,USD,9.02,USD,9.01,96908937-369f-46fe-be23-d9807ad1550d
+        04-04-2023,14:34,04-04-2023,ACME INC,NEWISIN12345,STOCK SPLIT: Buy 25 ACME Inc@4.8765 USD (NEWISIN12345),,USD,-121.91,USD,4.84,
+        04-04-2023,14:34,04-04-2023,ACME INC,OLDISIN12345,STOCK SPLIT: Sell 375 ACME Inc@0.3251 USD (OLDISIN12345),,USD,121.91,USD,126.75,
+        14-05-2021,15:54,14-05-2021,ACME INC,OLDISIN12345,FX Credit,1.2133,USD,460.70,USD,0.00,019fe8b6-772d-4fe4-89fd-1031045a3679
+        14-05-2021,15:54,14-05-2021,ACME INC,OLDISIN12345,FX Debit,,EUR,-379.71,EUR,1016.46,019fe8b6-772d-4fe4-89fd-1031045a3679
+        14-05-2021,15:54,14-05-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.56,EUR,1396.17,019fe8b6-772d-4fe4-89fd-1031045a3679
+        14-05-2021,15:54,14-05-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.50,EUR,1396.73,019fe8b6-772d-4fe4-89fd-1031045a3679
+        14-05-2021,15:54,14-05-2021,ACME INC,OLDISIN12345,Buy 170 ACME Inc@2.71 USD (OLDISIN12345),,USD,-460.70,USD,-460.70,019fe8b6-772d-4fe4-89fd-1031045a3679
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,FX Credit,1.1757,USD,165.25,USD,0.60,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,FX Debit,,EUR,-140.55,EUR,1258.17,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.17,EUR,1398.72,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,FX Credit,1.1757,USD,99.15,USD,-164.65,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,FX Debit,,EUR,-84.33,EUR,1398.89,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.10,EUR,1483.22,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.50,EUR,1483.32,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,Buy 50 ACME Inc@3.305 USD (OLDISIN12345),,USD,-165.25,USD,-263.80,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        25-03-2021,18:37,25-03-2021,ACME INC,OLDISIN12345,Buy 30 ACME Inc@3.305 USD (OLDISIN12345),,USD,-99.15,USD,-98.55,e4ec296d-8bb9-47ee-80d8-c230139a7f34
+        21-01-2021,15:35,21-01-2021,ACME INC,OLDISIN12345,FX Credit,1.2136,USD,612.50,USD,-0.51,d16a014f-cdb4-4a5d-b8ed-5dcdd0362c31
+        21-01-2021,15:35,21-01-2021,ACME INC,OLDISIN12345,FX Debit,,EUR,-504.70,EUR,217.95,d16a014f-cdb4-4a5d-b8ed-5dcdd0362c31
+        21-01-2021,15:35,21-01-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.41,EUR,722.65,d16a014f-cdb4-4a5d-b8ed-5dcdd0362c31
+        21-01-2021,15:35,21-01-2021,ACME INC,OLDISIN12345,DEGIRO Transaction and/or third party fees,,EUR,-0.50,EUR,723.06,d16a014f-cdb4-4a5d-b8ed-5dcdd0362c31
+        21-01-2021,15:35,21-01-2021,ACME INC,OLDISIN12345,Buy 125 ACME Inc@4.9 USD (OLDISIN12345),,USD,-612.50,USD,-613.01,d16a014f-cdb4-4a5d-b8ed-5dcdd0362c31"""
+
+        // Here we have:
+        // 125 + 30 + 50 + 170 = 375 shares bought
+        // then a stock split 15:1
+        // then 25 shares sold.
+
+        // If we use the multiplier of the stock split to compute the quantity of buys, we get:
+        // (125/15) + (30/15) + (50/15) + (170/15) = 8.33333 + 2 + 3.33333 + 11.33333 = 24.99999 shares bought
+        // which is different from the 25 shares sold.
+        // Hence we need to round up to the next unit the quantity of one of the buy transactions.
+
+        let rows = AccountCsv.Parse(testRows).Rows
+        let txnsGrouped = getRowsGroupedByOrderId rows
+
+        let txns = Seq.map buildTxn txnsGrouped |> Seq.toList
+        txns |> should haveLength 4
+
+        let sellTxns = getSellTxnsInPeriod txns 2024 Period.All
+        let splits = getStockChanges rows
+        splits |> should haveCount 1
+
+        let totCostBuys = 504.7m + 84.33m + 140.55m + 379.71m
+
+        let expectedEarning =
+            { Date = DateTime(2024, 7, 29, 15, 30, 0)
+              Product = "ACME INC"
+              ISIN = "NEWISIN12345"
+              ProdType = Shares
+              Value = 8.31m - totCostBuys
+              Percent = Math.Round(((8.31m - totCostBuys) / totCostBuys) * 100.0m, 2) }
+
+        getSellsEarnings sellTxns txns splits |> should equal [ expectedEarning ]
+
+
     [<Test>]
     let ``Get earnings of a stock that had an ISIN change`` () =
         let testRows =