handle stock split

internal/filler.go

@@ -9,19 +9,27 @@ import (
 type Filler struct {
 	Record
 
-	filled decimal.Decimal
+	filled   decimal.Decimal
+	quantity decimal.Decimal
+	price    decimal.Decimal
 }
 
 func NewFiller(r Record) *Filler {
 	return &Filler{
-		Record: r,
+		Record:   r,
+		quantity: r.Quantity(),
+		price:    r.Price(),
 	}
 }
 
+func (f *Filler) Quantity() decimal.Decimal { return f.quantity }
+
+func (f *Filler) Price() decimal.Decimal { return f.price }
+
 // Fill accrues some quantity. Returns how mutch was accrued in the 1st return value and whether
 // it was filled or not on the 2nd return value.
 func (f *Filler) Fill(quantity decimal.Decimal) (decimal.Decimal, bool) {
-	unfilled := f.Record.Quantity().Sub(f.filled)
+	unfilled := f.quantity.Sub(f.filled)
 	delta := decimal.Min(unfilled, quantity)
 	f.filled = f.filled.Add(delta)
 	return delta, f.IsFilled()
@@ -29,7 +37,15 @@ func (f *Filler) Fill(quantity decimal.Decimal) (decimal.Decimal, bool) {
 
 // IsFilled returns true if the fill is equal to the record quantity.
 func (f *Filler) IsFilled() bool {
-	return f.filled.Equal(f.Quantity())
+	return f.filled.Equal(f.quantity)
+}
+
+// ApplySplit adjusts the lot for a stock split by the given ratio (newQty/oldQty).
+// The total cost basis is preserved: quantity scales up, price scales down proportionally.
+func (f *Filler) ApplySplit(ratio decimal.Decimal) {
+	f.quantity = f.quantity.Mul(ratio)
+	f.filled = f.filled.Mul(ratio)
+	f.price = f.price.Div(ratio)
 }
 
 type FillerQueue struct {
@@ -86,6 +102,16 @@ func (fq *FillerQueue) frontElement() *list.Element {
 	return fq.l.Front()
 }
 
+// AdjustForSplit applies a stock split ratio to all lots in the queue.
+func (fq *FillerQueue) AdjustForSplit(ratio decimal.Decimal) {
+	if fq == nil || fq.l == nil {
+		return
+	}
+	for e := fq.l.Front(); e != nil; e = e.Next() {
+		e.Value.(*Filler).ApplySplit(ratio)
+	}
+}
+
 // Len returns how many elements are currently on the queue
 func (fq *FillerQueue) Len() int {
 	if fq == nil || fq.l == nil {

internal/filler_test.go

@@ -114,7 +114,7 @@ func TestFillerQueueNilReceiver(t *testing.T) {
 			t.Fatalf(`want panic message %q but got "%v"`, expMsg, r)
 		}
 	}()
-	rq.Push(NewFiller(nil))
+	rq.Push(NewFiller(&testRecord{}))
 }
 
 type testRecord struct {
@@ -122,11 +122,11 @@ type testRecord struct {
 
 	id       int
 	quantity decimal.Decimal
+	price    decimal.Decimal
 }
 
-func (tr testRecord) Quantity() decimal.Decimal {
+func (tr testRecord) Quantity() decimal.Decimal { return tr.quantity }
-	return tr.quantity
+func (tr testRecord) Price() decimal.Decimal    { return tr.price }
-}
 
 func TestFiller_Fill(t *testing.T) {
 	tests := []struct {
@@ -185,3 +185,89 @@ func TestFiller_Fill(t *testing.T) {
 		})
 	}
 }
+
+func TestFiller_ApplySplit(t *testing.T) {
+	tests := []struct {
+		name          string
+		qty           float64
+		price         float64
+		prefilled     float64
+		ratio         float64
+		wantQty       float64
+		wantPrice     float64
+		wantFilled    float64
+		wantCostBasis float64
+	}{
+		{
+			name: "5:1 split on unfilled lot preserves cost basis",
+			qty:  10, price: 100, prefilled: 0, ratio: 5,
+			wantQty: 50, wantPrice: 20, wantFilled: 0, wantCostBasis: 1000,
+		},
+		{
+			name: "5:1 split on partially filled lot",
+			qty:  10, price: 100, prefilled: 4, ratio: 5,
+			wantQty: 50, wantPrice: 20, wantFilled: 20, wantCostBasis: 1000,
+		},
+		{
+			name: "1:2 reverse split on unfilled lot preserves cost basis",
+			qty:  10, price: 100, prefilled: 0, ratio: 0.5,
+			wantQty: 5, wantPrice: 200, wantFilled: 0, wantCostBasis: 1000,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			f := NewFiller(&testRecord{
+				quantity: decimal.NewFromFloat(tt.qty),
+				price:    decimal.NewFromFloat(tt.price),
+			})
+			if tt.prefilled > 0 {
+				f.Fill(decimal.NewFromFloat(tt.prefilled))
+			}
+
+			f.ApplySplit(decimal.NewFromFloat(tt.ratio))
+
+			if !f.Quantity().Equal(decimal.NewFromFloat(tt.wantQty)) {
+				t.Errorf("want quantity %v but got %v", tt.wantQty, f.Quantity())
+			}
+			if !f.Price().Equal(decimal.NewFromFloat(tt.wantPrice)) {
+				t.Errorf("want price %v but got %v", tt.wantPrice, f.Price())
+			}
+			if !f.filled.Equal(decimal.NewFromFloat(tt.wantFilled)) {
+				t.Errorf("want filled %v but got %v", tt.wantFilled, f.filled)
+			}
+			costBasis := f.Quantity().Mul(f.Price())
+			if !costBasis.Equal(decimal.NewFromFloat(tt.wantCostBasis)) {
+				t.Errorf("want cost basis %v but got %v", tt.wantCostBasis, costBasis)
+			}
+		})
+	}
+}
+
+func TestFillerQueue_AdjustForSplit(t *testing.T) {
+	var fq FillerQueue
+	fq.Push(NewFiller(&testRecord{quantity: decimal.NewFromFloat(10), price: decimal.NewFromFloat(100)}))
+	fq.Push(NewFiller(&testRecord{quantity: decimal.NewFromFloat(5), price: decimal.NewFromFloat(200)}))
+
+	fq.AdjustForSplit(decimal.NewFromFloat(5))
+
+	lot1, _ := fq.Pop()
+	if !lot1.Quantity().Equal(decimal.NewFromFloat(50)) {
+		t.Errorf("lot1: want quantity 50 but got %v", lot1.Quantity())
+	}
+	if !lot1.Price().Equal(decimal.NewFromFloat(20)) {
+		t.Errorf("lot1: want price 20 but got %v", lot1.Price())
+	}
+
+	lot2, _ := fq.Pop()
+	if !lot2.Quantity().Equal(decimal.NewFromFloat(25)) {
+		t.Errorf("lot2: want quantity 25 but got %v", lot2.Quantity())
+	}
+	if !lot2.Price().Equal(decimal.NewFromFloat(40)) {
+		t.Errorf("lot2: want price 40 but got %v", lot2.Price())
+	}
+}
+
+func TestFillerQueue_AdjustForSplit_NilReceiver(t *testing.T) {
+	var fq *FillerQueue
+	fq.AdjustForSplit(decimal.NewFromFloat(5)) // must not panic
+}

internal/kind.go

@@ -24,6 +24,7 @@ func (d Kind) String() string {
 }
 
 // Is returns true when k equals o
-func (k Kind) Is(o Kind) bool {
+func (k Kind) Is(o any) bool {
-	return k == o
+	other, ok := o.(Kind)
+	return ok && k == other
 }

internal/report.go

@@ -168,6 +168,9 @@ func processRecord(ctx context.Context, q *FillerQueue, rec Record, sel Selector
 			}
 		}
 
+	case KindSplit:
+		q.AdjustForSplit(rec.Quantity())
+
 	default:
 		return fmt.Errorf("unknown side: %v", rec.Kind())
 	}

internal/trading212/record.go

@@ -18,7 +18,7 @@ import (
 type Record struct {
 	symbol    string
 	timestamp time.Time
-	side      internal.Kind
+	kind      internal.Kind
 	quantity  decimal.Decimal
 	price     decimal.Decimal
 	fees      decimal.Decimal
@@ -45,7 +45,7 @@ func (r Record) AssetCountry() int64 {
 }
 
 func (r Record) Kind() internal.Kind {
-	return r.side
+	return r.kind
 }
 
 func (r Record) Quantity() decimal.Decimal {
@@ -81,34 +81,26 @@ func NewRecordReader(r io.Reader, f *internal.OpenFIGI) *RecordReader {
 }
 
 const (
-	MarketBuy       = "market buy"
+	MarketBuy        = "market buy"
-	MarketSell      = "market sell"
+	MarketSell       = "market sell"
-	LimitBuy        = "limit buy"
+	LimitBuy         = "limit buy"
-	LimitSell       = "limit sell"
+	LimitSell        = "limit sell"
-	StockSplitOpen  = "Stock split open"
+	StockSplitOpen   = "stock split open"
-	StockSplitClose = "Stock split close"
+	StockSplitClose  = "stock split close"
+	StokDistribution = "stock distribution"
 )
 
 func (rr RecordReader) ReadRecord(ctx context.Context) (internal.Record, error) {
+	var splitRec *splitRecord
+
 	for {
 		raw, err := rr.reader.Read()
 		if err != nil {
 			return Record{}, fmt.Errorf("read record: %w", err)
 		}
 
-		var side internal.Kind
+		if strings.ToLower(raw[0]) == "action" {
-		switch strings.ToLower(raw[0]) {
-		case MarketBuy, LimitBuy:
-			side = internal.KindBuy
-		case MarketSell, LimitSell:
-			side = internal.KindSell
-		case StockSplitOpen, StockSplitClose:
-			// TODO: emit a special event that triggers a readjustment of unsold stock
 			continue
-		case "action": // TODO: this is the header, there's probably a better way to handle this
-			continue
-		default:
-			return Record{}, fmt.Errorf("parse record type: %s", raw[0])
 		}
 
 		qant, err := parseDecimal(raw[6])
@@ -141,9 +133,50 @@ func (rr RecordReader) ReadRecord(ctx context.Context) (internal.Record, error)
 			return Record{}, fmt.Errorf("parse record french transaction tax: %w", err)
 		}
 
+		var kind internal.Kind
+		switch strings.ToLower(raw[0]) {
+		case MarketBuy, LimitBuy:
+			kind = internal.KindBuy
+		case MarketSell, LimitSell:
+			kind = internal.KindSell
+		case StockSplitOpen:
+			if splitRec != nil {
+				return nil, fmt.Errorf("split already open")
+			}
+
+			splitRec = &splitRecord{
+				Record: Record{
+					symbol:       raw[2],
+					kind:         internal.KindSplit,
+					quantity:     qant,
+					price:        price,
+					fees:         conversionFee,
+					taxes:        stampDutyTax.Add(frenchTxTax),
+					timestamp:    ts,
+					natureGetter: figiNatureGetter(ctx, rr.figi, raw[2]),
+				},
+			}
+
+			continue
+
+		case StockSplitClose:
+			if splitRec == nil {
+				return nil, fmt.Errorf("missing split open")
+			}
+			splitRec.ratio = splitRec.Record.Quantity().Div(qant)
+			return splitRec, nil
+
+		case StokDistribution:
+			slog.Warn("Found stock distribution but can't handle it")
+			continue
+
+		default:
+			return Record{}, fmt.Errorf("parse record type: %s", raw[0])
+		}
+
 		return Record{
 			symbol:       raw[2],
-			side:         side,
+			kind:         kind,
 			quantity:     qant,
 			price:        price,
 			fees:         conversionFee,
@@ -190,3 +223,13 @@ func parseOptionalDecimal(s string) (decimal.Decimal, error) {
 
 	return parseDecimal(s)
 }
+
+type splitRecord struct {
+	Record
+
+	ratio decimal.Decimal
+}
+
+func (sr splitRecord) Quantity() decimal.Decimal {
+	return sr.ratio
+}

internal/trading212/record_test.go

@@ -30,7 +30,7 @@ func TestRecordReader_ReadRecord(t *testing.T) {
 			r:    bytes.NewBufferString(`Market buy,2025-07-03 10:44:29,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,2.4387014200,7.3690000000,USD,1.17995999,,"EUR",15.25,"EUR",0.25,"EUR",0.02,"EUR",,`),
 			want: Record{
 				symbol:       "XX1234567890",
-				side:         internal.KindBuy,
+				kind:         internal.KindBuy,
 				quantity:     ShouldParseDecimal(t, "2.4387014200"),
 				price:        ShouldParseDecimal(t, "7.3690000000"),
 				timestamp:    time.Date(2025, 7, 3, 10, 44, 29, 0, time.UTC),
@@ -44,7 +44,7 @@ func TestRecordReader_ReadRecord(t *testing.T) {
 			r:    bytes.NewBufferString(`Market sell,2025-08-04 11:45:30,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,2.4387014200,7.9999999999,USD,1.17995999,,"EUR",15.25,"EUR",,,0.02,"EUR",0.1,"EUR"`),
 			want: Record{
 				symbol:       "XX1234567890",
-				side:         internal.KindSell,
+				kind:         internal.KindSell,
 				quantity:     ShouldParseDecimal(t, "2.4387014200"),
 				price:        ShouldParseDecimal(t, "7.9999999999"),
 				timestamp:    time.Date(2025, 8, 4, 11, 45, 30, 0, time.UTC),
@@ -123,8 +123,8 @@ func TestRecordReader_ReadRecord(t *testing.T) {
 				t.Fatalf("want symbol %v but got %v", tt.want.symbol, got.Symbol())
 			}
 
-			if got.Kind() != tt.want.side {
+			if got.Kind() != tt.want.kind {
-				t.Fatalf("want side %v but got %v", tt.want.side, got.Kind())
+				t.Fatalf("want side %v but got %v", tt.want.kind, got.Kind())
 			}
 
 			if got.Price().Cmp(tt.want.price) != 0 {
@@ -154,6 +154,70 @@ func TestRecordReader_ReadRecord(t *testing.T) {
 	}
 }
 
+func TestRecordReader_ReadRecord_Split(t *testing.T) {
+	// open row has the NEW (post-split) position: more shares at lower price
+	// close row has the OLD (pre-split) position: fewer shares at higher price
+	// ratio = openQty / closeQty = 0.5 / 0.1 = 5 (a 5:1 split)
+	splitOpen  := `Stock split open,2025-06-03 05:34:16,XX1234567890,ABXY,"Aspargus Broccoli",EOF111111111,0.5000000000,20.0000000000,EUR,1.00000000,,,10.00,"EUR",,,,,,`
+	splitClose := `Stock split close,2025-06-03 05:34:16,XX1234567890,ABXY,"Aspargus Broccoli",EOF222222222,0.1000000000,100.0000000000,EUR,1.00000000,0.00,"EUR",10.00,"EUR",,,,,,`
+
+	t.Run("well-formed split pair returns split record with correct ratio", func(t *testing.T) {
+		rr := NewRecordReader(
+			bytes.NewBufferString(splitOpen+"\n"+splitClose),
+			NewFigiClientSecurityTypeStub(t, "Common Stock"),
+		)
+
+		got, err := rr.ReadRecord(t.Context())
+		if err != nil {
+			t.Fatalf("ReadRecord() failed: %v", err)
+		}
+
+		if got.Kind() != internal.KindSplit {
+			t.Errorf("want kind %v but got %v", internal.KindSplit, got.Kind())
+		}
+		if got.Symbol() != "NO0013536151" {
+			t.Errorf("want symbol NO0013536151 but got %v", got.Symbol())
+		}
+
+		wantTimestamp := time.Date(2025, 6, 3, 5, 34, 16, 0, time.UTC)
+		if !got.Timestamp().Equal(wantTimestamp) {
+			t.Errorf("want timestamp %v but got %v", wantTimestamp, got.Timestamp())
+		}
+
+		// ratio = openQty / closeQty = 0.1245045 / 0.0249009 ≈ 5
+		openQty := ShouldParseDecimal(t, "0.1245045000")
+		closeQty := ShouldParseDecimal(t, "0.0249009000")
+		wantRatio := openQty.Div(closeQty)
+		if !got.Quantity().Equal(wantRatio) {
+			t.Errorf("want ratio %v but got %v", wantRatio, got.Quantity())
+		}
+	})
+
+	t.Run("close without prior open errors", func(t *testing.T) {
+		rr := NewRecordReader(
+			bytes.NewBufferString(splitClose),
+			NewFigiClientSecurityTypeStub(t, "Common Stock"),
+		)
+
+		_, err := rr.ReadRecord(t.Context())
+		if err == nil {
+			t.Fatal("expected error but got none")
+		}
+	})
+
+	t.Run("two opens without close errors", func(t *testing.T) {
+		rr := NewRecordReader(
+			bytes.NewBufferString(splitOpen+"\n"+splitOpen),
+			NewFigiClientSecurityTypeStub(t, "Common Stock"),
+		)
+
+		_, err := rr.ReadRecord(t.Context())
+		if err == nil {
+			t.Fatal("expected error but got none")
+		}
+	})
+}
+
 func Test_figiNatureGetter(t *testing.T) {
 	tests := []struct {
 		name string // description of this test case