internal/trading212/README.md

@@ -0,0 +1,50 @@
+# Trading212
+
+Turns Trading212 account statements (CSV) into the internal record model used
+to build Anexo J reports. This file records how statement events are
+interpreted, and why.
+
+## Spin-off events
+
+A spin-off distributes shares of one company to the shareholders of another.
+Trading212 reports it as an `Action` of `Spin off`:
+
+```
+Spin off,2026-02-11 13:41:43+00:00,US9418481035,WAT,"Waters",EOF46451921527,0.0406450000,0E-10,USD,,,,0.00,"EUR",,,,,,
+```
+
+These rows are booked as **buys with a cost of zero**: quantity and timestamp
+are kept, and every cost column (price, result, total, stamp duty reserve tax,
+currency conversion fee, French transaction tax) is required to be zero —
+anything non-zero, or a non-positive quantity, is logged and returned as an
+error, stopping the run. A row that surprises the reader must be looked at by
+a human, not silently booked.
+
+### Why cost zero is fair
+
+No money was paid for the new shares, and their value came out of the value
+of the parent shares — on the distribution date the parent's price drops by
+roughly the distributed value. The position's total basis is unchanged; it is
+just partitioned over two lots: the spin-off lot at `0` and the parent lot at
+its original, unreduced basis. Selling the spin-off lot books full proceeds as
+gain; selling the parent lot shows the offsetting smaller gain or bigger loss.
+Netted together, the result equals the true economic gain.
+
+Example — buy 100 parent shares for 10,000; a spin-off delivers new shares
+worth 2,000; later sell the new shares for 2,200 and the parent for 8,100:
+
+| Lot    | Basis (cost 0) | Sold for | Gain (cost 0) | Gain (pro-rata) |
+| ------ | -------------: | -------: | ------------: | --------------: |
+| New    | 0              | 2,200    | 2,200         | 200             |
+| Parent | 10,000         | 8,100    | -1,900        | 100             |
+| Total  |                |          | **300**       | **300**         |
+
+Either partition of the same 10,000 basis yields the same net gain of 300
+(true gain: 10,300 received − 10,000 paid) — as long as gains and losses are
+assessed together, as they are in Anexo J within the same year. If the two
+lots are sold in different years the per-year attribution differs (the total
+does not). Trading212 statements don't report a pro-rata basis allocation, so
+cost 0 also keeps the report reproducible from the statement alone.
+
+> [!WARNING]
+> Verify any outputs of this tool on your own or with a certified accountant.

internal/trading212/record.go

@@ -92,6 +92,10 @@ const (
 	SpinOff          = "spin off"
 )
 
+// ReadRecord reads the next record from the statement, dispatching each
+// action to the parser that validates exactly the columns that matter for
+// it. Split open/close rows are paired up here because the pairing spans
+// rows: the ratio is only known once the close row is seen.
 func (rr RecordReader) ReadRecord(ctx context.Context) (internal.Record, error) {
 	var splitRec *splitRecord
 
@@ -101,13 +105,89 @@ func (rr RecordReader) ReadRecord(ctx context.Context) (internal.Record, error)
 			return Record{}, fmt.Errorf("read record: %w", err)
 		}
 
-		if strings.ToLower(raw[0]) == "action" {
+		action := strings.ToLower(raw[0])
+		if action == "action" { // header row
 			continue
 		}
 
-		symbol := raw[2]
+		switch action {
+		case MarketBuy, LimitBuy:
+			return rr.parseBuyRecord(ctx, raw)
 
-		qant, err := parseDecimal(raw[6])
+		case MarketSell, LimitSell:
+			return rr.parseSellRecord(ctx, raw)
+
+		case SpinOff:
+			return rr.parseSpinOff(ctx, raw)
+
+		case StockSplitOpen:
+			if splitRec != nil {
+				return nil, fmt.Errorf("split already open")
+			}
+
+			rec, err := rr.parseTradeRecord(ctx, raw)
+			if err != nil {
+				return nil, err
+			}
+			rec.kind = internal.KindSplit
+			splitRec = &splitRecord{Record: rec}
+			continue
+
+		case StockSplitClose:
+			if splitRec == nil {
+				return nil, fmt.Errorf("missing split open")
+			}
+
+			closeQty, err := parsePositiveDecimal(raw[6])
+			if err != nil {
+				return nil, fmt.Errorf("parse record quantity: %w", err)
+			}
+			splitRec.ratio = splitRec.Record.Quantity().Div(closeQty)
+			return splitRec, nil
+
+		case StokDistribution:
+			qant, err := parsePositiveDecimal(raw[6])
+			if err != nil {
+				return Record{}, fmt.Errorf("parse record quantity: %w", err)
+			}
+			slog.Warn("Skipping 'stock distribution' event!",
+				slog.Any("ISIN", raw[2]),
+				slog.Any("Quantity", qant),
+			)
+			continue
+
+		default:
+			return Record{}, fmt.Errorf("parse record type: %s", raw[0])
+		}
+	}
+}
+
+// parseBuyRecord parses a market/limit buy row into a KindBuy Record.
+func (rr RecordReader) parseBuyRecord(ctx context.Context, raw []string) (Record, error) {
+	rec, err := rr.parseTradeRecord(ctx, raw)
+	if err != nil {
+		return Record{}, err
+	}
+	rec.kind = internal.KindBuy
+	return rec, nil
+}
+
+// parseSellRecord parses a market/limit sell row into a KindSell Record.
+func (rr RecordReader) parseSellRecord(ctx context.Context, raw []string) (Record, error) {
+	rec, err := rr.parseTradeRecord(ctx, raw)
+	if err != nil {
+		return Record{}, err
+	}
+	rec.kind = internal.KindSell
+	return rec, nil
+}
+
+// parseTradeRecord parses the columns shared by trade-shaped rows (quantity,
+// price, timestamp, fees and taxes) into a Record with an unset Kind,
+// leaving the kind decision to the caller. Split-open rows reuse it because
+// their row shape is identical.
+func (rr RecordReader) parseTradeRecord(ctx context.Context, raw []string) (Record, error) {
+	qant, err := parsePositiveDecimal(raw[6])
 	if err != nil {
 		return Record{}, fmt.Errorf("parse record quantity: %w", err)
 	}
@@ -137,62 +217,81 @@ 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, SpinOff:
-			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:       symbol,
-					kind:         internal.KindSplit,
+	return Record{
+		symbol:       raw[2],
 		quantity:     qant,
 		price:        price,
 		fees:         conversionFee,
 		taxes:        stampDutyTax.Add(frenchTxTax),
 		timestamp:    ts,
-					natureGetter: figiNatureGetter(ctx, rr.figi, symbol),
-				},
+		natureGetter: figiNatureGetter(ctx, rr.figi, raw[2]),
+	}, nil
 }
 
-			continue
+// parseSpinOff books a 'Spin off' statement row as a zero-cost buy. The
+// zero-cost interpretation is only sound when no money moved with the
+// distribution — see README.md in this package for the rationale — so every
+// cost column is parsed as optional (an empty column means zero) and
+// required to be zero: anything unexpected is logged and returned as an
+// error so the run stops instead of silently booking a wrong lot.
+func (rr RecordReader) parseSpinOff(ctx context.Context, raw []string) (Record, error) {
+	symbol := raw[2]
 
-		case StockSplitClose:
-			if splitRec == nil {
-				return nil, fmt.Errorf("missing split open")
+	qant, err := parsePositiveDecimal(raw[6])
+	if err != nil {
+		return Record{}, fmt.Errorf("parse record quantity: %w", err)
 	}
-			splitRec.ratio = splitRec.Record.Quantity().Div(qant)
-			return splitRec, nil
 
-		case StokDistribution:
-			slog.Warn("Skipping 'stock distribution' event!",
+	ts, err := parseTimestamp(raw[1])
+	if err != nil {
+		return Record{}, fmt.Errorf("parse record timestamp: %w", err)
+	}
+
+	// A slice (not a map) keeps validation and error reporting deterministic.
+	// result and total are only ever read here, for this row kind.
+	costs := []struct {
+		name   string
+		column int
+	}{
+		{"price", 7},
+		{"result", 10},
+		{"total", 12},
+		{"stamp duty reserve tax", 14},
+		{"currency conversion fee", 16},
+		{"french transaction tax", 18},
+	}
+	for _, cost := range costs {
+		value, err := parseOptionalDecimal(raw[cost.column])
+		if err != nil {
+			return Record{}, fmt.Errorf("parse record %s: %w", cost.name, err)
+		}
+		if !value.IsZero() {
+			reason := fmt.Errorf("spin off with a non-zero %s (%s) for ISIN %s: refusing to book it as a zero-cost buy",
+				cost.name, value, symbol)
+			slog.Error("Unexpected 'spin off' row!",
+				slog.Any("err", reason),
+				slog.Any("ISIN", symbol),
+			)
+			return Record{}, reason
+		}
+	}
+
+	slog.Info("Booking 'spin off' event as a zero-cost buy!",
 		slog.Any("ISIN", symbol),
 		slog.Any("Quantity", qant),
 	)
-			continue
-
-		default:
-			return Record{}, fmt.Errorf("parse record type: %s", raw[0])
-		}
 
 	return Record{
 		symbol:       symbol,
-			kind:         kind,
+		kind:         internal.KindBuy,
 		quantity:     qant,
-			price:        price,
-			fees:         conversionFee,
-			taxes:        stampDutyTax.Add(frenchTxTax),
+		price:        decimal.Zero,
+		fees:         decimal.Zero,
+		taxes:        decimal.Zero,
 		timestamp:    ts,
 		natureGetter: figiNatureGetter(ctx, rr.figi, symbol),
 	}, nil
 }
-}
 
 func figiNatureGetter(ctx context.Context, of *ofigi.Client, isin string) func() internal.Nature {
 	return sync.OnceValue(func() internal.Nature {
@@ -231,6 +330,20 @@ func parseOptionalDecimal(s string) (decimal.Decimal, error) {
 	return parseDecimal(s)
 }
 
+// parsePositiveDecimal parses s as a decimal and requires it to be strictly
+// positive: empty, malformed, zero and negative values are rejected. A lot
+// (or a split-ratio denominator) is meaningless without a positive quantity.
+func parsePositiveDecimal(s string) (decimal.Decimal, error) {
+	value, err := parseDecimal(s)
+	if err != nil {
+		return decimal.Decimal{}, err
+	}
+	if !value.IsPositive() {
+		return decimal.Decimal{}, fmt.Errorf("non-positive value %q", s)
+	}
+	return value, nil
+}
+
 // parseTimestamp parses a Trading212 timestamp string. Trading212 has shipped
 // rows both without a timezone (treated here as UTC) and with an explicit
 // numeric offset (e.g. "+00:00", "-05:00", "+01:00") or a "Z" UTC marker.

internal/trading212/record_test.go

@@ -138,6 +138,69 @@ func TestRecordReader_ReadRecord(t *testing.T) {
 			r:       bytes.NewBufferString(`Market buy,not-a-timestamp,IE000GA3D489,ABXY,"Aspargus Broccoli",EOF987654321,2.4387014200,7.3690000000,USD,1.17995999,,"EUR",15.25,"EUR",0.25,"EUR",0.02,"EUR",,`),
 			wantErr: true,
 		},
+		{
+			name: "spin off booked as zero-cost buy",
+			r:    bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,0E-10,USD,,,,0.00,"EUR",,,,,,`),
+			want: Record{
+				symbol:       "XX1234567890",
+				kind:         internal.KindBuy,
+				quantity:     ShouldParseDecimal(t, "0.0406450000"),
+				price:        decimal.Zero,
+				fees:         decimal.Zero,
+				taxes:        decimal.Zero,
+				timestamp:    time.Date(2026, 2, 11, 13, 41, 43, 0, time.FixedZone("UTC", 0)),
+				natureGetter: func() internal.Nature { return internal.NatureG01 },
+			},
+		},
+		{
+			name:    "spin off with a non-zero price is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,45.2300000000,USD,,,,0.00,"EUR",,,,,,`),
+			wantErr: true,
+		},
+		{
+			name:    "spin off with a non-zero result is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,0E-10,USD,1.17995999,1.25,"EUR",0.00,"EUR",,,,,,`),
+			wantErr: true,
+		},
+		{
+			name:    "spin off with a non-zero total is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,0E-10,USD,,,,1.25,"EUR",,,,,,`),
+			wantErr: true,
+		},
+		{
+			name:    "spin off with a non-zero stamp duty reserve tax is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,0E-10,USD,,,,0.00,"EUR",0.25,"EUR",,,,,`),
+			wantErr: true,
+		},
+		{
+			name:    "spin off with a non-zero currency conversion fee is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,0E-10,USD,,,,0.00,"EUR",,,0.02,"EUR",,`),
+			wantErr: true,
+		},
+		{
+			name:    "spin off with a non-zero french transaction tax is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,0E-10,USD,,,,0.00,"EUR",,,,,0.1,"EUR"`),
+			wantErr: true,
+		},
+		{
+			name:    "spin off with a non-positive quantity is rejected",
+			r:       bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0E-10,0E-10,USD,,,,0.00,"EUR",,,,,,`),
+			wantErr: true,
+		},
+		{
+			name: "spin off with an empty price column is booked as a zero-cost buy",
+			r:    bytes.NewBufferString(`Spin off,2026-02-11 13:41:43+00:00,XX1234567890,ABXY,"Aspargus Broccoli",EOF987654321,0.0406450000,,USD,,,,0.00,"EUR",,,,,,`),
+			want: Record{
+				symbol:       "XX1234567890",
+				kind:         internal.KindBuy,
+				quantity:     ShouldParseDecimal(t, "0.0406450000"),
+				price:        decimal.Zero,
+				fees:         decimal.Zero,
+				taxes:        decimal.Zero,
+				timestamp:    time.Date(2026, 2, 11, 13, 41, 43, 0, time.FixedZone("UTC", 0)),
+				natureGetter: func() internal.Nature { return internal.NatureG01 },
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -377,10 +440,6 @@ func TestRecordReader_ReadRecord_SkipsCorporateActions(t *testing.T) {
 		name string
 		row  string
 	}{
-		{
-			name: "spin off",
-			row:  `Spin off,2026-02-11 13:41:43+00:00,US9418481035,WAT,"Waters",EOF46451921527,0.0406450000,0E-10,USD,,,,0.00,"EUR",,,,,,`,
-		},
 		{
 			name: "stock distribution",
 			row:  `Stock distribution,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",,`,