handle stock split

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 {
-	return tr.quantity
-}
+func (tr testRecord) Quantity() decimal.Decimal { 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
+}