feat(split): add payment splitting among recipients

internal/split/split.go

@@ -0,0 +1,46 @@
+// Package split allocates a payment amount among multiple recipients.
+package split
+
+import "fmt"
+
+// Recipient receives a share of a payment, expressed in basis points
+// (1/100th of a percent). Shares across all recipients must sum to 10000.
+type Recipient struct {
+	ID    string
+	Share int
+}
+
+// Allocation is the amount, in the smallest currency unit, owed to a
+// recipient after a split.
+type Allocation struct {
+	RecipientID string
+	Amount      int64
+}
+
+// Split divides amount among recipients according to their basis-point
+// shares. Any remainder from integer division is assigned to the last
+// recipient so the allocations always sum to amount exactly.
+func Split(amount int64, recipients []Recipient) ([]Allocation, error) {
+	if len(recipients) == 0 {
+		return nil, fmt.Errorf("split: no recipients")
+	}
+
+	var total int
+	for _, r := range recipients {
+		total += r.Share
+	}
+	if total != 10000 {
+		return nil, fmt.Errorf("split: shares must sum to 10000 basis points, got %d", total)
+	}
+
+	allocations := make([]Allocation, len(recipients))
+	var allocated int64
+	for i, r := range recipients {
+		share := amount * int64(r.Share) / 10000
+		allocations[i] = Allocation{RecipientID: r.ID, Amount: share}
+		allocated += share
+	}
+	allocations[len(allocations)-1].Amount += amount - allocated
+
+	return allocations, nil
+}