ledger.Transaction.Payee()

src/ledger/balances.go

@@ -37,9 +37,9 @@ func (balances Balances) Group(pattern string) Balances {
 	result := make(Balances)
 	p := regexp.MustCompile(pattern)
 	for k, v := range balances {
-		k2 := p.FindString(k)
-		if k2 == "" {
-			k2 = k
+		k2 := k
+		if p.MatchString(k) {
+			k2 = p.FindString(k)
 		}
 
 		was := result[k2]

src/ledger/transaction.go

@@ -39,6 +39,35 @@ func (deltas Deltas) Transactions() Transactions {
 	return result
 }
 
+func (transaction Transaction) Payee() string {
+	balances := Deltas(transaction).Balances()
+
+	candidates := []string{}
+
+	for name, balance := range balances {
+		everyoneElse := balances.NotLike(`^` + name + `$`).Group(`^`)[""]
+		matches := true
+		for currency, value := range balance {
+			matches = matches && everyoneElse[currency]*-1 == value
+		}
+		if matches {
+			candidates = append(candidates, name)
+		}
+	}
+	slices.Sort(candidates)
+
+	if len(candidates) == 0 {
+		panic(balances)
+	}
+
+	for _, candidate := range candidates {
+		if strings.HasPrefix(candidate, "Withdrawal") {
+			return candidate
+		}
+	}
+	return candidates[len(candidates)-1]
+}
+
 type transaction struct {
 	date        string
 	description string

src/ledger/transaction_test.go

@@ -8,6 +8,19 @@ import (
 	"testing"
 )
 
+func TestTransactionPayee(t *testing.T) {
+	given := Transaction{
+		Delta{Name: "x", Transaction: "a", Value: 1},
+		Delta{Name: "y", Transaction: "a", Value: 2},
+		Delta{Name: "Withdrawal:z", Transaction: "a", Value: -3},
+	}
+
+	got := given.Payee()
+	if got != "Withdrawal:z" {
+		t.Error(got)
+	}
+}
+
 func TestDeltasTransactions(t *testing.T) {
 	given := Deltas{
 		Delta{Date: "2", Name: "x", Transaction: "a"},