i am so pleased

ana/predictor.go

@@ -39,9 +39,26 @@ func NewInterestPredictor(namePattern string, currencyPattern string, apy float6
 
 func NewContributionPredictor(reg ledger.Register) Predictor {
 	monthlyRate := getMonthlyContributionRates(reg)
-	_ = monthlyRate
+	return newContributionPredictor(monthlyRate)
+}
+
+func newContributionPredictor(monthlyRate map[string]ledger.Balance) Predictor {
 	return func(given ledger.Balances, delta time.Duration) ledger.Balances {
-		panic(nil)
+		month := time.Hour * 24 * 365 / 12
+		months := float64(delta) / float64(month)
+		result := make(ledger.Balances)
+		for k, v := range given {
+			result[k] = maps.Clone(v)
+		}
+		for name := range monthlyRate {
+			if _, ok := result[name]; !ok {
+				result[name] = make(ledger.Balance)
+			}
+			for currency := range monthlyRate[name] {
+				result[name][currency] += monthlyRate[name][currency] * months
+			}
+		}
+		return result
 	}
 }
 

ana/predictor_test.go

@@ -105,3 +105,33 @@ func TestGetMonthlyContributionRate(t *testing.T) {
 		t.Error(got["y"])
 	}
 }
+
+func TestNewContributionPredictor(t *testing.T) {
+	name := "x"
+	currency := ledger.USD
+	predictor := newContributionPredictor(map[string]ledger.Balance{
+		name: {currency: 10},
+		"y":  {"XYZ": 3},
+	})
+	month := time.Hour * 24 * 365 / 12
+
+	if got := predictor(ledger.Balances{}, 2*month); got[name][currency] != 20 {
+		t.Error(got[name])
+	} else if got["y"]["XYZ"] != 6 {
+		t.Error(got["y"])
+	}
+
+	if got := predictor(ledger.Balances{name: {currency: 30}}, 2*month); got[name][currency] != 30+20 {
+		t.Error(got)
+	} else if got["y"]["XYZ"] != 6 {
+		t.Error(got["y"])
+	}
+
+	if got := predictor(ledger.Balances{"z": {"ABC": 100}}, 2*month); got[name][currency] != 20 {
+		t.Error(got)
+	} else if got["y"]["XYZ"] != 6 {
+		t.Error(got["y"])
+	} else if got["z"]["ABC"] != 100 {
+		t.Error(got["z"])
+	}
+}