ledger balances helpers like Sub, Sum, Invert

cmd/cli/main.go

@@ -4,7 +4,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"maps"
 	"os"
 	"slices"
 
@@ -101,14 +100,7 @@ func Main() {
 		for _, date := range register.Dates() {
 			balances := register[date]
 
-			newBalances := make(ledger.Balances)
+			if newBalances := balances.Sub(prev).Nonzero(); len(newBalances) > 0 {
-			for k, v := range balances {
-				if got := prev[k]; !maps.Equal(v, got) {
-					newBalances[k] = v
-				}
-			}
-
-			if len(newBalances) > 0 {
 				fmt.Println(date)
 				FPrintBalances(os.Stdout, newBalances)
 			}

src/ledger/balances.go

@@ -11,6 +11,30 @@ type Balances map[string]Balance
 
 type Balance map[Currency]float64
 
+func (balances Balances) Sub(other Balances) Balances {
+	result := make(Balances)
+	for k, v := range balances {
+		result[k] = v.Sub(other[k])
+	}
+	for k, v := range other {
+		if _, ok := balances[k]; ok {
+			continue
+		}
+		result[k] = v.Invert()
+	}
+	return result
+}
+
+func (balances Balances) Nonzero() Balances {
+	result := make(Balances)
+	for k, v := range balances {
+		if nonzero := v.Nonzero(); len(nonzero) > 0 {
+			result[k] = nonzero
+		}
+	}
+	return result
+}
+
 func (balances Balances) NotLike(pattern string) Balances {
 	result := make(Balances)
 	p := regexp.MustCompile(pattern)
@@ -98,6 +122,39 @@ func (balances Balances) Push(d Delta) {
 	balances[d.Name].Push(d)
 }
 
+func (balance Balance) Sub(other Balance) Balance {
+	return balance.Sum(other.Invert())
+}
+
+func (balance Balance) Sum(other Balance) Balance {
+	result := make(Balance)
+	for k, v := range balance {
+		result[k] += v
+	}
+	for k, v := range other {
+		result[k] += v
+	}
+	return result
+}
+
+func (balance Balance) Nonzero() Balance {
+	result := make(Balance)
+	for k, v := range balance {
+		if v != 0 {
+			result[k] = v
+		}
+	}
+	return result
+}
+
+func (balance Balance) Invert() Balance {
+	result := make(Balance)
+	for k, v := range balance {
+		result[k] = v * -1.0
+	}
+	return result
+}
+
 func (balance Balance) Push(d Delta) {
 	if _, ok := balance[d.Currency]; !ok {
 		balance[d.Currency] = 0