1 files changed, 120 insertions, 0 deletions
diff --git a/src/checkout/money/money.go b/src/checkout/money/money.go
new file mode 100644
index 0000000..ba21cd8
--- /dev/null
+++ b/src/checkout/money/money.go
@@ -0,0 +1,120 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+package money
+
+import (
+	"errors"
+
+	pb "github.com/open-telemetry/opentelemetry-demo/src/checkout/genproto/oteldemo"
+)
+
+const (
+	nanosMin = -999999999
+	nanosMax = +999999999
+	nanosMod = 1000000000
+)
+
+var (
+	ErrInvalidValue        = errors.New("one of the specified money values is invalid")
+	ErrMismatchingCurrency = errors.New("mismatching currency codes")
+)
+
+// IsValid checks if specified value has a valid units/nanos signs and ranges.
+func IsValid(m *pb.Money) bool {
+	return signMatches(m) && validNanos(m.GetNanos())
+}
+
+func signMatches(m *pb.Money) bool {
+	return m.GetNanos() == 0 || m.GetUnits() == 0 || (m.GetNanos() < 0) == (m.GetUnits() < 0)
+}
+
+func validNanos(nanos int32) bool { return nanosMin <= nanos && nanos <= nanosMax }
+
+// IsZero returns true if the specified money value is equal to zero.
+func IsZero(m *pb.Money) bool { return m.GetUnits() == 0 && m.GetNanos() == 0 }
+
+// IsPositive returns true if the specified money value is valid and is
+// positive.
+func IsPositive(m *pb.Money) bool {
+	return IsValid(m) && m.GetUnits() > 0 || (m.GetUnits() == 0 && m.GetNanos() > 0)
+}
+
+// IsNegative returns true if the specified money value is valid and is
+// negative.
+func IsNegative(m *pb.Money) bool {
+	return IsValid(m) && m.GetUnits() < 0 || (m.GetUnits() == 0 && m.GetNanos() < 0)
+}
+
+// AreSameCurrency returns true if values l and r have a currency code and
+// they are the same values.
+func AreSameCurrency(l, r *pb.Money) bool {
+	return l.GetCurrencyCode() == r.GetCurrencyCode() && l.GetCurrencyCode() != ""
+}
+
+// AreEquals returns true if values l and r are the equal, including the
+// currency. This does not check validity of the provided values.
+func AreEquals(l, r *pb.Money) bool {
+	return l.GetCurrencyCode() == r.GetCurrencyCode() &&
+		l.GetUnits() == r.GetUnits() && l.GetNanos() == r.GetNanos()
+}
+
+// Negate returns the same amount with the sign negated.
+func Negate(m *pb.Money) *pb.Money {
+	return &pb.Money{
+		Units:        -m.GetUnits(),
+		Nanos:        -m.GetNanos(),
+		CurrencyCode: m.GetCurrencyCode()}
+}
+
+// Must panics if the given error is not nil. This can be used with other
+// functions like: "m := Must(Sum(a,b))".
+func Must(v *pb.Money, err error) *pb.Money {
+	if err != nil {
+		panic(err)
+	}
+	return v
+}
+
+// Sum adds two values. Returns an error if one of the values are invalid or
+// currency codes are not matching (unless currency code is unspecified for
+// both).
+func Sum(l, r *pb.Money) (*pb.Money, error) {
+	if !IsValid(l) || !IsValid(r) {
+		return &pb.Money{}, ErrInvalidValue
+	} else if l.GetCurrencyCode() != r.GetCurrencyCode() {
+		return &pb.Money{}, ErrMismatchingCurrency
+	}
+	units := l.GetUnits() + r.GetUnits()
+	nanos := l.GetNanos() + r.GetNanos()
+
+	if (units == 0 && nanos == 0) || (units > 0 && nanos >= 0) || (units < 0 && nanos <= 0) {
+		// same sign <units, nanos>
+		units += int64(nanos / nanosMod)
+		nanos = nanos % nanosMod
+	} else {
+		// different sign. nanos guaranteed to not to go over the limit
+		if units > 0 {
+			units--
+			nanos += nanosMod
+		} else {
+			units++
+			nanos -= nanosMod
+		}
+	}
+
+	return &pb.Money{
+		Units:        units,
+		Nanos:        nanos,
+		CurrencyCode: l.GetCurrencyCode()}, nil
+}
+
+// MultiplySlow is a slow multiplication operation done through adding the value
+// to itself n-1 times.
+func MultiplySlow(m *pb.Money, n uint32) *pb.Money {
+	out := m
+	for n > 1 {
+		out = Must(Sum(out, m))
+		n--
+	}
+	return out
+}