From 43300547ae02d701d16915a3aab2e4979d60e881 Mon Sep 17 00:00:00 2001
From: James Harris <contact@jamesharris.id.au>
Date: Tue, 8 Feb 2022 11:52:08 +1000
Subject: [PATCH] Add `protomoney.Fmt()`.

---
 CHANGELOG.md              | 15 +++++++++++---
 protomoney/format.go      | 40 ++++++++++++++++++++++++++++++++++++
 protomoney/format_test.go | 43 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 95 insertions(+), 3 deletions(-)
 create mode 100644 protomoney/format.go
 create mode 100644 protomoney/format_test.go

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3838164..cea8560 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,15 +6,24 @@ The format is based on [Keep a Changelog], and this project adheres to
 [Semantic Versioning].
 
 <!-- references -->
-[Keep a Changelog]: https://keepachangelog.com/en/1.0.0/
-[Semantic Versioning]: https://semver.org/spec/v2.0.0.html
+
+[keep a changelog]: https://keepachangelog.com/en/1.0.0/
+[semantic versioning]: https://semver.org/spec/v2.0.0.html
+
+## [Unreleased]
+
+### Added
+
+- Add `protomoney.Fmt()`
+- Add original Google `money.proto` file to `protomoney` package
 
 ## [0.1.0] - 2021-06-14
 
 - Initial release
 
 <!-- references -->
-[Unreleased]: https://github.com/dogmatiq/dosh
+
+[unreleased]: https://github.com/dogmatiq/dosh
 [0.1.0]: https://github.com/dogmatiq/dogma/releases/tag/v0.1.0
 
 <!-- version template
diff --git a/protomoney/format.go b/protomoney/format.go
new file mode 100644
index 0000000..b72dfec
--- /dev/null
+++ b/protomoney/format.go
@@ -0,0 +1,40 @@
+package protomoney
+
+import (
+	"fmt"
+	"math/big"
+
+	"google.golang.org/genproto/googleapis/type/money"
+)
+
+// Fmt wraps a money value in a formatter allows it to be formatted using
+// standard fmt.Printf() verbs.
+func Fmt(m *money.Money) fmt.Formatter {
+	return formatter{m}
+}
+
+type formatter struct {
+	value *money.Money
+}
+
+func (f formatter) Format(st fmt.State, verb rune) {
+	m := normalize(f.value)
+
+	switch verb {
+	case 'f', 'F', // decimal notation
+		'e', 'E', // scientific notation
+		'g', 'G', // decimal notation, or scientific for large exponents
+		'v': // "default" format
+		// This is a subset of the verbs are supported by big.Float.Format().
+
+		s := fmt.Sprintf("%d.%09d", m.GetUnits(), m.GetNanos())
+		f := &big.Float{}
+		f.SetString(s)
+
+		fmt.Fprintf(st, "%s ", m.GetCurrencyCode())
+		f.Format(st, verb)
+
+	default:
+		fmt.Fprintf(st, "%%!%c(*money.Money=%s)", verb, f.value.String())
+	}
+}
diff --git a/protomoney/format_test.go b/protomoney/format_test.go
new file mode 100644
index 0000000..625c75c
--- /dev/null
+++ b/protomoney/format_test.go
@@ -0,0 +1,43 @@
+package protomoney_test
+
+import (
+	"fmt"
+
+	. "github.com/dogmatiq/dosh/protomoney"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+	"google.golang.org/genproto/googleapis/type/money"
+)
+
+func ExampleFmt() {
+	m := &money.Money{
+		CurrencyCode: "XYZ",
+		Units:        10,
+		Nanos:        129000000,
+	}
+
+	fmt.Printf("%.2f\n", Fmt(m))
+	// Output: XYZ 10.13
+}
+
+var _ = Describe("func Fmt()", func() {
+	It("returns a formatted representation of the amount", func() {
+		m := &money.Money{
+			CurrencyCode: "XYZ",
+			Units:        10,
+			Nanos:        129000000,
+		}
+		s := fmt.Sprintf("%0.2f", Fmt(m))
+		Expect(s).To(Equal("XYZ 10.13"))
+	})
+
+	It("returns a descriptive string if used with an unsupported verb", func() {
+		m := &money.Money{
+			CurrencyCode: "XYZ",
+			Units:        10,
+			Nanos:        129000000,
+		}
+		s := fmt.Sprintf("%d", Fmt(m))
+		Expect(s).To(Equal("%!d(*money.Money=" + m.String() + ")"))
+	})
+})