Implement fmt.Formatter

uuid.go

@@ -33,6 +33,8 @@ import (
 	"encoding/binary"
 	"encoding/hex"
 	"fmt"
+	"io"
+	"strings"
 	"time"
 )
 
@@ -156,6 +158,65 @@ func (u UUID) String() string {
 	return string(buf)
 }
 
+// Format implements fmt.Formatter for UUID values.
+//
+// The behavior is as follows:
+// The 'x' and 'X' verbs output only the hex digits of the UUID, using a-f for 'x' and A-F for 'X'.
+// The 'v', '+v', 's' and 'q' verbs return the canonical RFC-4122 string representation.
+// The 'S' verb returns the RFC-4122 format, but with capital hex digits.
+// The '#v' verb returns the "Go syntax" representation, which is a 16 byte array initializer.
+// All other verbs not handled directly by the fmt package (like '%p') are unsupported and will return
+// "%!verb(uuid.UUID=value)" as recommended by the fmt package.
+func (u UUID) Format(f fmt.State, c rune) {
+	switch c {
+	case 'x', 'X':
+		s := hex.EncodeToString(u.Bytes())
+		if c == 'X' {
+			s = strings.Map(toCapitalHexDigits, s)
+		}
+		_, _ = io.WriteString(f, s)
+	case 'v':
+		var s string
+		if f.Flag('#') {
+			s = fmt.Sprintf("%#v", [Size]byte(u))
+		} else {
+			s = u.String()
+		}
+		_, _ = io.WriteString(f, s)
+	case 's', 'S':
+		s := u.String()
+		if c == 'S' {
+			s = strings.Map(toCapitalHexDigits, s)
+		}
+		_, _ = io.WriteString(f, s)
+	case 'q':
+		_, _ = io.WriteString(f, `"`+u.String()+`"`)
+	default:
+		// invalid/unsupported format verb
+		fmt.Fprintf(f, "%%!%c(uuid.UUID=%s)", c, u.String())
+	}
+}
+
+func toCapitalHexDigits(ch rune) rune {
+	// convert a-f hex digits to A-F
+	switch ch {
+	case 'a':
+		return 'A'
+	case 'b':
+		return 'B'
+	case 'c':
+		return 'C'
+	case 'd':
+		return 'D'
+	case 'e':
+		return 'E'
+	case 'f':
+		return 'F'
+	default:
+		return ch
+	}
+}
+
 // SetVersion sets the version bits.
 func (u *UUID) SetVersion(v byte) {
 	u[6] = (u[6] & 0x0f) | (v << 4)

uuid_test.go

@@ -35,6 +35,7 @@ func TestUUID(t *testing.T) {
 	t.Run("Variant", testUUIDVariant)
 	t.Run("SetVersion", testUUIDSetVersion)
 	t.Run("SetVariant", testUUIDSetVariant)
+	t.Run("Format", testUUIDFormat)
 }
 
 func testUUIDBytes(t *testing.T) {
@@ -114,6 +115,43 @@ func testUUIDSetVariant(t *testing.T) {
 	}
 }
 
+func testUUIDFormat(t *testing.T) {
+	val := Must(FromString("12345678-90ab-cdef-1234-567890abcdef"))
+	tests := []struct {
+		u    UUID
+		f    string
+		want string
+	}{
+		{u: val, f: "%s", want: "12345678-90ab-cdef-1234-567890abcdef"},
+		{u: val, f: "%S", want: "12345678-90AB-CDEF-1234-567890ABCDEF"},
+		{u: val, f: "%q", want: `"12345678-90ab-cdef-1234-567890abcdef"`},
+		{u: val, f: "%x", want: "1234567890abcdef1234567890abcdef"},
+		{u: val, f: "%X", want: "1234567890ABCDEF1234567890ABCDEF"},
+		{u: val, f: "%v", want: "12345678-90ab-cdef-1234-567890abcdef"},
+		{u: val, f: "%+v", want: "12345678-90ab-cdef-1234-567890abcdef"},
+		{u: val, f: "%#v", want: "[16]uint8{0x12, 0x34, 0x56, 0x78, 0x90, 0xab, 0xcd, 0xef, 0x12, 0x34, 0x56, 0x78, 0x90, 0xab, 0xcd, 0xef}"},
+		{u: val, f: "%T", want: "uuid.UUID"},
+		{u: val, f: "%t", want: "%!t(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%b", want: "%!b(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%c", want: "%!c(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%d", want: "%!d(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%e", want: "%!e(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%E", want: "%!E(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%f", want: "%!f(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%F", want: "%!F(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%g", want: "%!g(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%G", want: "%!G(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%o", want: "%!o(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+		{u: val, f: "%U", want: "%!U(uuid.UUID=12345678-90ab-cdef-1234-567890abcdef)"},
+	}
+	for _, tt := range tests {
+		got := fmt.Sprintf(tt.f, tt.u)
+		if tt.want != got {
+			t.Errorf("Format(\"%s\") got %s, want %s", tt.f, got, tt.want)
+		}
+	}
+}
+
 func TestMust(t *testing.T) {
 	sentinel := fmt.Errorf("uuid: sentinel error")
 	defer func() {