time: format negative monotonic times correctly in Time.String

Fixes #18993

Change-Id: Ia1fa20b6d82384b07e9ba5512b909439e0bec2a5
Reviewed-on: https://go-review.googlesource.com/36611
Run-TryBot: Caleb Spare <cespare@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 27 insertions, 25 deletions

diff --git a/src/time/format.go b/src/time/format.go
index 7fe5b51bca..37e759f890 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -433,17 +433,13 @@ func (t Time) String() string {
 
 	// Format monotonic clock reading as m=±ddd.nnnnnnnnn.
 	if t.wall&hasMonotonic != 0 {
-		m2 := t.ext
-		m1, m2 := m2/1e9, m2%1e9
-		if m2 < 0 {
-			m2 += 1e9
-			m1--
-		}
+		m2 := uint64(t.ext)
 		sign := byte('+')
-		if m1 < 0 {
+		if t.ext < 0 {
 			sign = '-'
-			m1 = -m1
+			m2 = -m2
 		}
+		m1, m2 := m2/1e9, m2%1e9
 		m0, m1 := m1/1e9, m1%1e9
 		var buf []byte
 		buf = append(buf, " m="...)
diff --git a/src/time/mono_test.go b/src/time/mono_test.go
index 0bbb5c11f0..b5ae24f0ab 100644
--- a/src/time/mono_test.go
+++ b/src/time/mono_test.go
@@ -5,7 +5,7 @@
 package time_test
 
 import (
-	"regexp"
+	"strings"
 	"testing"
 	. "time"
 )
@@ -231,22 +231,28 @@ func TestMonotonicOverflow(t *testing.T) {
 	}
 }
 
-func TestMonotonicString(t *testing.T) {
-	t1 := Now()
-	re := regexp.MustCompile(` m=\+[0-9]+\.[0-9]{9}$`)
-	if !re.MatchString(t1.String()) {
-		t.Errorf("Now().String() = %q, want match for /%s/", t1.String(), re)
-	}
-
-	t2 := Now().Add(-5 * Hour)
-	re = regexp.MustCompile(` m=-[0-9]+\.[0-9]{9}$`)
-	if !re.MatchString(t2.String()) {
-		t.Errorf("Now().Add(-5*Hour).String() = %q, want match for /%s/", t2.String(), re)
-	}
+var monotonicStringTests = []struct {
+	mono int64
+	want string
+}{
+	{0, "m=+0.000000000"},
+	{123456789, "m=+0.123456789"},
+	{-123456789, "m=-0.123456789"},
+	{123456789000, "m=+123.456789000"},
+	{-123456789000, "m=-123.456789000"},
+	{9e18, "m=+9000000000.000000000"},
+	{-9e18, "m=-9000000000.000000000"},
+	{-1 << 63, "m=-9223372036.854775808"},
+}
 
-	t3 := Now().Add(1.2e18)
-	re = regexp.MustCompile(` m=\+120[0-9]{7}\.[0-9]{9}$`)
-	if !re.MatchString(t3.String()) {
-		t.Errorf("Now().Add(12e17).String() = %q, want match for /%s/", t3.String(), re)
+func TestMonotonicString(t *testing.T) {
+	for _, tt := range monotonicStringTests {
+		t1 := Now()
+		SetMono(&t1, tt.mono)
+		s := t1.String()
+		got := s[strings.LastIndex(s, " ")+1:]
+		if got != tt.want {
+			t.Errorf("with mono=%d: got %q; want %q", tt.mono, got, tt.want)
+		}
 	}
 }