time: add Duration.Truncate and Duration.Round

Fixes #18996

Change-Id: I0b0f7270960b368ce97ad4456f60bcc1fc2a8313
Reviewed-on: https://go-review.googlesource.com/36615
Run-TryBot: Caleb Spare <cespare@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/time.go

@@ -783,6 +783,45 @@ func (d Duration) Hours() float64 {
 	return float64(hour) + float64(nsec)/(60*60*1e9)
 }
 
+// Truncate returns the result of rounding d toward zero to a multiple of m.
+// If m <= 0, Truncate returns d unchanged.
+func (d Duration) Truncate(m Duration) Duration {
+	if m <= 0 {
+		return d
+	}
+	return d - d%m
+}
+
+// Round returns the result of rounding d to the nearest multiple of m.
+// The rounding behavior for halfway values is to round away from zero.
+// If the result exceeds the maximum (or minimum)
+// value that can be stored in a Duration,
+// Round returns the maximum (or minimum) duration.
+// If m <= 0, Round returns d unchanged.
+func (d Duration) Round(m Duration) Duration {
+	if m <= 0 {
+		return d
+	}
+	r := d % m
+	if d < 0 {
+		r = -r
+		if r+r < m {
+			return d + r
+		}
+		if d1 := d - m + r; d1 < d {
+			return d1
+		}
+		return minDuration // overflow
+	}
+	if r+r < m {
+		return d - r
+	}
+	if d1 := d + m - r; d1 > d {
+		return d1
+	}
+	return maxDuration // overflow
+}
+
 // Add returns the time t+d.
 func (t Time) Add(d Duration) Time {
 	dsec := int64(d / 1e9)

src/time/time_test.go

@@ -1058,6 +1058,65 @@ func TestDurationHours(t *testing.T) {
 	}
 }
 
+var durationTruncateTests = []struct {
+	d    Duration
+	m    Duration
+	want Duration
+}{
+	{0, Second, 0},
+	{Minute, -7 * Second, Minute},
+	{Minute, 0, Minute},
+	{Minute, 1, Minute},
+	{Minute + 10*Second, 10 * Second, Minute + 10*Second},
+	{2*Minute + 10*Second, Minute, 2 * Minute},
+	{10*Minute + 10*Second, 3 * Minute, 9 * Minute},
+	{Minute + 10*Second, Minute + 10*Second + 1, 0},
+	{Minute + 10*Second, Hour, 0},
+	{-Minute, Second, -Minute},
+	{-10 * Minute, 3 * Minute, -9 * Minute},
+	{-10 * Minute, Hour, 0},
+}
+
+func TestDurationTruncate(t *testing.T) {
+	for _, tt := range durationTruncateTests {
+		if got := tt.d.Truncate(tt.m); got != tt.want {
+			t.Errorf("Duration(%s).Truncate(%s) = %s; want: %s", tt.d, tt.m, got, tt.want)
+		}
+	}
+}
+
+var durationRoundTests = []struct {
+	d    Duration
+	m    Duration
+	want Duration
+}{
+	{0, Second, 0},
+	{Minute, -11 * Second, Minute},
+	{Minute, 0, Minute},
+	{Minute, 1, Minute},
+	{2 * Minute, Minute, 2 * Minute},
+	{2*Minute + 10*Second, Minute, 2 * Minute},
+	{2*Minute + 30*Second, Minute, 3 * Minute},
+	{2*Minute + 50*Second, Minute, 3 * Minute},
+	{-Minute, 1, -Minute},
+	{-2 * Minute, Minute, -2 * Minute},
+	{-2*Minute - 10*Second, Minute, -2 * Minute},
+	{-2*Minute - 30*Second, Minute, -3 * Minute},
+	{-2*Minute - 50*Second, Minute, -3 * Minute},
+	{8e18, 3e18, 9e18},
+	{9e18, 5e18, 1<<63 - 1},
+	{-8e18, 3e18, -9e18},
+	{-9e18, 5e18, -1 << 63},
+}
+
+func TestDurationRound(t *testing.T) {
+	for _, tt := range durationRoundTests {
+		if got := tt.d.Round(tt.m); got != tt.want {
+			t.Errorf("Duration(%s).Round(%s) = %s; want: %s", tt.d, tt.m, got, tt.want)
+		}
+	}
+}
+
 var defaultLocTests = []struct {
 	name string
 	f    func(t1, t2 Time) bool