make Format about twice as fast by removing allocations, using a bytes.Buffer

add a benchmark

R=rsc
CC=golang-dev
https://golang.org/cl/181164

diff --git a/src/pkg/time/format.go b/src/pkg/time/format.go
index 52d62a90862bfd8d98f8725a532d443564ac6708..28550aa364f83ff5a1caae09e2e5273226e426ba 100644 (file)
--- a/src/pkg/time/format.go
+++ b/src/pkg/time/format.go
@@ -1,6 +1,7 @@
 package time
 
 import (
+       "bytes"
        "strconv"
 )
 
@@ -12,16 +13,17 @@ const (
 
 // These are predefined layouts for use in Time.Format.
 // The standard time used in the layouts is:
-//     Mon Jan  2 15:04:05 PST 2006  (PST is GMT-0800)
+//     Mon Jan  2 15:04:05 MST 2006  (MST is GMT-0700)
 // which is Unix time 1136243045.
+// (Think of it as 01/02 03:04:05PM '06 -0700.)
 const (
        ANSIC    = "Mon Jan  2 15:04:05 2006"
-       UnixDate = "Mon Jan  2 15:04:05 PST 2006"
-       RFC850   = "Monday, 02-Jan-06 15:04:05 PST"
-       RFC1123  = "Mon, 02 Jan 2006 15:04:05 PST"
+       UnixDate = "Mon Jan  2 15:04:05 MST 2006"
+       RFC850   = "Monday, 02-Jan-06 15:04:05 MST"
+       RFC1123  = "Mon, 02 Jan 2006 15:04:05 MST"
        Kitchen  = "3:04PM"
        // Special case: use Z to get the time zone formatted according to ISO 8601,
-       // which is -0800 or Z for UTC
+       // which is -0700 or Z for UTC
        ISO8601 = "2006-01-02T15:04:05Z"
 )
 
@@ -46,7 +48,7 @@ const (
        stdZulu        = "1504"
        stdPM          = "PM"
        stdpm          = "pm"
-       stdTZ          = "PST"
+       stdTZ          = "MST"
        stdISO8601TZ   = "Z"
 )
 
@@ -112,26 +114,6 @@ func charType(c uint8) int {
        return separator
 }
 
-func pieces(s string) []string {
-       p := make([]string, 20)
-       i := 0
-       // Each iteration generates one piece
-       for n := range p {
-               if i >= len(s) {
-                       p = p[0:n]
-                       break
-               }
-               start := i
-               c := s[i]
-               pieceType := charType(c)
-               for i < len(s) && charType(s[i]) == pieceType {
-                       i++
-               }
-               p[n] = s[start:i]
-       }
-       return p
-}
-
 func zeroPad(i int) string {
        s := strconv.Itoa(i)
        if i < 10 {
@@ -146,9 +128,17 @@ func zeroPad(i int) string {
 // the time to be formatted.  Predefined layouts ANSIC, UnixDate,
 // ISO8601 and others describe standard representations.
 func (t *Time) Format(layout string) string {
-       pc := pieces(layout)
-       s := ""
-       for _, p := range pc {
+       b := new(bytes.Buffer)
+       // Each iteration generates one piece
+       for len(layout) > 0 {
+               c := layout[0]
+               pieceType := charType(c)
+               i := 0
+               for i < len(layout) && charType(layout[i]) == pieceType {
+                       i++
+               }
+               p := layout[0:i]
+               layout = layout[i:]
                switch p {
                case stdYear:
                        p = strconv.Itoa64(t.Year % 100)
@@ -218,9 +208,9 @@ func (t *Time) Format(layout string) string {
                case stdTZ:
                        p = t.Zone
                }
-               s += p
+               b.WriteString(p)
        }
-       return s
+       return b.String()
 }
 
 // String returns a Unix-style representation of the time value.

diff --git a/src/pkg/time/time_test.go b/src/pkg/time/time_test.go
index 4dfdea4456879ad8e58f07b916c22017195462b5..8f7c267cb607bd4ace53caec08b0742006c51b28 100644 (file)
--- a/src/pkg/time/time_test.go
+++ b/src/pkg/time/time_test.go
@@ -161,3 +161,10 @@ func BenchmarkNanoseconds(b *testing.B) {
                Nanoseconds()
        }
 }
+
+func BenchmarkFormat(b *testing.B) {
+       time := SecondsToLocalTime(1265346057)
+       for i := 0; i < b.N; i++ {
+               time.Format("Mon Jan  2 15:04:05 2006")
+       }
+}