utf8.String: provide an Init method to avoid unnecessary allocation

when creating an array of Strings.

R=rsc
CC=golang-dev
https://golang.org/cl/2267046

diff --git a/src/pkg/utf8/string.go b/src/pkg/utf8/string.go
index 59107ac19db26e10517874fab4dae7e292af5559..83b56b9448e968f4fe4609c8d6b8b134b32cc72f 100644 (file)
--- a/src/pkg/utf8/string.go
+++ b/src/pkg/utf8/string.go
@@ -25,20 +25,29 @@ type String struct {
 
 // NewString returns a new UTF-8 string with the provided contents.
 func NewString(contents string) *String {
+       return new(String).Init(contents)
+}
+
+// Init initializes an existing String to hold the provided contents.
+// It returns a pointer to the initialized String.
+func (s *String) Init(contents string) *String {
+       s.str = contents
+       s.bytePos = 0
+       s.runePos = 0
        for i := 0; i < len(contents); i++ {
                if contents[i] >= RuneSelf {
                        // Not ASCII.
-                       _, wid := DecodeRuneInString(contents)
-                       return &String{
-                               str:      contents,
-                               numRunes: RuneCountInString(contents),
-                               width:    wid,
-                               nonASCII: i,
-                       }
+                       s.numRunes = RuneCountInString(contents)
+                       _, s.width = DecodeRuneInString(contents)
+                       s.nonASCII = i
+                       return s
                }
        }
        // ASCII is simple.  Also, the empty string is ASCII.
-       return &String{str: contents, numRunes: len(contents), nonASCII: len(contents)}
+       s.numRunes = len(contents)
+       s.width = 0
+       s.nonASCII = len(contents)
+       return s
 }
 
 // String returns the contents of the String.  This method also means the