strings: add ReadRune to Reader

R=rsc
CC=golang-dev
https://golang.org/cl/940041

src/pkg/strings/reader.go

diff --git a/src/pkg/strings/reader.go b/src/pkg/strings/reader.go
index 7cefbd2..914faa0 100644
--- a/src/pkg/strings/reader.go
+++ b/src/pkg/strings/reader.go
@@ -4,9 +4,12 @@
 
 package strings
 
-import "os"
+import (
+	"os"
+	"utf8"
+)
 
-// A Reader satisfies calls to Read and ReadByte by
+// A Reader satisfies calls to Read, ReadByte, and ReadRune by
 // reading from a string.
 type Reader string
 
@@ -33,6 +36,26 @@
 	return
 }
 
+// ReadRune reads and returns the next UTF-8-encoded
+// Unicode code point from the buffer.
+// If no bytes are available, the error returned is os.EOF.
+// If the bytes are an erroneous UTF-8 encoding, it
+// consumes one byte and returns U+FFFD, 1.
+func (r *Reader) ReadRune() (rune int, size int, err os.Error) {
+	s := *r
+	if len(s) == 0 {
+		return 0, 0, os.EOF
+	}
+	c := s[0]
+	if c < utf8.RuneSelf {
+		*r = s[1:]
+		return int(c), 1, nil
+	}
+	rune, size = utf8.DecodeRuneInString(string(s))
+	*r = s[size:]
+	return
+}
+
 // NewReader returns a new Reader reading from s.
 // It is similar to bytes.NewBufferString but more efficient and read-only.
 func NewReader(s string) *Reader { return (*Reader)(&s) }

src/pkg/strings/strings_test.go

diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index 3c9dc584..2c99a6e 100644
--- a/src/pkg/strings/strings_test.go
+++ b/src/pkg/strings/strings_test.go
@@ -5,6 +5,7 @@
 package strings_test
 
 import (
+	"os"
 	. "strings"
 	"testing"
 	"unicode"
@@ -576,3 +577,25 @@
 		}
 	}
 }
+
+func TestReadRune(t *testing.T) {
+	testStrings := []string{"", abcd, faces, commas}
+	for _, s := range testStrings {
+		reader := NewReader(s)
+		res := ""
+		for {
+			r, _, e := reader.ReadRune()
+			if e == os.EOF {
+				break
+			}
+			if e != nil {
+				t.Errorf("Reading %q: %s", s, e)
+				break
+			}
+			res += string(r)
+		}
+		if res != s {
+			t.Errorf("Reader(%q).ReadRune() produced %q", s, res)
+		}
+	}
+}