path/filepath: avoid allocation in Clean of cleaned path even on windows (fix build)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6344049

src/pkg/path/filepath/path.go

@@ -18,26 +18,28 @@ import (
 // and retrieving the final string. It does not allocate a buffer
 // to hold the output until that output diverges from s.
 type lazybuf struct {
-	s   string
-	buf []byte
-	w   int
+	path       string
+	buf        []byte
+	w          int
+	volAndPath string
+	volLen     int
 }
 
 func (b *lazybuf) index(i int) byte {
 	if b.buf != nil {
 		return b.buf[i]
 	}
-	return b.s[i]
+	return b.path[i]
 }
 
 func (b *lazybuf) append(c byte) {
 	if b.buf == nil {
-		if b.w < len(b.s) && b.s[b.w] == c {
+		if b.w < len(b.path) && b.path[b.w] == c {
 			b.w++
 			return
 		}
-		b.buf = make([]byte, len(b.s))
-		copy(b.buf, b.s[:b.w])
+		b.buf = make([]byte, len(b.path))
+		copy(b.buf, b.path[:b.w])
 	}
 	b.buf[b.w] = c
 	b.w++
@@ -45,9 +47,9 @@ func (b *lazybuf) append(c byte) {
 
 func (b *lazybuf) string() string {
 	if b.buf == nil {
-		return b.s[:b.w]
+		return b.volAndPath[:b.volLen+b.w]
 	}
-	return string(b.buf[:b.w])
+	return b.volAndPath[:b.volLen] + string(b.buf[:b.w])
 }
 
 const (
@@ -77,14 +79,15 @@ const (
 // Getting Dot-Dot Right,''
 // http://plan9.bell-labs.com/sys/doc/lexnames.html
 func Clean(path string) string {
-	vol := VolumeName(path)
-	path = path[len(vol):]
+	originalPath := path
+	volLen := volumeNameLen(path)
+	path = path[volLen:]
 	if path == "" {
-		if len(vol) > 1 && vol[1] != ':' {
+		if volLen > 1 && originalPath[1] != ':' {
 			// should be UNC
-			return FromSlash(vol)
+			return FromSlash(originalPath)
 		}
-		return vol + "."
+		return originalPath + "."
 	}
 	rooted := os.IsPathSeparator(path[0])
 
@@ -94,7 +97,7 @@ func Clean(path string) string {
 	//	dotdot is index in buf where .. must stop, either because
 	//		it is the leading slash or it is a leading ../../.. prefix.
 	n := len(path)
-	out := lazybuf{s: path}
+	out := lazybuf{path: path, volAndPath: originalPath, volLen: volLen}
 	r, dotdot := 0, 0
 	if rooted {
 		out.append(Separator)
@@ -146,7 +149,7 @@ func Clean(path string) string {
 		out.append('.')
 	}
 
-	return FromSlash(vol + out.string())
+	return FromSlash(out.string())
 }
 
 // ToSlash returns the result of replacing each separator character
@@ -448,3 +451,11 @@ func Dir(path string) string {
 	}
 	return vol + dir
 }
+
+// VolumeName returns leading volume name.  
+// Given "C:\foo\bar" it returns "C:" under windows.
+// Given "\\host\share\foo" it returns "\\host\share".
+// On other platforms it returns "".
+func VolumeName(path string) (v string) {
+	return path[:volumeNameLen(path)]
+}

src/pkg/path/filepath/path_plan9.go

@@ -11,10 +11,10 @@ func IsAbs(path string) bool {
 	return strings.HasPrefix(path, "/") || strings.HasPrefix(path, "#")
 }
 
-// VolumeName returns the leading volume name on Windows.
-// It returns "" elsewhere.
-func VolumeName(path string) string {
-	return ""
+// volumeNameLen returns length of the leading volume name on Windows.
+// It returns 0 elsewhere.
+func volumeNameLen(path string) int {
+	return 0
 }
 
 // HasPrefix exists for historical compatibility and should not be used.

src/pkg/path/filepath/path_unix.go

@@ -13,10 +13,10 @@ func IsAbs(path string) bool {
 	return strings.HasPrefix(path, "/")
 }
 
-// VolumeName returns the leading volume name on Windows.
-// It returns "" elsewhere.
-func VolumeName(path string) string {
-	return ""
+// volumeNameLen returns length of the leading volume name on Windows.
+// It returns 0 elsewhere.
+func volumeNameLen(path string) int {
+	return 0
 }
 
 // HasPrefix exists for historical compatibility and should not be used.

src/pkg/path/filepath/path_windows.go

@@ -14,29 +14,27 @@ func isSlash(c uint8) bool {
 
 // IsAbs returns true if the path is absolute.
 func IsAbs(path string) (b bool) {
-	v := VolumeName(path)
-	if v == "" {
+	l := volumeNameLen(path)
+	if l == 0 {
 		return false
 	}
-	path = path[len(v):]
+	path = path[l:]
 	if path == "" {
 		return false
 	}
 	return isSlash(path[0])
 }
 
-// VolumeName returns leading volume name.  
-// Given "C:\foo\bar" it returns "C:" under windows.
-// Given "\\host\share\foo" it returns "\\host\share".
-// On other platforms it returns "".
-func VolumeName(path string) (v string) {
+// volumeNameLen returns length of the leading volume name on Windows.
+// It returns 0 elsewhere.
+func volumeNameLen(path string) int {
 	if len(path) < 2 {
-		return ""
+		return 0
 	}
 	// with drive letter
 	c := path[0]
 	if path[1] == ':' && ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z') {
-		return path[:2]
+		return 2
 	}
 	// is it UNC
 	if l := len(path); l >= 5 && isSlash(path[0]) && isSlash(path[1]) &&
@@ -56,13 +54,13 @@ func VolumeName(path string) (v string) {
 							break
 						}
 					}
-					return path[:n]
+					return n
 				}
 				break
 			}
 		}
 	}
-	return ""
+	return 0
 }
 
 // HasPrefix exists for historical compatibility and should not be used.