bytes, strings: restore O(1) behavior of IndexAny(s, "") and LastIndexAny(s, "")

CL 65851 (bytes) and CL 65910 (strings) “improve[d] readability”
by removing the special case that bypassed the whole function body
when chars == "". In doing so, yes, the function was unindented a
level, which is nice, but the runtime of that case went from O(1) to O(n)
where n = len(s).

I don't know if anyone's code depends on the O(1) behavior in this case,
but quite possibly someone's does.

This CL adds the special case back, with a comment to prevent future
deletions, and without reindenting each function body in full.

Change-Id: I5aba33922b304dd1b8657e6d51d6c937a7f95c81
Reviewed-on: https://go-review.googlesource.com/78112
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

2 files changed, 9 insertions, 1 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index 260f32500a..9af177fa88 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -144,6 +144,10 @@ func IndexRune(s []byte, r rune) int {
 // code points in chars. It returns -1 if chars is empty or if there is no code
 // point in common.
 func IndexAny(s []byte, chars string) int {
+	if chars == "" {
+		// Avoid scanning all of s.
+		return -1
+	}
 	if len(s) > 8 {
 		if as, isASCII := makeASCIISet(chars); isASCII {
 			for i, c := range s {
@@ -176,6 +180,10 @@ func IndexAny(s []byte, chars string) int {
 // the Unicode code points in chars. It returns -1 if chars is empty or if
 // there is no code point in common.
 func LastIndexAny(s []byte, chars string) int {
+	if chars == "" {
+		// Avoid scanning all of s.
+		return -1
+	}
 	if len(s) > 8 {
 		if as, isASCII := makeASCIISet(chars); isASCII {
 			for i := len(s) - 1; i >= 0; i-- {
diff --git a/src/bytes/example_test.go b/src/bytes/example_test.go
index 00e39cadcb..5b7a46058f 100644
--- a/src/bytes/example_test.go
+++ b/src/bytes/example_test.go
@@ -155,7 +155,7 @@ func ExampleCount() {
 
 func ExampleEqual() {
 	fmt.Println(bytes.Equal([]byte("Go"), []byte("Go")))
-	fmt.Println(bytes.Equal([]byte("Go"), []byte("go")))
+	fmt.Println(bytes.Equal([]byte("Go"), []byte("C++")))
 	// Output:
 	// true
 	// false