2 files changed, 34 insertions, 54 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index aa07b9fbc1..ce52649f13 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -227,19 +227,26 @@ func IndexAny(s []byte, chars string) int {
 			continue
 		}
 		r, width = utf8.DecodeRune(s[i:])
-		if r == utf8.RuneError {
-			for _, r = range chars {
-				if r == utf8.RuneError {
+		if r != utf8.RuneError {
+			// r is 2 to 4 bytes
+			if len(chars) == width {
+				if chars == string(r) {
 					return i
 				}
+				continue
+			}
+			// Use bytealg.IndexString for performance if available.
+			if bytealg.MaxLen >= width {
+				if bytealg.IndexString(chars, string(r)) >= 0 {
+					return i
+				}
+				continue
 			}
-			continue
 		}
-		// r is 2 to 4 bytes. Using strings.Index is more reasonable, but as the bytes
-		// package should not import the strings package, use bytealg.IndexString
-		// instead. And this does not seem to lose much performance.
-		if chars == string(r) || bytealg.IndexString(chars, string(r)) >= 0 {
-			return i
+		for _, ch := range chars {
+			if r == ch {
+				return i
+			}
 		}
 	}
 	return -1
@@ -304,19 +311,26 @@ func LastIndexAny(s []byte, chars string) int {
 		}
 		r, size := utf8.DecodeLastRune(s[:i])
 		i -= size
-		if r == utf8.RuneError {
-			for _, r = range chars {
-				if r == utf8.RuneError {
+		if r != utf8.RuneError {
+			// r is 2 to 4 bytes
+			if len(chars) == size {
+				if chars == string(r) {
 					return i
 				}
+				continue
+			}
+			// Use bytealg.IndexString for performance if available.
+			if bytealg.MaxLen >= size {
+				if bytealg.IndexString(chars, string(r)) >= 0 {
+					return i
+				}
+				continue
 			}
-			continue
 		}
-		// r is 2 to 4 bytes. Using strings.Index is more reasonable, but as the bytes
-		// package should not import the strings package, use bytealg.IndexString
-		// instead. And this does not seem to lose much performance.
-		if chars == string(r) || bytealg.IndexString(chars, string(r)) >= 0 {
-			return i
+		for _, ch := range chars {
+			if r == ch {
+				return i
+			}
 		}
 	}
 	return -1
diff --git a/src/internal/bytealg/index_generic.go b/src/internal/bytealg/index_generic.go
index 83345f1013..98e859f925 100644
--- a/src/internal/bytealg/index_generic.go
+++ b/src/internal/bytealg/index_generic.go
@@ -16,42 +16,8 @@ func Index(a, b []byte) int {
 
 // IndexString returns the index of the first instance of b in a, or -1 if b is not present in a.
 // Requires 2 <= len(b) <= MaxLen.
-func IndexString(s, substr string) int {
-	// This is a partial copy of strings.Index, here because bytes.IndexAny and bytes.LastIndexAny
-	// call bytealg.IndexString. Some platforms have an optimized assembly version of this function.
-	// This implementation is used for those that do not. Although the pure Go implementation here
-	// works for the case of len(b) > MaxLen, we do not require that its assembly implementation also
-	// supports the case of len(b) > MaxLen. And we do not guarantee that this function supports the
-	// case of len(b) > MaxLen.
-	n := len(substr)
-	c0 := substr[0]
-	c1 := substr[1]
-	i := 0
-	t := len(s) - n + 1
-	fails := 0
-	for i < t {
-		if s[i] != c0 {
-			o := IndexByteString(s[i:t], c0)
-			if o < 0 {
-				return -1
-			}
-			i += o
-		}
-		if s[i+1] == c1 && s[i:i+n] == substr {
-			return i
-		}
-		i++
-		fails++
-		if fails >= 4+i>>4 && i < t {
-			// See comment in src/bytes/bytes.go.
-			j := IndexRabinKarp(s[i:], substr)
-			if j < 0 {
-				return -1
-			}
-			return i + j
-		}
-	}
-	return -1
+func IndexString(a, b string) int {
+	panic("unimplemented")
 }
 
 // Cutover reports the number of failures of IndexByte we should tolerate